The essence for knowledgeable practitioners
The amplifier is assembled according to the “dual mono” principle; the circuit diagram of one channel is shown in Fig.1. The first stage on transistors VT1-VT4 is a voltage amplifier with a coefficient of about 2.9, the second stage on VT5 is a current amplifier (emitter follower). With an input voltage of 1 V, the output power is about 0.5 W into a 16 Ohm load. Operating frequency range at -1 dB level is approximately from 3 Hz to 250 kHz. The input impedance of the amplifier is 6.5...7 kOhm, the output impedance is 0.2 Ohm.
THD graphs at 1 kHz with output powers of 0.52 W and 0.15 W are shown in Fig.2 And Fig.3(the signal is supplied to the sound card through a “30:1” divider).
On Fig.4 shows the result of intermodulation distortion when measured with two tones of equal level (19 kHz and 20 kHz).
The amplifier is assembled in a suitable-sized housing taken from another amplifier. The fan control unit ( Fig.5), controlling the temperature of one of the output transistor heatsinks (the surface-mount circuit board is visible in the center on Figure 6).
The sound rating by ear is “not bad”. The sound is not “linked” to the speakers, there is a panorama, but its “depth” is less than what I’m used to. I haven’t figured out what this is connected with yet, but it’s possible (options with other transistors, changing the quiescent current of the output stages and searching for connection points for input/output “grounds” were tested).
Now for those who are interested, a little about experiments
The experiments took quite a long time and were carried out a little chaotically - transitions from one to another were made as some questions were solved and others appeared, so some discrepancies may be noticeable in the diagrams and measurements. In the diagrams this is reflected as a violation of the numbering of elements, and in measurements - as a change in the level of noise, interference from the 50 Hz network, 100 Hz ripple and their products (different power supplies were used). But in most cases, measurements were taken several times, so inaccuracies should not be particularly significant.
All experiments can be divided into several. The first was carried out to assess the fundamental performance of the TND stage, the next ones were carried out to check such characteristics as load capacity, gain, linearity dependence, and operation with the output stage.
Quite complete theoretical information about the operation of the TND cascade can be found in the articles by G.F. Prishchepov in the magazines “Scheme Engineering” No. 9 2006 and “Radio Hobby” No. 3 2010 (the texts there are approximately the same), so only its practical application will be considered here.
So, the first thing is to assess the fundamental performance
First, a circuit was assembled using KT315 transistors with a gain of about three ( Fig.7). When checking, it turned out that with the values of R3 and R4 shown in the diagram, the amplifier only works with low-level signals, and when 1 V is applied, an overload occurs at the input (1 V is the level that the PCD and the computer sound card can output, therefore, almost all measurements are reduced to it). On Figure 8 The lower graph shows the spectrum of the output signal, the upper graph shows the input signal and distortions are visible on it (THI should be about 0.002-0.006%). Looking at the graphs and comparing the levels in the channels, we must take into account that the output signal enters the sound card through a 10:1 divider (with an input resistance of about 30 kOhm, resistors R5 and R6 at Fig.7) – below in the text, the divisor parameters will be different and this will always be indicated).
If we assume that the appearance of distortion in the input signal indicates a change in the input resistance of the cascade (which is usually caused by an incorrectly selected DC mode), then to work with larger input signals, the resistance R4 should be increased and, accordingly, to maintain Kus equal to three, increase R3 .
After setting R3=3.3 kOhm, R4=1.1 kOhm, R1=90 kOhm and increasing the supply voltage to 23V, it was possible to obtain a more or less acceptable THD value ( Fig.9). It also turned out that the TND cascade “does not like” low-resistance loads, i.e. the greater the resistance of the next stage, the lower the harmonic levels and the closer to the calculated value the gain becomes (another example will be considered below).
Then the amplifier was assembled on a printed circuit board and an emitter follower based on a composite transistor KT829A was connected to it (circuit on Figure 1). After installing the transistor and board on the radiator ( Fig.10), the amplifier was tested when operating into an 8 ohm load. On Figure 11 it can be seen that the SOI value has increased significantly, but this is the result of the operation of the emitter follower (the signal from the amplifier input (top graph) is taken directly to the computer, and from the output through a 3:1 divider (bottom graph)).
On Figure 12 shows the THD graph with an input signal of 0.4 V:
After this, two more variants of repeaters were tested - with a composite transistor made of bipolar KT602B + KT908A and with a field-effect IRF630A (it required an increase in the quiescent current by installing + 14.5 V on the gate and reducing the resistance R7 to 5 Ohms at a constant voltage across it of 9. 9 V (quiescent current about 1.98 A)). The best results obtained with input voltages of 1 V and 0.4 V are shown in pictures 13 And 14 (KT602B+KT908A), 15 And 16 (IRF630A):
After these checks, the circuit returned to the version with the KT829 transistor, the second channel was assembled, and after listening to the prototype when powered from laboratory sources, the amplifier shown in Figure 6. It took two or three days of listening and minor modifications, but this had almost no effect on the sound and characteristics of the amplifier.
Load Capacity Assessment
Since the desire to test the TND cascade for “load capacity” has not yet disappeared, a new prototype was assembled using 4 transistors in a chain ( Fig.17). Supply voltage +19 V, divider at the cascade output 30 kOhm “10:1”, input signal – 0.5 V, output – 1.75 V (gain is 3.5, but if the divider is turned off, the output voltage is about 1.98 V, which indicates Kus = 3.96):
By selecting the resistance of resistor R1, you can obtain a certain minimum SOI, and this graph with a load of 30 kOhm is shown in Figure 18. But if we now install another one of the same value (54 kOhm) in series with resistor R5, then the harmonics take the form shown in Figure 19– the second harmonic increases by about 20 dB relative to the fundamental tone and in order to return it to a low value, you need to change the resistance R1 again. This indirectly indicates that in order to obtain the most stable SOI values, the cascade power supply must be stabilized. It is easy to check - changing the supply voltage approximately also changes the appearance of the harmonic “tail”.
Okay, so this stage works with 0.5V input. Now we need to check it at 1 V and, say, with a gain of “5”.
Gain Estimation
The cascade is assembled using KT315 transistors, supply voltage +34.5 V ( Fig.20). To obtain Kus = 5, resistors R3 and R4 with nominal values of 8.38 kOhm and 1.62 kOhm were installed. On a load in the form of a 10:1 resistor divider with an input resistance of about 160 kOhm, the output voltage was about 4.6 V.
On Figure 21 it can be seen that the SOI is less than 0.016%. A high level of interference of 50 Hz and other multiples of higher frequencies means poor power filtering (works to the limit).
