Principles of common power amplifier circuit – Solution – Huaqiang Electronic Network
The power amplifier circuit output of a single triode output is small, the efficiency is low, and it is rare in the daily appliance. It is often used in the form of push-pull circuits. Figure 1 is a schematic diagram of a push-pull circuit using a coupling transformer. Its feature is that the triode static working current is close to zero, the amplifier power consumption and less. When there is a letter input, the circuit operating current is large, but most of the power is output to the load, its own loss is not large, so the power supply utilization is high. Each tripode is only turned on within half a cycle of the signal, in order to avoid distortion, it uses two three-pole coordination. The secondary of the input transformer B1 in the figure has one
The output of the power amplifier output of single triode output is small, low efficiency, rare in the daily appliance. It is often used in the form of push-pull circuits.
FIG. 1 is a schematic diagram of a push-pull circuit using a coupling transformer. Its feature is that the triode static working current is close to zero, the amplifier power consumption and less. When there is a letter input, the circuit operating current is large, but most of the power is output to the load, its own loss is not large, so the power supply utilization is high. Each tripode is only turned on within half a cycle of the signal, in order to avoid distortion, it uses two three-pole coordination. In the figure, enter the secondary of the transformer B1 with a grounded center tap. At the time of the audio signal input, the B1 secondary size is equal, and the signals opposite to the polarity are sent to the emission knots of BG1 and BG2, respectively. In the positive half cycle of the input signal, the BG1 tube is turned off due to the reverse bias, only the BG2 enlays the signal, output from the collector; the signal is negative half-week, the BG1 is highly biased, which can be used for half The signal amplifies the output, and BG2 is closed. Although the two triodes in the circuit although the half of the signals are amplified, their output current is divided by the output transformer B2, so the induction current obtained at the secondary of B2 can be fully an complete output signal.
In this power amplifier circuit, in order to solve the problem of impedance 配 and signal phase, input It is indispensable with the output transformer. However, the production of high-quality transformers is more difficult in materials and processes. Its itself has to consume some energy, reduce the efficiency of the circuit, and the frequency characteristics of the transformer are not good, so that the circuit is not uniform to different frequency signals, which will result Distortion, so in order to improve the quality of power amplifier, people use a non-transformer (OTL) power amplifying circuit.
FIG. 2 is a schematic diagram of a complementary symmetric push-pull amplifier circuit. Two magnificent properties are used here, and the two amplifier (referred to as a complementary tube) of the conductive polarity. In the figure, BG1 is an NPN tube. When the amplifier inputs the positive half week of the AC signal, the BG1 tube is used, the base voltage is positive and the emission is extremely negative, and the emitting isPositive bias, the triode can work. However, BG2 is turned off due to transmitting a reverse bias. Therefore, the positive half of the signal is amplified by the BG1 tube. At the time of the signal negative, the situation is in contrast, the BG2 tube can operate, and amplify the signal of the signal. The zoomed signal is sent from the two triode wheels, and the complete signal is recoached on the speaker.
The two triodes in the push circuit are half a cycle of the signal, which As requested that the two tubes are similar to the amplification performance (within 10% of the beta value), otherwise the amplification signal is different from half a cycle, and it will have obvious distortion. Duecent the distortion is also a unique problem of the push-pull circuit. The triodes in the above schematic did not add static bias. When the input signal is very weak, the triode enlargement is small, and even lost the enlargement due to the transmission knot. This time, when the input signal is close to zero, the output signal cannot be touched and the output signal is not very good, and severe distortion occurs. In order to solve these problems, in many actual application circuits, the triode is small and a small positive bias, so that the circuit is efficient and decreased.
FIG. 3 is a power amplifier circuit commonly used in the radio. Its static working current is adjusted by bias resistor R8, and generally two tubes total static collector current is 4 to 8 mA. R10 is a negative feedback resistor, which is used to reduce distortion and reduce the requirements of the triode “pair”. In order to reduce the loss of the input signals on the two resistors of R9 and R10, their resistance is relatively small. Capacitor C7 is used to improve sound quality.
Fig. 4 is a red rock board television sound power amplifier circuit. Compared to the principle, it has a few different:
schematic Powering with two sets of power supplies, it is inconvenient to use, where a large capacity capacitor C64 is stringed on the load speaker. For audio current, C64 can be seen as a path. When the signal is in the square, the output current of the BG13 tube is flow.The speaker is charged to be C64, which produces a voltage of polarity “left right right” on it. At the time of the signal negative half, the BG13 is cut off, and the capacitor C64 is discharged by BG14 and speaker, which acts as a power source of BG14. This will use a set of power to work normally.
In order to reduce distortion, the circuit also provides a quiescent current for the triode. The resistor R73 is a part of the front-level voltage amplifier BG12 (not shown) in the figure, and is a base bias resistance of complementary power vents. When the output current of the BG12 passes R73, and the diode BG39, the voltage drop produced above the upper surface is only Bg13, and the BG14 two tube emission knot bias (two tube emitter resistance is small, negligible). This voltage is determined by the working current of the complementary work. R73 resistance changes or changes in the working current by its pre-stage operating current, which is noted when adjusting.
The diode BG39 in series with R73 is used to stabilize the static operating point of complementary tube. It is a silicon diode that generates a voltage drop of about 0.7V above it when it passes. When the ambient temperature rises, the forward resistance of the diode is reduced, and the voltage drop at both ends will also decrease, so that the base bias of the complementary tube is reduced, and the working current increases due to temperature rise. The resistor R74 and the diode are parallel, and when the diode is broken, the power amplifier is burned due to excessive current excessive.
In the circuit, the capacitor C63 has an important role. Because of the audio signal, the power supply can be seen as a path, so the collector and BG14 of the BG13 are “AC contact”. If there is no C63, the signal will be sent between the base and the collector. This “co-collector connection” gain in the common electrode of the input and output signal is low, and it is not advisable to use in the power amplifier circuit. After the access C63, it can also view the audio signal as a path, so the input signal is added to the base and emitter by R72; the BG14 is applied to the base and emitter by R73, R72. In this way, the circuit has become a “common emitter connection” with high gain, which greatly increases the output power. The effect of resistance R71 is an isolation, does not make the current collector of DG13Pole with the emitter AC short circuit.
Original article，author：zongmuLi，If reprinted，Please indicate the source：http://madedesign.net/index.php/2020/12/25/principles-of-common-power-amplifier-circuit-solution-huaqiang-electronic-network/