Basic knowledge of power amplifier

The power amplifier (PA) circuit in general audio-visual circuits is after the voltage amplifier, which further amplifies the low-frequency signal to obtain a larger output power, which is ultimately used to drive the speaker to play sound or provide deflection current in a television.

1. Characteristics of power amplifier current

The understanding or evaluation of power amplifier circuits mainly considers three aspects: output power, efficiency and distortion.

1. In order to obtain the required output power, the circuit must select a transistor with a sufficiently large collector power consumption, and the working current and collector voltage of the power amplifier tube are also high. In circuit design and use, the first thing to consider is how to fully exert the function of the transistor without damaging the transistor. Since the working state of the power amplifier tube in the circuit is often close to the limit value, the power amplifier current should be adjusted and used with caution, and it is not advisable to exceed the limit.

2. From the perspective of energy consumption, the power output of the amplifier is ultimately provided by the power supply. For example, the power consumption of the amplifier in a radio accounts for 2/3 of the entire machine. Therefore, great attention should be paid to improving the circuit efficiency, that is, the ratio of output power to power consumption.

3. The input signal of the power amplifier circuit has been amplified several times and has sufficient strength, which will cause the working point of the power amplifier tube to move significantly, so the power amplifier circuit is required to have a large dynamic range. If the working point of the power amplifier tube is not selected properly, the output will be seriously distorted.

2. Principles of commonly used power amplifier circuits

The power amplifier circuit with a single triode output has a small output and low efficiency, and is rarely seen in daily electrical appliances. At present, the push-pull circuit is commonly used.

Figure 1 is a schematic diagram of a push-pull circuit using a coupling transformer. Its characteristics are that the static working current of the transistor is close to zero, and the amplifier consumes little power. When there is a signal input, although the circuit working current is large, most of the power is output to the load, and the loss itself is not large, so the power utilization rate is high. In this circuit, each transistor is only turned on and works in half a cycle of the signal. In order to avoid distortion, two transistors are used to work in coordination. The secondary of the input transformer B1 in the figure has a grounded center tap. When the audio signal is input, two signals of equal size and opposite polarity from the secondary of B1 are sent to the emitter junctions of BG1 and BG2 respectively. In the positive half cycle of the input signal, the BG1 tube is cut off due to the reverse bias, and only BG2 can amplify the signal and output it from the collector; in the negative half cycle of the signal, BG1 obtains a positive high bias and can amplify and output the signal of this half cycle, while BG2 is cut off. Although the two transistors in the circuit each amplify half of the signal, their output currents pass through the output transformer B2 one after another, so the induced current obtained at the secondary of B2 can be fully transformed into a complete output signal.

In this power amplifier circuit, input and output transformers are indispensable to solve problems such as impedance matching and signal phase. However, it is difficult to make high-quality transformers in terms of materials and processes. It always consumes some energy, reducing the efficiency of the circuit. In addition, the frequency characteristics of the transformer are not good, which makes the circuit output very uneven for signals of different frequencies, causing distortion. Therefore, in order to improve the quality of the power amplifier, people use more transformerless (OTL) power amplifier circuits.

Figure 2 is a schematic diagram of a complementary symmetrical push-pull amplifier circuit. Two transistors with the same amplification performance but opposite conduction polarities (called complementary transistors) are used here. In the figure, BG1 is an NPN transistor. When the amplifier inputs the positive half cycle of the AC signal, for the BG1 tube, the base voltage is positive, the emitter is negative, the emitter junction has a forward bias, and the transistor can work. However, BG2 is cut off because the emitter junction is reverse biased. Therefore, the positive half cycle of the signal is amplified by the BG1 tube. In the negative half cycle of the signal, the situation is just the opposite, and the BG2 tube can work and amplify the negative half cycle of the signal. The amplified signal is sent out by the two transistors in turn, and the complete signal is resynthesized on the speaker.

Three actual circuit analysis

The two transistors in the push-pull circuit each amplify half a cycle of the signal, which requires the two tubes to have similar amplification performance (the difference in β value is within 10%), otherwise the amplitude of the two half cycles of the amplified signal is different, and obvious distortion will occur. Crossover distortion is also a unique problem of the push-pull circuit. For example, the transistors in the schematic diagram above do not have static bias current added. When the input signal is very weak, the transistor's amplification ability is very small, and it may even lose its amplification effect because the emitter junction cannot be turned on. In this way, whenever the input signal amplitude is close to zero, that is, when the two push-pull tubes start and end their rotation work, the output signal cannot be well connected and serious distortion occurs. In order to solve these problems, in many practical application circuits, a very small positive bias voltage must be added to the transistor to make the circuit both efficient and reduce distortion.

Figure 3 is a commonly used power amplifier circuit in radio. Its static operating current is adjusted by bias resistor R8. Generally, the total static collector current of the two tubes is 4-8mA. R10 is a negative feedback resistor to reduce distortion and reduce the requirements for "pairing" of the transistors. In order to reduce the loss of input signals on the two resistors R9 and R10, their resistance values ​​are relatively small. Capacitor C7 is used to improve the sound quality.

Figure 4 is the Hongyan TV sound amplifier circuit. Compared with the schematic diagram 3, it has the following differences:

The schematic diagram uses two sets of power supplies, which is very inconvenient in actual use. Here, a large-capacity capacitor C64 is connected in series to the load speaker. For the audio current, C64 can be regarded as a path. During the positive half cycle of the input signal, the output current of the BG13 tube charges C64 through the speaker, generating a voltage with a polarity of "positive on the left and negative on the right" on it. During the negative half cycle of the signal, BG13 is cut off, and the capacitor C64 is discharged through BG14 and the speaker, acting as the power supply of BG14. In this way, only one set of power supplies can make the circuit work normally.

In order to reduce distortion, the circuit also needs to provide static current for the triode. Resistor R73 is both a part of the load of the pre-stage voltage amplifier tube BG12 (not shown in the figure) and the base bias resistor of the complementary power amplifier tube. When the output current of BG12 passes through R73 and diode BG39, the voltage drop generated on them is the sum of the emitter junction bias voltages of BG13 and BG14 (the emitter resistance of the two tubes is very small and can be ignored). The size of this voltage determines the working current of the complementary power amplifier tube. When the resistance value of R73 changes or the pre-stage working current through it changes, it will affect the working point of the power amplifier tube, which should be paid attention to during adjustment.

The diode BG39 connected in series with R73 is used to stabilize the static working point of the complementary tube. It is a silicon diode, and a voltage drop of about 0.7V is generated on it when current passes through it. When the ambient temperature rises, the forward resistance of the diode decreases, and the voltage drop at both ends will also decrease, so that the base bias of the complementary tube will decrease accordingly, offsetting the trend of the working current increasing due to temperature rise. The resistor R74 is connected in parallel with the diode to prevent the power amplifier tube from burning out due to excessive current when the diode is broken.

In the circuit, capacitor C63 plays a very important role. Because the power supply can be regarded as a path for audio signals, the collector of BG13 is "AC contact ground" like BG14. If there is no C63, the signal will be sent from the base and collector. This "common collector connection method" with the collector as the common terminal of the input and output signals has a low gain and is not suitable for use in power amplifier circuits. After connecting to C63, it can also be regarded as a path for audio signals, so the input signal is added to the base and emitter of BG13 through R72; for BG14, it is added to the base and emitter through R73 and R72. In this way, the circuit becomes a "common emitter connection method" with much higher gain, which greatly improves the output power. The role of resistor R71 is to isolate and prevent the collector and emitter of DG13 from AC short circuit.