A KP303+KT829 repeater was connected to this stage ( Fig.22) and then the characteristics of the entire amplifier were taken when operating into an 8 Ohm load ( Fig.23). Supply voltage 26.9 V, gain about 4.5 (4.5 V AC output into an 8 Ohm load is approximately 2.5 W). When setting the repeater to the minimum SOI level, it was necessary to change the bias voltage of the TND stage, but since its distortion level is much lower than that of the repeater, this did not affect the hearing in any way - two channels were assembled and listened to in a prototype version. There were no differences in sound with the half-watt version of the amplifier described above, but since the amplification of the new version was excessive and it generated more heat, the circuit was disassembled.
When adjusting the bias voltage TND of the cascade, you can find such a position that the harmonic “tail” has a more even decay, but becomes longer and at the same time the level of the second harmonic increases by 6-10 dB (the total THD becomes about 0.8-0.9%) .
With such a large SOI repeater, by changing the value of resistor R3, you can safely change the gain of the first stage, both up and down.
Checking a cascade with a higher quiescent current
The circuit was assembled using a KTS613B transistor assembly. The cascade's quiescent current of 3.6 mA is the highest of all tested options. The output voltage at the 30 kOhm resistor divider turned out to be 2.69V, with a THD of about 0.008% (( Fig.25). This is approximately three times less than shown in Figure 9 when checking the cascade on KT315 (with the same gain and approximately the same supply voltage). But since it was not possible to find another similar transistor assembly, the second channel was not assembled and the amplifier, accordingly, did not listen.
When the resistance R5 is doubled and without adjusting the bias voltage, the SOI becomes about 0.01% ( Fig.26). We can say that the appearance of the “tail” changes slightly.
An attempt to estimate the operating frequency band
First, the prototype assembled on a transistor assembly was checked. When using the GZ-118 generator with an output frequency band from 5 Hz to 210 kHz, no “blockages at the edges” were detected.
Then the already assembled half-watt amplifier was checked. It attenuated the 210 kHz signal by about 0.5 dB (with no change at 180 kHz).
There was nothing to estimate the lower limit; at least, it was not possible to see the difference between the input and output signals when running the program sweep generator, starting with frequencies of 5 Hz. Therefore, we can assume that it is limited by the capacitance of the coupling capacitor C1, the input resistance of the TND stage, as well as the capacitance of the “output” capacitor C7 and the load resistance of the amplifier - an approximate calculation in the program shows -1 dB at a frequency of 2.6 Hz and -3 dB at a frequency 1.4 Hz ( Fig.27).
Since the input impedance of the TND stage is quite low, the volume control should be selected no more than 22...33 kOhm.
A replacement for the output stage can be any repeater (current amplifier) with a sufficiently large input impedance.
Attached to the text are files of two versions of printed circuit boards in the format of the program version 5 (the drawing must be “mirrored” when making boards).
Afterword
A few days later, I increased the power supply to the channels by 3 V, replaced the 25-volt electrolytic capacitors with 35-volt ones, and adjusted the bias voltages of the first stages to the minimum SOI. The quiescent currents of the output stages became about 1.27 A, the values of SOI and IMD at 0.52 W output power decreased to 0.028% and 0.017% ( Fig.28 And 29 ). The graphs show that the ripples at 50 Hz and 100 Hz have increased, but they are not audible.
Literature:
1. G. Prishchepov, “Linear broadband TND amplifiers and repeaters,” magazine “Scheme Engineering” No. 9, 2006.
Andrey Goltsov, r9o-11, Iskitim
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad | |
|---|---|---|---|---|---|---|---|
| Figure No. 1, details for one channel | |||||||
| VT1...VT4 | Bipolar transistor | PMSS3904 | 4 | To notepad | |||
| VT5 | Bipolar transistor | KT829A | 1 | To notepad | |||
| VD1...VD4 | Diode | KD2999V | 4 | To notepad | |||
| R1 | Resistor | 91 kOhm | 1 | smd 0805, select the exact value when configuring | To notepad | ||
| R2 | Resistor | 15 kOhm | 1 | smd 0805 | To notepad | ||
| R3 | Resistor | 3.3 kOhm | 1 | smd 0805 | To notepad | ||
| R4 | Resistor | 1.1 kOhm | 1 | smd 0805 | To notepad | ||
| R5, R6 | Resistor | 22 Ohm | 2 | smd 0805 | To notepad | ||
| R7 | Resistor | 12 ohm | 1 | dial from PEV-10 | To notepad | ||
| R8, R9 | Resistor | ||||||
Pre-amplification stages. A typical signal source used to develop an output voltage of 50-200 mV. High-quality amplifiers were oriented towards this voltage. Correction circuits were previously located between the input sockets and the grid of the first lamp, in which the signal was attenuated by at least half (6 dB) at the most sensitive input. In the fine-compensated volume control, the minimum signal attenuation is another 6 dB. Tone controls that provide ±20dB of control typically attenuate the signal by another 30-40dB. If there were cathode followers in the input circuits, the signal loss increased by another 3-6 dB. So, the total signal attenuation used to be 45-58 dB. The signal voltage on the grids of the final stage lamps averages 10-20 V. The ratio of this value to the input signal voltage is 10/0.05 = 200 (46 dB). So, the amplification of the preliminary stages, taking into account the signal attenuation and the required voltage on the grids of the final stage lamps, should previously have been on the order of 90-100 dB. In other words, the gain of the preliminary stages should be approximately 100,000. This is quite a significant value for a low-frequency amplifier. If the voltage gain of each of the amplifier stages is approximately 10, then, obviously, the number of stages should be equal to 5. If the gain of each stage is about 100, the total number of stages will be equal to 3 (with some margin). Since a gain of 10 per stage is provided by almost any modern low-frequency tube triode, and a gain of 100 per stage is the limit even for good low-frequency pentodes, it can be argued that for tube amplifiers the number of pre-amplification stages should range from three until five.
How many cascades should you make: 3 or 5? The first answer, of course, is “3”. However, there is no need to rush. Three cascades - this means the minimum gain of the cascade is equal to the third root of 10000. Note that this is not the μ of the lamp, but the gain of the cascade, which rarely exceeds 50% of the μ of the lamp. Therefore, triodes are no longer needed. This means there will be three cascades on pentodes or, in extreme cases, two on pentodes and one on a triode. The latter circuit, which does not have any gain margin, does not allow the use of negative feedback in the circuit, i.e. practically unsuitable for Hi-Fi amplifiers, because without negative feedback it is impossible to reduce the coefficient of nonlinear distortion and expand the frequency range to the required values. Three stages on pentodes can allow the introduction of negative feedback, but then the first, input stage is also assembled on the pentode, and in this case, as experience shows, it is almost impossible to achieve a complete absence of microphone effect and a background level below 60 dB. The other extreme - five stages on triodes - always provides the required gain even on the worst tubes, however, using tubes with an average gain of about 20-50, it is easy to obtain the required gain with a sufficient margin with four triodes (i.e. on two double lamps). This scheme is the most common. True, many foreign companies produce a specially designed pentode for the input stage with a low level of self-noise and not prone to microphone effects (EF-184, EF-804, etc.). Using such a pentode and subsequent triodes with a large μ (90-120) of the ECC-83 type, it is possible to obtain the required gain on three stages using the pentode - triode - triode system, but firstly, such a system requires the use of special lamps, and - secondly - very high quality transformer steel, highly sensitive end lamps, etc. Therefore, this scheme is not suitable.
