The power amplification (referred to as power amplifier) ​​circuit in the general audio-visual circuit is to further amplify the low-frequency signal after the voltage amplifier to obtain a larger output power, which is ultimately used to push the speaker to play or provide deflection current in the TV.
1. Characteristics of power amplification current
The understanding or evaluation of the power amplifier circuit is mainly considered from the three aspects of output power, efficiency and distortion.
1. In order to obtain the required output power, the circuit must select a triode with sufficient collector power consumption, and the working current and collector voltage of the power amplifier tube are also high. In the circuit design and use, we must first consider how to fully play the function of the transistor without damaging the transistor. Because the working state of the power amplifier tube in the circuit is often close to the limit value, be careful when adjusting and using the power amplifier current, and should not be used beyond the limit.
2. From the perspective of energy consumption, the power output by the power amplifier is ultimately provided by the power supply. For example, the power consumption of the power amplifier in the radio should account for 2/3 of the whole machine. ratio.
3. The input signal of the power amplifier circuit has been amplified by several stages and has sufficient strength, which will cause the working point of the power amplifier tube to move greatly, so the power amplifier circuit is required to have a larger dynamic range. If the working point of the power amplifier tube is not selected properly, the output will be seriously distorted.
Second, the principle of commonly used power amplifier circuit
The output of the power amplifier circuit with a single transistor output is small and low in efficiency, which is rare in household appliances. At present, the form of push-pull circuit is often used.
Figure 1 is a schematic diagram of a push-pull circuit using a coupling transformer. Its characteristic is that the triode's static working current is close to zero, and the amplifier consumes less power. When there is a letter input, although the working current of the circuit is large, most of the power is output to the load, but the loss itself is not large, so the power supply utilization rate is high. In this circuit, each transistor only works within half a period 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 equal-sized and opposite-polarity signals of B1 secondary are sent to the transmitting junctions of BG1 and BG2, respectively. During the positive half cycle of the input signal, the BG1 tube is cut off due to the application of reverse bias. Only BG2 can amplify the signal and output it from the collector; while in the negative half of the signal, BG1 gets a positive high bias, which can The signal of the period is amplified and output, but BG2 is cut off. Although the two transistors in the circuit each amplify the signal for half of the same period, their output currents are passed through the output transformer B2, so the induced current obtained at the secondary of B2 can all become a complete output signal.
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In this power amplifier circuit, in order to solve the problems of impedance turns and signal phase, the input and output transformers are indispensable. However, the production of high-quality transformers is difficult in terms of materials and processes. It always consumes a part of energy itself, reducing the efficiency of the circuit, and the frequency characteristics of the transformer are not good, making the circuit very uneven to the signal output of different frequencies, which will cause Distortion, so in order to improve the quality of the power amplifier, people use transformerless (OTL) power amplifier circuits more.
Figure 2 is a schematic diagram of a complementary symmetric push-pull power amplifier circuit. Two transistors with the same amplification performance and opposite conductive polarities (called complementary tubes) are used here. In the picture, BG1 is an NPN tube. During the positive half of the input AC signal of the amplifier, for the BG1 tube, the base voltage is positive, the emitter is negative, and the emitter junction is forward biased. The triode can work. But BG2 is terminated due to the reverse bias applied to the launch junction. Therefore, the positive half of the signal is amplified by the BG1 tube. In the negative half cycle of the signal, the situation is reversed. The BG2 tube can work to amplify the negative half cycle of the signal. The amplified signal is sent by two transistors in turn, and the complete signal is synthesized again on the speaker.
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The two transistors in the push-pull circuit each amplify the signal for a half period, which requires that the amplification performance of the two transistors are similar (with a β value within 10% of the difference), otherwise the amplified signal will have different amplitudes in the two half periods, and obvious distortion will occur. Crossover distortion is also unique to push-pull circuits. Like the transistors in the above schematic diagram, there is no static bias current. When the input signal is weak, the transistor's amplification capacity is very small, and the amplifier will lose its amplification because the transmitting junction cannot be turned on. In this way, whenever the input signal amplitude is close to zero, that is, when the work of the two push-pull tubes is started and ended, the output signal cannot be well connected, and serious distortion occurs. In order to solve these problems, in many practical application circuits, it is necessary to add a small positive bias voltage to the transistor, so that the circuit is both efficient and can reduce distortion.
Figure 3 is a commonly used power amplifier circuit in the radio. Its static working current is adjusted by the bias resistor R8. Generally, the total static collector current of the two tubes is 4 ～ 8mA. R10 is a negative feedback resistor, used to reduce distortion and reduce the "pairing" requirements of the transistor. In order to reduce the loss of the input signal on the two resistors R9 and R10, their resistance values ​​are relatively small. Capacitor C7 is used to improve sound quality.
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In order to reduce distortion, the circuit must also provide quiescent current for the transistor. Resistor R73 is not only a part of the load of the front-stage voltage amplifier BG12 (not shown in the figure), but also the base bias resistance 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 bias voltages of the emitter junctions of the two tubes BG13 and BG14 (the emitter resistance of the two tubes is very small and can be ignored). The magnitude of this voltage determines the working current of the complementary power amplifier tube. When the resistance value of R73 changes or the working current changes through it, it will affect the working point of the power amplifier tube. This should be paid attention to when adjusting.
Diode BG39 in series with R73 is used to stabilize the static operating point of the complementary tube. It is a silicon diode, which produces a voltage drop of about 0.7V when current passes through it. When the ambient temperature increases, the forward resistance of the diode decreases, and the voltage drop across the diode also decreases, which reduces the base bias voltage of the complementary tube, counteracting the trend of increasing operating current due to temperature rise. Resistor R74 is connected in parallel with the diode to prevent the power amplifier tube from being burned due to excessive current when the diode is broken and damaged.
In the circuit, the capacitor C63 plays a very important role. Because for the audio signal, the power supply can be regarded as a path, so the collector of BG13 is "AC contact ground" like BG14. If there is no C63, the signal will be sent between the base and collector. This kind of "common collector connection" 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 it is connected to C63, it can also be regarded as a path for the audio signal, so the input signal is added to the base and emitter for 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" with much higher gain, which greatly increases the output power. The function of the resistor R71 is to play an isolation role, so as not to make the collector of the DG13 and the emitter short-circuited.
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