Primary analog circuit: 5-5 class A and B power amplifier


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1. Basic working principle

The main structure of class A and B power amplifiers (class AB) is similar to class B power amplifiers, but it can eliminate the crossover distortion of class B power amplifiers. The corresponding cost is to increase a little static power consumption, so its efficiency is slightly lower than that of pure class B power amplifiers.

There are many ways to eliminate crossover distortion. The simplest way is to use two diodes to counteract transistor Q1And Q2V ofBEOn voltage, as shown in the figure below:

Figure 5-05.01

In the figure above, make R1And r2The resistance is equal. Ideally, the voltage between the two diodes is 0, so there is no need to be viAdd the input coupling capacitance.

Due to the action of two diodes, q1And Q2V ofBEIt is no longer zero, but consistent with the diode (approximately 0.7V), so Q1And Q2Static collector current ICQNo longer exactly zero, but slightly greater than zero.

When viWhen increasing from 0, due to diode D1The voltage rise effect of the voltage will make Q1Turn on immediately; when viIt’s the same thing to reduce hours from zero, which makes Q2Turn on immediately, thus eliminating the crossover distortion. Its input and output waveforms are shown in the following figure:

Figure 5-05.02

The class A and class B power amplifiers used to eliminate the crossover distortion described above are sufficient for explaining the concept, but the effect may not always be very ideal in practical application. There are two reasons: one is that the PN junction voltage drop parameters of BJT transistor and diode must match very well. If the performance of the element device is more discrete, the circuit will be unbalanced and static power consumption will be increased; The other reason is that the heat generated by BJT power tube is often much larger than that of diode, so the temperature of BJT power tube will be inconsistent, and the parameter balance between BJT power tube and diode will be destroyed due to the inconsistent temperature, which is called thermal runaway.

The improved class A and class B power amplifiers use current source to generate bias to solve the above mentioned defects. If you are interested, please refer to relevant professional books.

2. Single power supply

The class B power amplifier and class a power amplifier described above need to use dual power supply (i.e. vCCAnd – VCC)The circuit cost is relatively high. A push-pull amplifier using a single ended power supply scheme is shown in the following figure:

Figure 5-05.03

In the above circuit, when there is no input voltage viTransistor Q1And Q2The static voltage in the middle is no longer zero, but vCC/2, diode D1And D2The voltage in the middle is also vCC/Therefore, the input and output terminals need to add coupling capacitance to isolate the DC component.

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