Note. In the 21st century, the situation has changed significantly. Nowadays no one is using physical analogue pre-amplifier stages. Pre-processing of the signal is trusted to high-quality DACs. The input signal is considered normal at 1-2 volts. Therefore, for a tube terminal, an amplification of 20-50 times is sufficient. And this task is handled by one vacuum tube in the pre-amplifier stage. This is, for example, a double triode, which combines the functions of a bass reflex. That is why all the garbage from numerous successive cascades remains in the distant past. Evgeny Bortnik.
Bass reflexes. If the phase inverter is assembled according to a circuit in which each arm is also an amplifier (for example, according to the circuit in Fig. 1), then the gain of this arm is taken into account in the overall gain of the path. We remind you that you need to take into account the gain of only one arm, since the second arm of the inverter is only a matcher for the second arm of the push-pull final stage and is not part of the general amplification path.
If the phase inverter is assembled according to a symmetrical cathode follower circuit (Fig. 2), then its gain is always less than unity, so such a stage is not only not an amplification stage, but also requires an additional increase in the total gain by 4-6 dB.
The method for selecting the gain for a transistor amplifier is exactly the same. Now specifically about the circuits of the pre-amplifier stages themselves. These are the simplest resistive amplifiers without any circuit features. Typical for all stages, both triodes and pentodes, are the anode (collector) loads reduced by 2-5 times compared to the optimal calculated values for expanding the bandwidth towards higher frequencies, increased to 0.1-0. 25 μF transition capacitors and up to 1-1.5 MΩ grid leakage resistors to reduce the frequency response rolloff at low frequencies, the use of negative current feedback in all stages except the one on which the frequency response control unit is assembled. As for the amplification elements themselves, in recent years many different new types of lamps and transistors with excellent parameters have appeared. Thus, the value of S for low-power lamps became equal to 30-50 mA/V against the usual values of 3-10 mA/V, and therefore the sensitivity of the lamps increased sharply. Calculations show that theoretically all the pre-amplification can be obtained even on two stages with such lamps. However, it would be useful to warn amateurs against haste in choosing such lamps. And the point here is not conservatism, but the fact that an increase in, say, the slope of the lamps is achieved by a sharp decrease in the gap between the control grid and the cathode, which significantly increases the tendency of the lamp to generate thermal currents and the resulting huge nonlinear distortions. Also important are the high cost and lower durability of such lamps. It can be argued that such tubes as 6N1P, 6N2P, 6NZP, 6N23P, 6N24P, 6Zh1P, 6Zh5P, proven by many years of practice, are quite suitable for the preliminary stages of even the best, most modern amplifiers. For example, below are shown several circuits of the CPU on lamps in their normal modes
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In Fig.3. tube pre-amp stages are shown. a - two-stage amplifier with interstage internal feedback; b - cascade with linearizing feedback in the protective grid circuit.
Final and pre-final stages – power amplifiers. Formally, pre-terminal cascades (drivers, from the English word drive - excite, set, swing) are classified as voltage amplifiers, i.e., preliminary cascades, but they are discussed in this, and not in the previous paragraph, in order to emphasize that by the nature of the work and In terms of modes of use, drivers are much closer to final amplifiers, i.e. power amplifiers. Hi-Fi amplifiers are characterized by a significant output power of the order of 15-50W. This means that to excite (drive) the final stage without noticeable nonlinear distortions, a power of the order of 1-5 W is already required, at a voltage of up to 25-35 V, and if we take into account the requirements for reducing nonlinear distortions, it becomes clear that conventional low-power triodes cannot provide excitation of powerful terminal lamps. Therefore, it becomes logical and justified to use high-power lamps in the last voltage amplification stage. It is possible that, theoretically, it would be more correct to make the pre-terminal cascades in all cases transformer or choke in order to obtain the highest value of the anode voltage utilization factor ξ, but there are several reasons why this should not be done. The transformer cascade always introduces noticeable frequency distortions, and at powers above 1-2 W, noticeable nonlinear distortions. In addition, transformers are relatively expensive, complex and labor-intensive to manufacture, heavy and bulky, sensitive to magnetic interference and at the same time a source of audio frequency interference for other amplifier circuits (primarily input ones).
At the same time, radio amateurs now have medium-power, broadband and economical lamps at their disposal, which make it possible to easily obtain undistorted power of about 2-4 W at an active load resistance. These primarily include lamps of types 6P15P, 6E5P, 6F3P, 6F4P, 6F5P, 6Zh5P, 6Zh9P, etc. However, this issue needs to be approached more carefully. In some cases, for reasons of simpler coordination, it is still advisable to use a transformer connection. Pre-amplifier circuits are shown below
For final low-frequency cascades with a power of up to 10-12 W, radio amateurs in most cases use 6P14P type lamps, partly because they quite easily provide the specified power. In addition, unfortunately, there are no other lamps suitable for this purpose. Such an outdated, although very good lamp, like 6P3S (6L6) Nowadays It cannot be recommended, and the industry does not produce more powerful special lamps for ULF final stages like the German EL-34. [Strange conclusion, without any reason, in 1980-90 the use of 6P3S cannot be recommended! Pure voluntarism from the Soviet of Deputies. In the 21st century, for example, 6P3S lamps can be strongly recommended for designing a tube amplifier. It is important to find specimens in good preservation. E.B.] People often try to get more power from the same 6P14P tubes by forcing the mode, but this path is completely unacceptable due to the sharp deterioration in the reliability of the amplifier and the increase in nonlinear distortions when a grid thermal current appears.
Taking into account the above, we can recommend that radio amateurs use 6P14P lamps in any push-pull circuits only at powers not exceeding 10 watts. [An amazingly meaningless recommendation in the style of “since there is nothing good, then do what you do.” The author seems to be a cool authority, but he writes nonsense. E.B.] With a higher output power, it is necessary to switch to such obviously not “low-frequency” lamps as 6P31S, 6P36S, 6P20S, GU-50, 6N13S (6N5S) both in classic push-pull and ultra-linear circuits, and in bridge circuits less familiar to radio amateurs circuits, also called push-pull-parallel. The first three of these lamps are intended for use in the final cascades of horizontal scanning televisions and allow you to extract power up to 25 W from two lamps; a GU-50 generator lamp with an anode voltage of 500-750 V (and according to its passport it has Ua.work = 1000 V) is easy delivers power of 40-60W in a push-pull circuit; double triode 6N13S, designed specifically as a control lamp in electronic voltage stabilizer circuits, has a very low internal resistance and, with a relatively low anode voltage, makes it possible to obtain a power of at least 15 W (per one cylinder) in a conventional push-pull circuit, and when switched on, two in each arm triodes in parallel (two cylinders) in conventional push-pull and bridge circuits provide an output power of up to 25 W. Using the listed lamps, the radio amateur has a wide choice for creative activities.
