Primary analog circuit: 5-4 class B power amplifier


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1. Basic working principle

Because the efficiency of class a amplifier is too low, it is usually only used for small power amplification or signal amplification. Class B (class B) or class A (class AB) power amplifier is really used for high power amplification. Class B power amplifier is characterized by nearly zero DC bias current, so that there is no input signal viWhen the bias current is maintained, the power consumption is not wasted and the work efficiency is improved. The cost of this is that the circuit will be a little more complex, requiring two complementary amplifier circuits. Next, we analyze a conceptual class B power amplifier circuit, as shown in the following figure:

Figure 5-04.01

In the picture above, Q1And Q2A pair of complementary NPN and PNP transistors with the same performance constitute two separate amplifiers, one is responsible for amplifying the positive half cycle signal and the other is responsible for amplifying the negative half cycle signal. This way of working is called:push-pull(push pull) working mode. Next, we analyze its on state

A when there is no input signal (or input signal ViWhen it is less than 0.7V and greater than – 0.7V), transistor Q1、Q2All cut-off, load RLThere is no current on the circuit and there is no DC bias current (iCIs 0).

B. when the input signal ViWhen it is greater than 0.7V, the transistor Q1Conduction, q2Cut off, load current from VCCThrough Q1Flow direction RL

C. when the input signal ViWhen q is less than – 7V2The load current passes through R from groundLAnd Q2Flow to negative supply (- V)CC)。

2. Crossover distortion

The defect of class B amplifier is that the output voltage and output current are blank when the input signal is – 0.7V to 0.7V, as shown in the following figure:

Figure 5-04.02

This distortion of the output waveform is calledCrossover distortion(crossover distortion)。 If you want to eliminate the crossover distortion, you need to use the class A and B power amplifiers described in the next section.

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