According to frequency range.
1. DC(Direct coupled ) amplifiers: From zero frequency dc on wards.
2.Audio frequency amplifiers : 20Hz to 20kHz
3.Video Frequency amplifiers : up to few MHz
4.Radio frequency (RF) amplifiers : From a few kHz to hundreds of MHz.
5. Ultra high frequency (UHF) and Microwave amplifiers: Upto hundred or thousands of MHz,
The position of the zero signal (quiescent) operating point and the extent of characteristic curves being used determine the category of operation. Thus amplifiers (using either BJT or FET) may be classified as class A, class AB, class B or class C amplifiers as per following definition.
(1).Class A amplifiers.: In these operating point and the input signal magnitude are so selected that the output circuit current (collector current or drain current) flows all the time (for full cycle of the a.c. signal). A class A amplifier operates essentially over the linear portion of characteristic curves of the device.
(ii) Class B amplifier. In-these operating point is placed at an extreme end of its characteristic curve with the result that zero signal current (or zero signal voltage) is almost zero. Hence zero signal output power is very small. Witha sinusoidal input signal voltage, amplification takes place for only half the cycle. Thus if output circuit quiescent current is zero, on application of signal, the output current remains zero for half the cycle and flows for the remaining half cycle.
(iii) Class AB amplifier. In these operation lies in between class A and class B operations. Thus in a class AB amplifier, output current flows for more than half but less than the complete cycle of input sinusoidal signal.
(iv) Class C amplifier. In these operating point is so chosen that the output current flows for less than one-half of the input sinusoidal signal cycle.
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