[Another recommendation in a vague state of consciousness. I wonder why twin or triple lamps are not suitable for creative activities? Maybe the author simply does not know the rules for parallel connection of radioelements? Namely, a parallel connection, with a high-quality selection of copies, gives a lot of intermediate options for very powerful amplifiers with decent characteristics. It is strange to read the recommendation of a 6P31S lamp, which is not at all more powerful than 6P14P, but is much worse in characteristics. And it’s also disappointing to see quick recommendations for the use of 6N13C lamps (paralleled, by the way). An amazing demonstration of frivolity, since the author is completely unaware of practice, because 6N13C lamps are rare guano. The spread of characteristics of the halves has a range of 100% or more. It is almost impossible to accurately select them for parallel connection, so the amplifier cannot deliver significant power to the load without overheating one of the halves, and the utilization factor is unlikely to exceed 40-50%. And simple parallel circuits for 6N13S, without leveling body kits, are unsuitable. And the discussions about lamps are touching, because there are a large number of other excellent lamps, in contrast to the recommended ones, for example 6P13S, 6P44S, 6P45S, G807; in extreme cases, 6P3S lamps are suitable. E.B.]
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Fig.5. Powerful final stages of the low-frequency ULF path. a - on 6P36S lamps in ultralinear switching; b - on GU-50 lamps in a push-pull parallel circuit; c - on 6N13S lamps with fixed bias balancing
Since all circuits were considered as low-frequency, i.e. designed for a limited bandwidth (no more than 5-8 kHz), nothing was said about output transformers, chokes, and autotransformers. All of them are the most common, assembled on W-shaped or strip cores made of simple transformer steel 0.35 mm thick. There are no increased requirements for the frame design and windings, with the exception of a high degree of symmetry of the individual halves of the primary winding. This requirement is especially important for ultra-linear circuits for switching terminal lamps. The values of leakage inductance and capacitance of the primary winding are not significant. Secondary windings with powers above 10 W should be wound with as thick a wire as possible to reduce active losses. It is advisable to make several taps to select the best operating mode for the final stage. This issue is discussed in more detail in the next paragraph. The high-frequency final stages of two-channel Hi-Fi amplifiers are significantly different from the low-frequency ones, so the recommendations regarding them will be different. First of all, this applies to the types of lamps. [ Amazing reasoning. The author invented his own classification of LF and HF. Even to a complete amateur who has read the section on vacuum tubes, first of all, it is obvious that the invented frequency division has nothing to do with vacuum tubes at all; their range goes into hundreds of megahertz. The 6P14P lamp is purple, which frequency signals should be amplified, be it 0.1 kHz, 1 kHz, 5 kHz, 8 kHz, 16 kHz or 32 kHz. But with regard to the matching transformer, this question is already relevant. But there is no need to worry here either, because... up to 18-20 kHz, ordinary transformers are suitable; you don’t need to wind anything at all. And for frequencies above 20 kHz you should switch to ferrites. It seems that the author has not heard anything about sectioning the windings to improve the frequency response, and recommends a thick wire for the secondary winding. And the concept of ACTIVE LOSSES is absolute bullshit, since there are no passive losses and there are no reactive losses either. E.B.]
Since the power of high-frequency channels, even in top-class amplifiers, is in the range of 10-12 W, the most suitable lamps are 6P14P and 6N13S. The best switching circuits are push-pull ultralinear, bridged on 6P14P in triode switching, and “two-story” on 6N13S. Regarding the last scheme, the most common version of which is shown in Fig. 6, we can say that although it is not new in a theoretical sense, it became widespread in broadcasting equipment only in the 60s of the last century. As often happens, the scheme has become very widespread, and when talking about the advantages of the scheme, they usually remain silent about its disadvantages. Let's try to objectively evaluate both.
[First of all, I propose to sensibly evaluate the most important consequence of the creation of transformerless circuits. The past 50 years have shown that such schemes have not received any distribution, and could not have received them. As the standard of living rises, the value of health increases. Therefore, the main and insurmountable disadvantage of transformerless circuits - the lack of galvanic isolation from a high voltage source - will never allow such circuits to achieve at least some distribution among the human population. And let the dreamers study and analyze the modes of such circuitry until they are blue in the face.]
Fig.6. One of the most common final stage circuits with series connection of DC lamps
Connecting two lamps in series for direct current is equivalent to the fact that for alternating current both of them are connected in parallel relative to the load, due to which their total internal resistance is actually four times less than that of a conventional push-pull cascade. If for such a circuit we take lamps whose internal resistance is lower than usual, and use relatively high-impedance loudspeakers as a load, then it turns out that the output transformer, according to calculations, would in this case have a transformation coefficient close to unity or, in any case, measured in units. It is then possible to connect the load to the lamps directly, without an output transformer. This, of course, is an unconditional advantage of the scheme. However, this dignity comes at a high price. First of all, direct switching on of the load still turns out to be impossible due to the presence at the points of its switching on of half the voltage of the power source (120-150V). Therefore, the loudspeakers have to be switched on through a decoupling capacitor, the capacitance of which is directly related to the load resistance and the lower limit of the passband. Indeed, if the permissible voltage loss of the useful signal on the separating capacitor is 10% of the value of the signal itself, then at Rн=20 Ohm and flow=40 Hz the reactance of the capacitor should not exceed 2 Ohms, from which its capacitance is equal to
It is clear that only an electrolytic capacitor can have such a capacitance, but it must be remembered that its operating voltage must be at least not lower than the full voltage of the power source, i.e. 300-350V. And then it turns out that the cost of such a capacitor is not at all lower than the cost of the output transformer, especially since, unlike a capacitor, a radio amateur can always make a transformer himself, if necessary. Of course, it is possible to make a loudspeaker with a voice coil resistance of not 20, but 200 Ohms, which, under the same conditions, will allow reducing the capacitance of the coupling capacitor to 200 μF, but in this case the cost of the loudspeaker increases sharply. However, this is not the only drawback of this scheme. The second is that when the lamps are connected in series with direct current, only half the voltage of the anode source is applied to each of them, so the circuit can only work well on special lamps whose rated anode voltage does not exceed 100-150V. However, most lamps of this type have an insignificant maximum output power, rarely exceeding a few watts. In addition, studies have shown that when using pentodes, this circuit is fundamentally somewhat asymmetrical, which makes it unsuitable for the final low-frequency stages of Hi-Fi amplifiers. In high-frequency cascades, the first drawback immediately disappears, since with the values selected in the previous calculation and the lower limit of the HF channel flow = 2 kHz, the capacitance value of the separating capacitor
Moreover, in this case, a ten percent signal loss will occur only in the worst, practically non-working part of the passband, and at ftop = 20 kHz the signal loss will be only 1%. In addition, the required output power for the final RF stage is significantly less than for the LF stage, which allows the use of a 6N13C double triode in this circuit, which has low internal resistance and works well at low anode voltages. A practical diagram of such a cascade is shown in Fig. 7.
