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A Pure Class B Audio Amplifier

Updated on January 2, 2013

Introduction


There are many audio amplifier options you can choose for your electronics project. Low distortion, reasonable efficiency, low quiescent current, no thermal runaway and good supply line noise rejection are desirable characteristics. Even with IC based solutions it can be difficult to get a good balance between the competing factors.


Figure 1.

A pure class B audio amplifier circuit you can use for your electronics projects.
A pure class B audio amplifier circuit you can use for your electronics projects.

The Circuit


Figure 1 shows a simple circuit that has a number of useful characteristics including a distortion level of less than 0.1%, a quiescent current requirement of only 6 mA, high supply line noise rejection and good stability.

Q1 is a bootstrapped common emitter amplifier. The bootstrapping of R1 increases the gain to in excess of 1000. Q2 is an emitter follower circuit that provides sufficient current to drive the output transistors Q3 and Q4. D1 reduces the emitter voltage difference between Q3 and Q4, thus reducing distortion while still ensuring the amplifier remains in pure class B mode.

Q1 is biased through R3. R8 and R9 determine the negative feedback ratio. The circuit is arranged to give a voltage gain of 1. Meaning that an input signal of about 2 volts will give a good loud output into a 8 Ohm loudspeaker connected between the output and ground. The gain of the amplifier can be increased by increasing R9. The input impedance of the amplifier can be increased by increasing R8 and R9 equally.

The high level of negative feedback gives both low distortion and high supply line noise rejection. The circuit is very stable with basic supply line decoupling precautions (C1 and C6).

The output transistors should have a Hfe in excess of 100 or else the value of R6 should be reduced to provide adequate drive current to the bases of Q3 and Q4. The output transistors can range from common 400 mW plastic case transistors to power transistors depending on the output power requirements. As the circuit lacks output limiting protection you should make sure the input audio signal is limited to what the output transistors can handle. If necessary a clipping circuit based on anti-parallel connected LED’s could be constructed to limit the input signal. Normally though you can arrange the supply voltage to a preamplifier circuit (for example) low enough to prevent any possibility of the amplifier being overdriven.


Conclusion

This circuit makes a nice alternative to common IC based output stages such as the LM386. All the components are easy to find and by selecting different output transistors you can create a range of amplifiers with different output power capabilities. The circuit itself is simple and easy to construct.

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