For every watt delivered to the load, the amplifier itself, at best, uses an extra watt. If high output power is needed from a class-A circuit, the power supply and accompanying heat becomes significant. 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. In a power amplifier, this not only wastes power and limits operation with batteries, but increases operating costs and requires higher-rated output devices. A maximum theoretical efficiency of 25% is obtainable using usual configurations, but 50% is the maximum for a transformer or inductively coupled configuration.
This, however, incurs higher signal distortion. Subclass A2 is sometimes used to refer to vacuum-tube class-A stages that drive the grid slightly positive on signal peaks for slightly more power than normal class A (A1 where the grid is always negative ). A class-A amplifier is distinguished by the output stage devices being biased for class A operation. The active element remains conducting all of the time.Īmplifying devices operating in class A conduct over the entire range of the input cycle. In a class-A amplifier, 100% of the input signal is used (conduction angle Θ = 360°). However, the same attributes are found with MOSFETs or vacuum tubes. In the illustrations below, a bipolar junction transistor is shown as the amplifying device. The angle of flow is closely related to the amplifier power efficiency. If it is on for only half of each cycle, the angle is 180°.
If the device is always on, the conducting angle is 360°.
The image of the conduction angle derives from amplifying a sinusoidal signal. The classes are based on the proportion of each input cycle (conduction angle) during which an amplifying device passes current. Power amplifier circuits (output stages) are classified as A, B, AB and C for linear designs-and class D and E for switching designs.