Fig.7. Practical diagram of a “two-story” final stage based on a double triode 6N13S (6N5S)
If the power of the RF channel does not exceed 2-3W, you can assemble the final stage according to the circuit in Fig. 8 using lamps of types 6F3P or 6F5P. The output transformer for this circuit is assembled on a tape core with a tape thickness of no more than 0.2 mm or on an W-shaped permalloy. In order for the ultralinear circuit to give a noticeable result and for nonlinear distortions to actually be on the order of 0.2-0.5%, the tap point of the primary winding must in each case be selected empirically directly from the results of measurements of the r.n.i. in the process of setting up an amplifier. To do this, when winding a transformer, 4-6 taps must be provided for each half of the primary winding.
Fig.8. Push-pull high-frequency final stage using 6F3P or 6F5P lamps (Pout = 2.5 W)
For transistor amplifiers, the “two-story” circuit, on the contrary, turns out to be preferable to all others. This is explained by the low internal resistance of high-power transistors and collector voltage (compared to lamps). Therefore, excellent matching of the cascade with the load is ensured even when using conventional low-impedance loudspeakers, for example, the 4GD-35 type. In addition, the decoupling capacitor turns out to be small in size even with a capacity of 2000-5000 μF, since its operating voltage does not exceed 20-30V. Such schemes are widespread and well known to radio amateurs.
As a general conclusion, I can cite several considerations that in the 21st century will certainly be perceived as rational. The first consideration is whether it is correct for the author to discuss only push-pull amplifiers, since single-ended circuits are intended for beginners. Secondly, the thoroughness of the approach to systematizing the circuitry of cascades also deserves respect. Third, the author’s indisputable qualifications in some cases border on astounding prejudices, and lapses in thinking are apparently a consequence of the author’s high theoretical preparation and insufficient practical experience. Fourth, the past decades have significantly changed the situation, both in basic concepts and in circuit design, especially with regard to the output stages of high-performance amplifiers. And there is no longer any excessive ceremony. Much has become simpler and clearer. Some show-offs died without showing resilience. But they will be replaced by new show-offs, like oxygen-free copper. It seems very important to understand the fact that changes in the technological structure of society should not change the fundamental values of life, for example, the Slavic civilization. Prepared a publication based on materials from Gendin’s book downloaded online.
Evgeny Bortnik, Krasnoyarsk, Russia, March 2018
Electrical signal amplifier - is an electronic device designed to increase the power, voltage, or current of a signal applied to its input without significantly distorting its waveform. Electrical signals can be harmonic oscillations of emf, current or power, signals of rectangular, triangular or other shapes. Frequency and waveform are significant factors in determining the type of amplifier. Since the signal power at the output of the amplifier is greater than at the input, then according to the law of conservation of energy amplification device must include a power source. Thus, the energy for operating the amplifier and load is supplied from the power source. Then the generalized block diagram of the amplifier device can be depicted as shown in Fig. 1.
Figure 1. Generalized block diagram of the amplifier.
Electrical vibrations come from the signal source to the input of the amplifier , to the output of which a load is connected, Energy for operation of the amplifier and load is supplied from the power source. The amplifier takes power from the power source Ro - necessary to amplify the input signal. The signal source provides power to the amplifier input R in output power P out allocated to the active part of the load. In the power amplifier, the following inequality holds: R in < P out< Ро . Therefore, amplifier- it is input driven converter power source energy into output signal energy. Energy conversion is carried out using amplifying elements (AE): bipolar transistors, field-effect transistors, electronic tubes, integrated circuits (ICs). varicaps and others.
The simplest amplifier contains one reinforcing element. In most cases, one element is not enough and several active elements are used in the amplifier, which are connected in a stepwise manner: oscillations amplified by the first element are fed to the input of the second, then the third, etc. The part of the amplifier that makes up one amplification stage is calledcascade. The amplifier consists ofactive and passive elements: k active elementsinclude transistors, el. microcircuits and other nonlinear elements that have the property of changing the electrical conductivity between the output electrodes under the influence of a control signal at the input electrodes.Passive elementscopsare resistors, capacitors, inductors and other elements that form the required oscillation range, phase shifts and other amplification parameters.Thus, each amplifier stage consists of the minimum required set of active and passive elements.
The block diagram of a typical multistage amplifier is shown in Fig. 2.
Figure 2. Multistage amplifier circuit.
Input stage And preamplifier are designed to amplify the signal to the value required to feed it to the input of a power amplifier (output stage). The number of pre-amplification stages is determined by the required gain. The input stage provides, if necessary, matching with the signal source, noise parameters of the amplifier and the necessary adjustments.
Output stage (power amplification stage) is designed to deliver a given signal power to the load with minimal distortion of its shape and maximum efficiency.
Sources of amplified signals there may be microphones, reading heads of magnetic and laser information storage devices, various converters of non-electrical parameters into electrical ones.
Load are loudspeakers, electric motors, warning lights, heaters, etc. Power supplies generate energy with specified parameters - nominal values of voltages, currents and power. Energy is consumed in the collector and base circuits of transistors, in the incandescent circuits and anode circuits of lamps; used to maintain the specified operating modes of the amplifier elements and load. Often, the energy of power supplies is also required for the operation of input signal converters.
Classification of amplification devices.
Amplification devices are classified according to various criteria.
By mind amplified electrical signals amplifiers are divided into amplifiers harmonic (continuous) signals and amplifiers pulse signals.
Based on the bandwidth and absolute values of the amplified frequencies, amplifiers are divided into the following types:
- DC Amplifiers (UPT) are designed to amplify signals ranging from the lowest frequency = 0 to the upper operating frequency. The UPT amplifies both the variable components of the signal and its constant component. UPTs are widely used in automation and computer devices.
- Voltage Amplifiers, in turn, they are divided into low, high and ultra high frequency amplifiers.
Width bandwidth amplified frequencies are distinguished:
- electoral amplifiers (high frequency amplifiers - UHF), for which the frequency ratio is valid /1 ;
- broadband amplifiers with a large frequency range, for which the frequency ratio />>1 (for example, ULF - low frequency amplifier).
