Distortion Measurements
Standard class-B circuit.
This
is the distortion at 20kHz of a standard class-B amplifier with output stage
quiescent current set to 6mA. Large crossover spikes can be seen.
This is the distortion at the optimum quiescent current
setting of 10mA. Crossover spikes can still be seen. Even a small variation in
the current gave a rapid increase in distortion. Thermal effects make it
difficult to keep the current accurately at this optimum value.
Above the optimum quiescent current the distortion
increases again. This is the distortion at 60mA. Increasing the current further would eventually eliminate the distortion and the amplifier would then be operating in class-A at this signal level.
Modified circuit.
The modified circuit with feedforward error correction gave this remarkable result for any
quiescent current over 15mA (a maximum of 120mA was used in the
tests). The 'distortion' was not visibly different with the 20kHz test signal
switched off, and is primarily the noise of the amplifier and
test equipment.
The full details and circuit diagram of the design are presented here. The
idea is very simple, but appears not to have been discovered previously. It was published in Electronics World, April 1998, with the title "Class-B in a New Class".
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