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.Class-A designs can be simpler than other classes in so far as class-AB and -B designs require two connected devices in the circuit ( push–pull output), each to handle one half of the waveform whereas class A can use a single device ( single-ended).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. 1.4 Suffix numbers for vacuum tube amplifiers.1.1.3 Single-ended and triode class-A amplifiers.1.1.2 Disadvantage of class-A amplifiers.
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