From Valve Amplifiers, Third Edition
Classes of Amplifiers
The class of an amplifier refers to the proportion of quiescent anode current to signal current. Until now, we have only looked at Class A amplifiers, although the fact was not explicitly stated. If we relax that restriction, we will need some definitions.
Class A
Anode current is set at a level such that, even with maximum allowable input signal, anode current never falls to zero. In other words, the valve never switches off. (Maximum theoretical efficiency for sine wave output is 50%.)
Class B
There is zero quiescent anode current, and anode current only flows during the positive half cycle of the input waveform. The valve is therefore switched off for the negative half cycle of the input waveform, and considerable distortion of the signal occurs, since it has been half-wave rectified. Additional measures will need to be taken to deal with this problem. (Maximum theoretical efficiency for sine wave output is 78.5% for a push pull Class B amplifier.)
Class C
Anode current flows for less than half a cycle of the input waveform. This method is only used in radio frequency amplifiers where resonant techniques can be used to restore the missing portion of the signal, and results in even greater efficiency and distortion than Class B.
Radio frequency engineers refer to the conduction angle to specify the proportion of time in which anode current flows. Using this description, we see that Class A amplifiers have a conduction angle of 360 , Class...
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