High-impedance drive for the elimination of crossover distortion

Crossover distortion in a class B transistor power amplifier can be greatly reduced by driving the output stage from a high-impedance source. Doing this capitalizes upon the fact that the current-to-current gain characteristic of a class B stage is much more linear in the crossover region than the voltage-to-current gain. High-impedance drive can be most easily applied to a complementary output stage, but can also be applied to two transistors of the same polarity if a driver transformer is used. In circuits of this type, no temperature-compensated bias arrangements are necessary, and "thermal runaway" is virtually impossible, as only one of the output transistors can be biased on at a time. Moreover, reverse bias is applied to the "off" transistor, reducing its turn-off time and minimizing the increase in power supply drain at high signal frequencies. Amplifiers designed according to this principle have operated with very low distortion and have exhibited unequalled thermal stability.