Tung-Sol.com:: FAQs What are Class A, Class B and Class AB amplifiers?

FAQs

Question:
What are Class A, Class B and Class AB amplifiers?
Answer:
The Class A amplifier is an amplifier biased and supplied with excitation (an input signal) of such amplitude that plate current flows continuously (360 degrees of the exciting waveform) and grid current does not flow at any time. All preamp stages and single ended output stages of guitar and hi-fi amplifiers are Class A. A classic example of a single ended Class A amplifier is the Fender Champ. The Vox AC-30 is a Class A amplifier with a push-pull output. Class A amplifiers are normally operated in the center of the grid-voltage plate-current transfer characteristic and gives an output waveform that is a substantial replica of the input waveform. When these amplifiers are overdriven into saturation and cutoff, they have a somewhat spongy feel with a singing response and an interesting harmonic content that is highly desired by many musicians. They are, however, very inefficient and at least half of the power consumed is generated as heat. Plate efficiency is about 20% to about 35%.

The Class AB amplifier is an amplifier that is operated under such conditions of grid bias and exciting voltage that plate current flows for more than one-half the input voltage cycle but for less than the complete cycle. In the Class AB1 amplifier grid current does not flow over any portion of the input cycle. In the Class AB2 amplifier the exciting voltage is of such amplitude that grid current flows over a portion of the input cycle. A single ended Class AB audio amplifier would be impractical because of the excessive amount of second harmonic energy that would be generated when the output tube is not conducting. All Class AB guitar and hi-fi amplifiers are push-pull amplifiers using at least pair of output tubes. One output tube is used to amplify the positive portion of the input signal and the other output tube amplifies the negative portion of the input signal. Addition power can be obtained by placing additional tubes in parallel with the existing pair of output tubes. A phase inverter circuit is used to split the positive and negative parts of the input signal between the two output tubes. When one output tube is conducting, the other is in cut-off (not conducting) for a portion of the input cycle. Push-pull Class AB operation allows a plate circuit efficiency of 45% to 60%. Practically every guitar amplifier with an output power over 30 watts employs push-pull Class AB operation. All Fender and Marshall amplifiers with an output of more than 40 watts are Class AB types.

The Class B amplifier is substantially biased to cut-off of plate current when no input signal voltage is applied so that plate current flows essentially over one-half the input voltage cycle. The operating angle of plate current flow is essentially 180 degrees. The Class B amplifier is almost always driven with a signal to the extent that grid current flows. Class B push-pull amplifiers are the most efficient tube type audio amplifiers. Full-signal plate-circuit efficiencies of 60% to 70% are readily available. Class B push-pull amplifiers must be carefully designed to minimize the crossover distortion as plate current flow switches from one tube to the other in the push-pull pair during the positive and negative cycles of the input signal. The most notable Class B guitar amplifiers are those made by Music Man. These employed a cathode driven circuit using driver transistors to provide the signal to the output tubes. These amplifiers have an output of 75 watts with two output tubes and 150 watts with four output tubes, where a Class AB amplifier would be rated at 50 watts for two output tubes and 100 watts when using the same type of tubes.

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