Amplifier Classes

Amplifier circuits can be categorized as class A, class AB, class B, and class C. Each class has its own special characteristics, and works best in its own unique set of applications.

The Class A Amplifier

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With the previously mentioned component values, the amplifier circuits in above figure 1 and figure 2 operate in class A. This type of amplifier is linear, meaning that the output waveform has the same shape as (although a much greater amplitude than) the input waveform.
 
For class A operation with a bipolar transistor, the bias must be such that, with no signal input, the device is near the middle of the straight-line portion of the IC versus IB (collector current versus base current) curve. This is shown for an NPN transistor in below figure 3. For PNP, reverse the polarity signs. With a JFET or MOSFET, the bias must be such that, with no signal input, the device is near the middle of the straight-line part of the ID versus EG (drain current versus gate voltage) curve. This is shown in below figure 4 for an N-channel JFET. For P channel, reverse the polarity signs.
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In a class A amplifier, it is important that the input signal not be too strong. An excessively strong input signal will drive the device out of the straight-line part of the characteristic curve during part of the cycle. When this occurs, the output waveshape will not be a faithful reproduction of the input wave shape, and the amplifier will become nonlinear. Class A amplifiers are supposed to operate in a linear fashion at all times.

The Class AB Amplifier

Class A operation is inefficient because the transistor draws current whether there is a signal input or not. For weak-signal work, efficiency is not too critical; the things that matter are the gain and the sensitivity. In power amplifiers, efficiency is more important, and gain and sensitivity don’t matter as much.
 
When a bipolar transistor is biased close to cutoff under no-signal conditions, or when an FET is near pinchoff, the input signal always drives the device into the nonlinear part of the operating curve. Typical bias zones for class AB are shown in above figure 3 and above figure 4. A small collector or drain current flows when there is no input, but it is less than the no-signal current that flows in a class A amplifier. This is called class AB operation. It’s more efficient than class A, but the gain and sensitivity are not as high.
 
There are two modes of class AB amplification. If the bipolar transistor or FET is never driven into cutoff/pinchoff during any part of the signal cycle, the amplifier is working in class AB1. If the device goes into cutoff pinchoff for any part of the cycle (up to almost half ), the amplifier is working in class AB2.
 
In a class AB amplifier, the output signal waveform is not identical with the input signal waveform. But if the signal wave is modulated, such as in a voice radio transmitter, the waveform of the modulating signal comes out undistorted anyway. Thus, class AB operation is useful in RF power amplifiers.

The Class B Amplifier

When a bipolar transistor is biased exactly at cutoff, or an FET is biased exactly at pinchoff under zero-input-signal conditions, an amplifier is working in class B. These operating points are labeled on the curves in above figure 3 and above figure 4. The class B scheme lends itself well to RF power amplification. In class B operation, there is no collector or drain current when there is no signal. This saves energy, because the circuit does not consume power unless there is a signal going into it. (Class A and class AB amplifiers consume some power even when the input is zero.) When there is an input signal, current flows in the device during exactly half of the cycle. The output signal waveform is greatly different from the input waveshape in a class B amplifier. In fact, it is half-wave rectified. You’ll sometimes hear of class AB or class B “linear amplifiers,” especially in ham radio. The term “linear” refers to the fact that the modulation waveform is not distorted by such an amplifier, even though the carrier waveform is distorted because the transistor is not biased in the straight-line part of the operating curve. Class AB2 and class B amplifiers draw power from the input signal source. Engineers say that such amplifiers require a certain amount of drive or driving power to function. Class A and class AB1 amplifiers theoretically need no driving power, although there must be an input voltage.

The Class B Push-Pull Amplifier

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A class B push-pull amplifier using NPN bipolar transistors. Component designators are discussed in the text.
Sometimes two bipolar transistors or FETs are used in a class B circuit, one for the positive half of the cycle and the other for the negative half. In this way, signal waveform distortion is eliminated. This is called a class B push-pull amplifier. This type of circuit, using two NPN bipolar transistors, is illustrated in above figure . Resistor R1 limits the current through the transistors. Capacitor C1 keeps the input transformer center tap at signal ground, while allowing for some dc base bias. Resistors R2 and R3 bias the transistors precisely at their cutoff points. The two transistors must be identical. Not only should their part numbers be the same, but ideally they should be chosen by experiment to ensure that their characteristic curves are as closely matched as possible.
 
Class B push-pull is a popular arrangement for audio-frequency (AF) power amplification. It combines the efficiency of class B with the low distortion of class A. Its main disadvantage is that it needs two center-tapped transformers, one at the input and the other at the output. This makes push-pull amplifiers rather bulky and expensive compared to other types.

The Class C Amplifier

A bipolar transistor or FET can be biased past cutoff or pinchoff, and it will still work as a power amplifier (PA), provided that the drive is sufficient to overcome the bias during part of the cycle. This is known as class C operation. Bias points for class C are labeled in figure 3 and figure 4. Class C amplifiers are nonlinear, even for amplitude modulation waveforms. Because of this, a class C circuit is useful only for signals that are either full-on or full-off. Such signals include oldfashioned Morse code, and digital schemes in which the frequency or phase (but not the amplitude) of the signal is varied.
 
A class C amplifier needs a lot of driving power. The gain is low. For example, it might take 300 W of signal drive to get 1 kW of signal power output. However, the efficiency is better than that of class A, AB, or B amplifiers. Let’s take a closer look, now, at what amplifier efficiency is all about.