Transistor Tutorial Power Amplifiers Part 4

Posted on Feb 6, 2014

An audio power amplifier can boost weak signals from a tuner, CD player, or tape deck to fill a room with sound. This article focuses on the operating principles and circuitry of low-frequency power amplifiers based on the bipolar junction transistor (BJT). Other articles in this series have discussed multivibrators, oscillators, audio preamplifie

Transistor Tutorial Power Amplifiers Part 4
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rs, and tone-control circuits, all based on the BJT. A transistorized audio power amplifier converts the medium-level, medium-impedance AC signal into a high-level, amplified signal that can drive a low-impedance audio transducer such as a speaker. A properly designed power amplifier will do this with minimal signal distortion. Audio can be amplified with one or more power transistors in either of three configurations: Class A, Class B, and Class AB. Figure 1-a shows a single BJT Class A amplifier in a common-emitter configuration with a speaker as its collector load. A Class A amplifier can be identified by the way its input base is biased. Fig. 1-a shows that BJT Q1`s collector current has a quiescent value that is about halfway between the zero bias and cutoff positions. (The quiescent value is that value of transistor bias at which the negative- and positive-going AC input signals are zero. ) This bias permits the positive and negative swings of the output collector AC current to reach their highest values without distortion. If the AC and DC impedances of the speaker load are equal, the collector voltage will assume a quiescent value that is about half the supply voltage. The Class A circuit amplifies audio output with minimum distortion, but transistor Q1 consumes current continuously-even in the quiescent state-giving it low efficiency. Amplifier efficiency is defined as the ratio of AC power input to the load...

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