Amplifier classes

Updated July 13, 2008

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New for July 2007! Here we will give a brief explanation of the different classes of power amplifiers, which mainly relate to bias point.

I-V curves of a transistor

The curves of a transistor are shown in the following two figures. First is a field effect transistor. Here the current source is voltage controlled, which means that there is virtually no current on the input (gate) terminal.

In the case of a bipolar device (such as an HBT) the device acts like a current controlled current source.

The bias point, also known as the quiescent point "Q" is a function of the voltage (or current) on the input terminal, and the voltage on the output terminal. We've shown the bias points for class A, AB, B on the IV curve for comparison.

Class A

Class A small signal operation is linear. Bias point is at halfway between saturated current and pinch-off. When you hear he words "gain block" or "linear amplifier", think Class A. The output signal uses 100% of the input signal waveform.

Class B

A "complimentary amplifier", or push-pull amplifier uses opposite polarity (PNP and NPN) transistors that operate class B yet still provide the full waveform. There is distortion in that the transistors each have threshold voltages to overcome, which can be reduced by bias circuit on the input.

Class AB

Here the devices is biased somewhere between linear (Class A) and Class B, perhaps at 25% of the maximum current IMAX. This is by far the most popular bias point for a power amplifier, offering a good compromise between gain, power and efficiency. Quite often the gain of a solid state device is reduced the closer you are to a zero-current bias point.

In practice, the AB bias point drifts (or self-biases) under drive. Usually there is an appreciable resistance between the input terminal and its power supply, so under large-signal conditions, DC current starts to flow, either from breakdown or forward conduction. In both cases the bias point will shift in Class AB, the bias point is closer to breakdown than to...

Class C

This is similar to Class B but a tuned circuit brings only the intended frequency out of the device.

Class D, E and F are considered "switching amplifiers". We'll save that discussion for another day, when someone wants to help us out!