Class A means that the bias on the 'device' (transistor or tube) is set to allow the device to be "on" 100% of the time. Very linear for the signal (that's why they sound good), but poor efficency. That's why real Class A amps are huge and expensive.
Class B amps are used in a "push-pull" arrangement - two tubes or transistors are used. While the signal is positive, only one tube is 'on' while the other is 'off'. Visa versa for the neg signal. Not used in transistor
audophile amps because of crossover distortion.
Crossover distortion is caused by the fact that the input of a transistor needs 0.7 volts to forward bias the junction. By keeping a small dc voltage at the junction to prevent the signal itself from being used to turn on the junction, this eliminates crossover distortion.
Class AB is a compromise between pure class A (sounds great, but runs very hot and is expensive) and class B (which runs cooler - more efficent - but doesn't quite sound as great as class A).
I don't believe that negative feedback is used to eliminate crossover distortion, but rather "regular" or non-linear distortion in general. Feeding back part of the signal in a 'negative' or degenerative manner increases amplifier linearity somewhat, but can produce other kinds of distortion (intermodulation, odd-order harmonic distortion, etc.)
Class C is for RF amplifiers almost exclusively (except bullhorns)
Class B amps are used in a "push-pull" arrangement - two tubes or transistors are used. While the signal is positive, only one tube is 'on' while the other is 'off'. Visa versa for the neg signal. Not used in transistor
audophile amps because of crossover distortion.
Crossover distortion is caused by the fact that the input of a transistor needs 0.7 volts to forward bias the junction. By keeping a small dc voltage at the junction to prevent the signal itself from being used to turn on the junction, this eliminates crossover distortion.
Class AB is a compromise between pure class A (sounds great, but runs very hot and is expensive) and class B (which runs cooler - more efficent - but doesn't quite sound as great as class A).
I don't believe that negative feedback is used to eliminate crossover distortion, but rather "regular" or non-linear distortion in general. Feeding back part of the signal in a 'negative' or degenerative manner increases amplifier linearity somewhat, but can produce other kinds of distortion (intermodulation, odd-order harmonic distortion, etc.)
Class C is for RF amplifiers almost exclusively (except bullhorns)