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A BIAS SENSING CIRCUIT FOR CLASS-AB AMPLIFIERS

IP.com Disclosure Number: IPCOM000005676D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2001-Oct-25

Publishing Venue

Motorola

Related People

Authors:
Walt Rozanski

Abstract

The output transistors of a class-AB power amplifier are required to be quiescently biased 'on' even when no AC signal is present. When an AC signal is present, the current in the output devices must increase so that useful output power can be generated The quiescent bias is frequently established by using resistors and diode(s), or some other temperature sensitive device. The temperature sensitive device(s) are placed in thermal contact with the output transistors. A kind of thermal balancing act must take place between the output devices and the biasing circuit so that bias will be properly maintained as the temperature varies. What follows is adescrip- tion of a method for sensing the quiescent bias of the output transistors that does not rely on temperature.