LINEAR CLASS B AUDIO AMPLIFIER OUTPUT-POWER CAPABILITY; ANALYSIS OF LOAD-IMPEDANCE AND WAVEFORM EFFECTS.

Large amounts of electrical power at audiofrequency are usually provided by some form of class B electronic amplifier. Unfortunately, changes in load impedance and in waveform can result in very large reductions in the output-power capability of such amplifiers. The design equations for class B amplifiers are developed with respect to impedance and signal waveshape. Manipulation of the equations leads to proof that class B amplifier operation with any signal waveshape can be assumed to be the same as operation with a sine wave of the same root-mean-square value. Therefore, once the sine-wave capability of a class B amplifier is determined, its capability for other waveshapes is determined by application of a simple derived multiplication factor. The sine-wave capability of class B amplifiers has been determined and presented in the form of graphs of power versus magnitude of impedance for fixed power factors and device peak-voltage capabilities. In general, the optimum sine-wave-output capability is determined as the power-dissipation capability of each device multiplied by 4.93, provided the current limit is not exceeded. The actual output capability will be reduced in magnitude by load and signal effects and system efficiency. This reduction is of the order of ten for loads with power factor of 0.7 and impedance swing of 41 and for device efficiency of 70 percent and circuit efficiency of 92 percent. For signals other than sine waves, the sine-wave average-power capability is then further reduced and is determined, in the worst case, by multiplying the sine-wave capacity by two and dividing by the square of the crest factor. Author