Push-pull class-AB transformerless power amplifiers

One of the major problems in power-amplifier design is to combine high efficiency with thermal stability. Class-C operation with current drive is one possibility but distortion level is high due to crossover and nonlinearity of hFE. A new diode-compensation method is described which allows high efficiency Class-AB operation for maximum output and lowest distortion and with excellent thermal stability. In the output stage the variation of the quiescent current vs temperature is kept small by a diode-resistance network in the emitter of each driver transistor, thereby eliminating the need for large thermal compensating resistance in the emitters of the output transistors. A 10-watt amplifier was designed using the described power-dissipation criteria and the new biasing scheme. Performance of the amplifier demonstrates the effectiveness of the design. It operates with maximum power dissipation up to 65°C ambient with a sine wave input. Calculations are also included in the paper which show the voltage levels of maximum power dissipation for sine, triangular and square waves.