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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
Document File: 1 page(s) / 62K

Publishing Venue

Motorola

Related People

Walt Rozanski: AUTHOR

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.

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MO7VROLA Technical Developments Volume 7 October 1987

A BIAS SENSING CIRCUIT FOR CLASS-AB AMPLIFIERS

by Walt Rozanski

   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.

   The basic idea behind this method is that the quiescent bias will be equal to the average current through the output transistors as long as the current through the pull-up device is substantially equal to the current in the pull-down device. This will be true as long as the amplifier is AC coupled to its load and the output tran- sistors are stacked in such a way that the DC currents of the pull-up and pull-down devices have no other places to go. Figure 1 shows one possible embodiment of the bias sensing circuit.

   The combination of Rl, R2's and op-amp Al act to measure the current of pull...