- Power amplifiers - ULF final stage with transformer isolation. To ensure maximum power R int. To= Rn, those. the load resistance must be equal to the internal resistance of the collector circuit of the key element (transistor).
By design amplifiers can be divided into two large groups: amplifiers made using discrete technology, that is, by surface-mounted or printed circuit mounting, and amplifiers made using integrated technology. Currently, analog integrated circuits (ICs) are widely used as active elements.
Amplifier performance indicators.
Performance indicators of amplifiers include input and output data, gain, frequency range, distortion factor, efficiency and other parameters that characterize its quality and operational properties.
TO input data refer to the nominal value of the input signal (voltage Uinput= U 1 , current Iinput= I 1 or power Pinput= P 1 ), input resistance, input capacitance or inductance; they determine the suitability of the amplifier for specific practical applications. Input fromoppositionRinput compared to signal source impedance RAnd predetermines the type of amplifier; Depending on their ratio, voltage amplifiers are distinguished (with Rinput >> RAnd), current amplifiers (with Rinput << RAnd) or power amplifiers (if Rinput = RAnd). Input eatboneS input, being a reactive component of resistance, has a significant impact on the width of the operating frequency range.
Output - these are the nominal values of the output voltage U out = U 2, current I out =I 2, output power P out =P 2 and output resistance. The output impedance should be significantly less than the load impedance. Both input and output resistances can be active or have a reactive component (inductive or capacitive). In general, each of them is equal to the impedance Z, containing both active and reactive components
Gain is called the ratio of the output parameter to the input parameter. Voltage gains are differentiatedK u= U 2/ U 1 , by current K i= I 2/ I 1 and power Kp= P2/ P 1 .
Amplifier characteristics.
The characteristics of an amplifier reflect its ability to amplify signals of various frequencies and shapes with a certain degree of accuracy. The most important characteristics include amplitude, amplitude-frequency, phase-frequency and transition.
Rice. 3. Amplitude characteristic.
Amplitude the characteristic is the dependence of the amplitude of the output voltage on the amplitude of a harmonic oscillation of a certain frequency supplied to the input (Fig. 3.). The input signal changes from a minimum to a maximum value, and the level of the minimum value must exceed the level of internal noise UP created by the amplifier itself. In an ideal amplifier (amplifier without interference), the amplitude of the output signal is proportional to the amplitude of the input U out= K*Uinput and the amplitude characteristic has the form of a straight line passing through the origin. In real amplifiers it is not possible to get rid of interference, so its amplitude characteristic differs from straight line.
Rice. 4. Amplitude-frequency response.
Amplitude- And phase-frequency characteristics reflect the dependence of the gain on frequency. Due to the presence of reactive elements in the amplifier, signals of different frequencies are amplified unequally, and the output signals are shifted relative to the input signals at different angles. Amplitude-frequency The characteristic in the form of a dependence is presented in Figure 4.
Operating frequency range amplifier is called the frequency interval within which the modulus of the coefficient K remains constant or varies within predetermined limits.
Phase-frequency characteristic is the frequency dependence of the phase shift angle of the output signal relative to the phase of the input signal.
Feedback in amplifiers.
Feedback (OS) call the connection between electrical circuits, through which signal energy is transferred from a circuit with a higher signal level to a circuit with a lower signal level: for example, from the output circuit of an amplifier to the input circuit or from subsequent stages to previous ones. The block diagram of the feedback amplifier is shown in Figure 5.
Rice. 5. Structural (left) and circuit diagram with negative current feedback (right).
Signal transmission from the output to the input of the amplifier is carried out using a four-port network IN. A four-terminal feedback network is an external electrical circuit consisting of passive or active, linear or nonlinear elements. If the feedback covers the entire amplifier, then the feedback is called general: if the feedback covers individual stages or parts of the amplifier, it is called local. Thus, the figure shows a block diagram of an amplifier with general feedback.
Model of the amplifier stage.
Amplifier nal cascade - amplifier structural unit - contains one or more active (amplifying) elements and a set of passive elements. In practice, for greater clarity, complex processes are studied using simple models.
One of the options for a transistor cascade for amplifying alternating current is shown in the figure on the left. Transistor V1 p-p-p type connected according to a common emitter circuit. The input base-emitter voltage is created by a source with EMF E c and internal resistance R c source. Resistors are installed in the base circuit R 1 And R 2 . The collector of the transistor is connected to the negative terminal of the source E to through resistors R To And R f. The output signal is taken from the collector and emitter terminals and through the capacitor C 2 enters the load R n. Capacitor Sf together with a resistor Rf forms RС -filter link ( positive feedback - POS), which is required, in particular, to smooth out supply voltage ripples (with a low-power source E to with high internal resistance). Also, for greater stability of the device, a transistor is added to the emitter circuit V1 (negative feedback - OOC) can be additionally enabled R.C. - a filter that will prevent part of the output signal from being transferred back to the amplifier input. In this way, the effect of self-excitation of the device can be avoided. Usually artificially created external environmental protection allows you to achieve good amplifier parameters, but this is generally true only for amplification of direct current or low frequencies.
Low frequency amplifier circuit based on a bipolar transistor.
An amplification stage based on a bipolar transistor connected in a circuit with an OE is one of the most common asymmetric amplifiers. A schematic diagram of such a cascade, made on discrete elements, is shown in the figure below.
In this circuit the resistor Rк , included in the main circuit of the transistor, serves to limit the collector current, as well as to provide the required gain. Using a voltage divider R1R2 sets the initial bias voltage at the base of the transistor VT, required for class A amplification mode.
Chain ReSe performs the function of emitter thermal stabilization of the resting point; capacitors C1 And C2 are separating for direct and alternating current components. Capacitor Se bypasses the resistor Re according to alternating current, since the capacity Se significant.
When a signal of constant amplitude is applied to the input of a voltage amplifier at different frequencies, the output voltage, depending on the frequency of the signal, will change, since the resistance of the capacitors C1 , C2 different at different frequencies.
The dependence of the gain on the signal frequency is called amplitude-frequency amplifier characteristics (frequency response).
Low Frequency Amplifiers most widely apply to amplify signals carrying audio information, in these cases they are also called audio frequency amplifiers; in addition, ULFs are used to amplify the information signal in various fields: measuring technology and flaw detection; automation, telemechanics and analog computer technology; in other electronics industries. An audio amplifier usually consists of preamp And power amplifier (MIND). Pre-amplifier designed to increase power and voltage and bring them to the values necessary for the operation of the final power amplifier, often includes volume controls, tone controls or an equalizer, sometimes it can be structurally designed as a separate device.
Amplifier must deliver the specified power of electrical oscillations to the load (consumer) circuit. Its load can be sound emitters: acoustic systems (speakers), headphones (headphones); radio broadcast network or radio transmitter modulator. A low-frequency amplifier is an integral part of all sound reproducing, recording and radio broadcasting equipment.
