Utilizing the high gain and high input impedance of short channel power MOSFETs, a simple circuit has been devised to provide sufficient drive for amplifiers up to 200 watts output. The circuit described features automatic control of the quiescent bias level and offers performance which meets criteria for high quality audio reproduction.

A Simple Direct--Coupled Power MOSFET Audio Amplifier Topology Featuring Bias Stabilization

Method and apparatus are disclosed for automatically biasing a push-pull amplifier. Generally, the auto-biasing circuit of the present invention includes a resistor circuit for sensing the supply current to the push-pull amplifier. A control circuit responds to a minimum value of the supply current to increase charge which is stored in a charge storage circuit. A source circuit responds to the increase in stored charge of the charge storage circuit to increase a control current and, consequently, increase the supply or bias current of the push-pull amplifier. In an alternative embodiment a dynamic control circuit is used to continually modulate the supply current so that the difference between the average supply current and the average output current is maintained at a predetermined value.

Bill Roehr

Papers

The Autobias Amplifier: A New Topology for Automatically Biased Audio Amplifiers Using Power MOSFETs