What is a Class A Amplifier?


The most commonly used type of power amplifier configuration is the Class A Amplifier. The Class A amplifier is the simplest form of a power amplifier. It uses a single switching transistor in the standard common emitter circuit configuration, as seen previously to produce an inverted output. The transistor is always biased “ON” so that it conducts during one complete cycle of the input signal waveform. It produces minimum distortion and maximum amplitude of the output signal. This means then that the Class A Amplifier configuration is the ideal operating model. This is because there can be no crossover or switch-off distortion to the output waveform even during the negative half of the cycle. Class A power amplifier output stages may use a single power transistor or pairs of transistors connected to share the high load current.


Input Signal


Class A amplifier uses 100% of the input signal (conduction angle Θ = 360°). The active element remains to conduct all of the time. Amplifying devices operating in class A conduct over the entire range of the input cycle. A class-A amplifier is distinguished by the output stage devices being biased for class A operation. Subclass A2 sometimes refers to vacuum-tube class-A stages that drive the grid slightly positive on signal peaks for slightly more power than standard class A (A1, where the grid is always negative). This, however, incurs higher signal distortion.


The Design


Class-A designs can be simpler than other classes. Class -AB and -B designs require two connected devices in the circuit (push-pull output), each to handle one half of the waveform. In contrast, class A can use a single device (single-ended). The amplifying element is biased, so the device is always conducting, the quiescent (small-signal) collector current (for transistors; drain current for FETs or anode/plate current for vacuum tubes) is close to the most linear portion of its transconductance curve.

Because the device is never ‘off,’ there is no “turn on” time, no problems with charge storage. Generally, there is better high-frequency performance and feedback loop stability (and usually fewer high-order harmonics). The point where the device comes closest to being ‘off’ is not at ‘zero signal’. So, the problem of crossover distortion associated with class-AB and -B designs does not exist. This is best for low signal levels of radio receivers due to low distortion.




Class-A amplifiers are inefficient. The maximum theoretical efficiency of 25% is obtainable using usual configurations. However, 50% is the maximum for a transformer or inductively coupled configuration. In a power amplifier, this not only wastes power and limits operation with batteries. But it increases operating costs and requires higher-rated output devices. Inefficiency comes from the standing current, which must be roughly half the maximum output current, and a large part of the power supply voltage is present across the output device at low signal levels. If the class-A circuit needs high output power, the power supply and accompanying heat becomes significant. For every watt delivered to the load, the amplifier itself, at best, uses an extra watt. For high power amplifiers, this means enormous and expensive power supplies and heat sinks.

Because the output devices are in full operation at all times (unlike a Class A/B amplifier), they will not have as long life unless the amplifier is specifically over-designed to take this into account, adding to the cost of maintaining or designing the amplifier.


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