The operation of the amplifier stage is analyzed using an equivalent circuit (in the figure below), in which the transistor is replaced by a T-shaped equivalent circuit.
In this equivalent circuit, all the physical processes occurring in the transistor are taken into account using the small-signal H-parameters of the transistor, which are given below.
To power the amplifiers, voltage sources with low internal resistance are used, so we can assume that, in relation to the input signal, resistors R1 And R2 are included in parallel and can be replaced by one equivalent Rb = R1R2/(R1+R2) .
An important criterion for choosing resistor values Re, R1 And R2 is to ensure temperature stability of the static operating mode of the transistor. A significant dependence of the transistor parameters on temperature leads to an uncontrolled change in the collector current Ik , as a result of which nonlinear distortions of the amplified signals may occur. To achieve the best temperature stabilization of the regime, it is necessary to increase the resistance Re . However, this leads to the need to increase the supply voltage E and increases the power consumed from it. By decreasing the resistance of the resistors R1 And R2 power consumption also increases, reducing the efficiency of the circuit, and the input resistance of the amplifier stage decreases.
Integrated DC amplifier.
An integrated amplifier (op-amp) is the most common universal microcircuit (IC). An op-amp is a device with highly stable quality indicators that allow processing analog signals according to an algorithm specified using external circuits.
Operational amplifier (op-amp) - unified multistage DC amplifier (UPT), satisfying the following requirements for electrical parameters:
· voltage gain tends to infinity;
· input resistance tends to infinity;
· output resistance tends to zero;
· if the input voltage is zero, then the output voltage is also zero Uin = 0, Uout = 0;
· endless band of amplified frequencies.
The op-amp has two inputs, inverting and non-inverting, and one output. The input and output of the UPT are made taking into account the type of signal source and external load (unbalanced, symmetrical) and the values of their resistances. In many cases, DC amplifiers, like AC amplifiers, provide a high input impedance to reduce the impact of the DC amplifier on the signal source, and a low output impedance to reduce the influence of the load on the DC amplifier's output signal.
Figure 1 shows the circuit of an inverting amplifier, and Figure 2 shows a non-inverting amplifier. In this case, the gain is equal to:
For inverting Kiou = Roс / R1
For non-inverting Know = 1 + Roс / R1
The inverting amplifier is covered by an OOS parallel in voltage, which causes a decrease in Rin and Rout. The non-inverting amplifier is covered by a voltage-series feedback loop, which ensures an increase in Rin and a decrease in Rout. Based on these op-amps, you can build various circuits for analog signal processing.
The UPT is subject to high requirements for the lowest and highest input resistance. A spontaneous change in the output voltage of the UPT with a constant voltage of the input signal is called amplifier drift . The causes of drift are instability of the circuit supply voltages, temperature and time instability of the parameters of transistors and resistors. These requirements are met by an op-amp in which the first stage is assembled using a differential circuit, which suppresses all common-mode interference and provides high input impedance. This cascade can be assembled on field-effect transistors and on composite transistors, where a GCT (stable current generator) is connected to the emitter (source) circuit, which enhances the suppression of common-mode interference. To increase the input resistance, deep series OOS and a high collector load are used (in this case, Jin tends to zero).
DC amplifiers are designed to amplify signals that vary slowly over time, that is, signals whose equivalent frequency approaches zero. Therefore, UPT must have amplitude-frequency response
in the form shown in the figure on the left. Since the gain of the op-amp is very high, its use as an amplifier is possible only if it is covered by deep negative feedback (in the absence of negative feedback, even an extremely small “noise” signal at the op-amp input will produce a voltage close to the saturation voltage at the op-amp output).
The history of the operational amplifier is connected with the fact that direct current amplifiers were used in analog computing technology to implement various mathematical operations, such as summation, integration, etc. Currently, although these functions have not lost their importance, they constitute only a small part of the list of possible applications of op amps .
Power amplifiers.
What is it like? amplifier- further, for brevity, we will call it MIND? Based on the above, the block diagram of the amplifier can be divided into three parts:
- Input stage
- Intermediate stage
- Output stage (power amplifier)
All these three parts perform one task - to increase the power of the output signal without changing its shape to such a level that it is possible to drive a low-impedance load - a dynamic head or headphones.
There are transformer And transformerless mind circuits.
1. Transformer power amplifiers.
Let's consider single-cycle transformer MIND, in which the transistor is connected according to the circuit with an OE (Fig. on the left).
Transformers TP1 and TP2 are designed to match the load and output impedance of the amplifier and the input impedance of the amplifier with the impedance of the input signal source, respectively. Elements R and D provide the initial operating mode of the transistor, and C increases the variable component supplied to transistor T.
Since the transformer is an undesirable element of power amplifiers, because. has large dimensions and weight, and is relatively difficult to manufacture, then currently the most widespread transformerless power amplifiers.
2. Transformerless power amplifiers.
Let's consider push-pull PA on bipolar transistors with different types of conductivity. As noted above, it is necessary to increase the power of the output signal without changing its shape. To do this, the direct power supply current of the PA is taken and converted into alternating current, but in such a way that the shape of the output signal repeats the shape of the input signal, as shown in the figure below:
If the transistors have a sufficiently high transconductance value, then it is possible to construct circuits that operate on a load of one ohm without the use of transformers. Such an amplifier is powered by a bipolar power supply with a grounded midpoint, although it is also possible to construct circuits for unipolar power supply.
Schematic diagram of complementary emitter follower
- amplifier with additional symmetry - shown in the figure on the left. Given the same input signal, current flows through the npn transistor during the positive half cycles. When the input voltage is negative, current will flow through the pnp transistor. By combining the emitters of both transistors, loading them with a common load and supplying the same signal to the combined bases, we obtain a push-pull power amplification stage.
Let's take a closer look at the inclusion and operation of transistors. The amplifier's transistors operate in class B mode. In this circuit, the transistors must be absolutely identical in their parameters, but opposite in planar structure. When a positive half-wave voltage is received at the input of the amplifier Uin transistor T1 , operates in amplification mode, and the transistor T2 - in cut-off mode. When a negative half-wave arrives, the transistors switch roles. Since the voltage between the base and emitter of the open transistor is small (about 0.7 V), the voltage Uout close to voltage Uin . However, the output voltage turns out to be distorted due to the influence of nonlinearities in the input characteristics of the transistors. The problem of nonlinear distortion is solved by applying an initial bias to the base circuits, which switches the cascade to AB mode.
For the amplifier in question, the maximum possible voltage amplitude across the load is Um equal to E . Therefore, the maximum possible load power is determined by the expression
It can be shown that at maximum load power, the amplifier consumes power from the power supplies, determined by the expression
Based on the above, we obtain the maximum possible UM efficiency factor: n max = P n.max/ P consumptionmax = 0,78.
