Class AB Power Amplifier Using MOSFET

CLASS AB POWER AMPLIFIER USING MOSFET

Class AB amplifier can be designed using MOSFET to generate an output power of 100 W. This amplifier has high efficiency and less harmonics.

This circuit consists of drivers, bias circuit and power amplification using MOSFET with class AB amplifier MOSFET is preferred over BJT because of its simple drive circuit, less susceptibility to thermal stability and high input impedance.

BIAS CIRCUIT

It consists of two V_BE multipliers formed by transistors Q₅ and Q₆ with their associated resistors. Q₆ is connected in direct thermal contact with the output transistors by connecting Q₆ on their common heat sink. The bias voltage V_GG can be reduced by selecting V_BE multiplication factor of Q₆ appropriately so V_BE decreases with temperature same rate as that of sum of threshold voltages |V_in| + |V_tp| of output MOSFET. Thus quiescent current of output transistors is stabilized against temperature variations.

DRIVER CIRCUIT

Complementary Darlington emitter followers are formed by the transistors Q₁ through Q₄. It provides the low output resistance necessary for driving the output MOSFET at high speeds.

V_GG is given by

V_BE is thermally coupled to the output devices and other BJT remain at constant temperature.

The other V_BE multiplier is adjusted to field the value of V_GG required for the desired quiescent current in Q_N and Q_P.

The Class AB amplifier using MOSFET is known as Hi-fi amplifier circuit and is suitable for applications such as keyboard amplifiers, general purpose amplifiers, guitar amplifiers and subwoofer amplifiers.