The block diagram of a complete low-frequency ULF amplifier is shown in Fig. 14.
Fig. 14 Block diagram of ULF.
Input stage separated from the group of pre-amplification stages, since it is subject to additional requirements for coordination with the signal source.
To reduce signal source shunting R i low input impedance amplifier R IN~ the following condition must be met: R IN~ >> R i
Most often, the input stage is an emitter follower, in which R IN~ reaches 50 kOhm or more, or field-effect transistors are used that have a very high input resistance.
In addition, the input stage must have a maximum signal-to-noise ratio, since it determines the noise properties of the entire amplifier.
Adjustments allow you to quickly set the output power level (volume, balance) and change the shape of the frequency response (timbre).
Final stages provide the required output power in the load with minimal nonlinear signal distortion and high efficiency. The requirements for the final cascades are determined by their characteristics.
1. The operation of a power amplifier for a low-impedance load of speaker systems requires optimal matching of the final stage with the total sound impedance of the speakers: ROUT~ ≈ R H .
2. The final stages consume the bulk of the energy of the power source and efficiency for them is one of the main parameters.
3. The share of nonlinear distortions introduced by the final stages is 70...90%. This is taken into account when choosing their operating modes.
Pre-terminal cascades. At high output powers of the amplifier, the purpose and requirements for the pre-final stages are similar to the final stages.
Besides this, if two-stroke the final stages are made of transistors the same structures, then the pre-terminal cascades should be phase inverted .
Requirements to preamp stages stem from their purpose - to amplify the voltage and current created by the signal source at the input to the value necessary to excite the power amplification stages.
Therefore, the most important indicators for a multistage preamplifier are: voltage and current gain, frequency response (AFC) and frequency distortion.
Basic properties of pre-amp stages:
1. The signal amplitude in the preliminary stages is usually small, so in most cases nonlinear distortions are small and can be ignored.
2. The construction of pre-amplifier stages using single-ended circuits requires the use of non-economical mode A, which has virtually no effect on the overall efficiency of the amplifier due to the low values of the quiescent currents of the transistors.
3. The most widely used circuit in preliminary stages is the connection of a transistor with a common emitter, which makes it possible to obtain the greatest gain and has a sufficiently large input resistance so that the stages can be connected without matching transformers without losing gain.
4. Of the possible methods of stabilizing the mode in preliminary stages, emitter stabilization has become the most widespread as the most effective and simplest in circuit.
5. To improve the noise properties of the amplifier, the transistor of the first stage is chosen to be low-noise with a high value of the static current gain h 21e >100, and its direct current mode should be low-current I ok = 0.2...0.5 mA, and the transistor itself To increase the input impedance, the ULF is switched on according to a circuit with a common collector (CC).
To study the properties of preliminary amplification stages, a equivalent their electrical circuit for alternating current. To do this, the transistor is replaced by an equivalent circuit (an equivalent generator E OUT, internal resistance R OUT,pass-through capacity S K), and all elements of the external circuit that affect the gain and frequency response (frequency distortion) are connected to it.
The properties of the preliminary amplification stages are determined by the scheme of their construction: with capacitive or galvanic connections, on bipolar or field-effect transistors, differential, cascode and other special circuits.
In amplifiers based on bipolar transistors, as a rule, a connection circuit with a common emitter is used, which provides amplification in both voltage and current (Fig. 2.4). In the circuit, resistors and are connected between the body and the point +E K, form a divider for the supply voltage, which fixes the operating mode of the transistor - its operating point R" (Fig. 2.5). Current I d flowing through must create a voltage drop corresponding to the voltage between the base and emitter of the transistor UBEr, therefore
Where I d – current divider formed by resistors and
Rice. 2.4.
Rice. 2.5. Effect of operating point positionR to operate the amplifier
Through resistance R" B current flows I Σ equal to the sum of the current I d and current flowing into the base of the transistor, therefore
Divider current is selected I d = (2...5)I Br. The more I d, the more stable the cascade operates, since changes in the collector currents I k and emitter I uh, and therefore the base current I B = I To - I e will not lead to a significant change in voltage on R"B :
Thus, the voltage UBEr will change slightly. At the same time, you should not choose the divider current too high, because this reduces the efficiency of the cascade due to energy losses in the divider.
Let us first assume that UBX = 0. Under the influence of voltage UBer, a constant base current flows through the open emitter-base p-n junction I Br. The decoupling capacitor Cp1 prevents direct current from flowing through the input signal source.
The transistor is open and located in the active region. Its condition will be determined by the point R intersection of the load line drawn through the points E to and E K /R K, cut off on the axes (see Fig. 2.5), with a characteristic corresponding to the current I Br. Constant collector current I Kp corresponding to the point R, will determine the initial voltage between the emitter and collector U KEp. Since DC voltage does not pass through the isolation capacitor Cp2, the output voltage Uout = 0. The considered state of the circuit is called DC operating mode.
Let now the input of the circuit receive a signal in the form of a sinusoidal voltage with an amplitude Uin max. This signal will already pass through the isolation capacitor and cause a change in the control voltage of the NBE. Under its action, the base, emitter and collector currents will change. We will estimate the change in base current using the input characteristic I B = f (UBE) and determine the amplitude values of the current + I Bmah and - I Bmah. If the input characteristic were linear, then changes in the base current, both upward and downward, would be the same, but due to the nonlinearity of the characteristic, the amplitude + I Bmah is more than - I Bmah. Since the output characteristics I k = f (Uke) are built for different base currents, from them you can determine the collector currents corresponding to the currents + I Bmax and - I Bmah. Changes in collector current relative to the average value I Kr from ( I Kr+ I Kmax) to (I Kr – I Kmax) will lead to voltage fluctuations across the resistance R K and therefore on the collector of the transistor. These vibrations can be easily assessed using a load line. Indeed, the operating point R will move along the load line between the points of intersection of this line with the output characteristics corresponding to the base currents ( I Br + I Bmah) and ( I Bmin – I Bmax). Thus, fluctuations in the input signal led to proportional fluctuations in the collector-emitter voltage Uke with amplitude UKEmax = I KmaxRk. Through capacitor Cp2, these oscillations are sent to the output of the amplifier. The output signal is therefore equal to
This mode is called operating mode by alternating current.
From the plots shown in the characteristics, it is clear that Uвx.max = 0.1 V, Uout.max = 5 V and, therefore, the voltage gain of such a cascade
Please note that the positive half-cycle of the input voltage (when UBEr + Uвx.max) corresponds to the negative half-cycle of the output voltage (i.e. UКЭр – Uout.max). In other words, there is a phase shift of 180° between the input and output voltages. To obtain the least distortion of the amplified signal, the operating point is R should be located in the middle of the linear section of the input characteristic.
