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A MOSFET AMPLIFIER AUTOTRACKING BIASING CIRCUIT

IP.com Disclosure Number: IPCOM000006069D
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2001-Nov-30
Document File: 1 page(s) / 73K

Publishing Venue

Motorola

Related People

H.O. Granberg: AUTHOR

Abstract

Figure 1 shows a closed loop system for MOSFET biasing. It provides an automatic and precise temperature compensation to any MOSFET regardless of its electrical size and g,. No temperature sensing elements need to be coimected to the heat sink or to the device housing. In fact FETs with different gate threshold voltages can be changed in the amplifier without affecting the level of the idle current. The gate threshold voltage range is about *0.5V with a single initial setting of the idle current. This means that the gate threshold voltage can vary within these limits in short or long term for other reasons than temperature effects as well. These include such as moisture, atmospheric pressure, etc.

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MOTOROLA INC. Technical Developments Volume 12 April 1991

A MOSFET AMPLIFIER ~ AUTOTRACKING BIASING CIRCUIT

by H.O. Granberg

   Figure 1 shows a closed loop system for MOSFET biasing. It provides an automatic and precise temperature compensation to any MOSFET regardless of its electrical size and g,. No temperature sensing elements need to be coimected to the heat sink or to the device housing. In fact FETs with different gate threshold voltages can be changed in the amplifier without affecting the level of the idle current. The gate threshold voltage range is about *0.5V with a single initial setting of the idle current. This means that the gate threshold voltage can vary within these limits in short or long term for other reasons than temperature effects as well. These include such as moisture, atmospheric pressure, etc.

  The principle of operation of circuit in Figure 6 is as follows: The idle current of the MOSFET amplifier is initially set to class A, AB or anywhere in between these by R8, which also provides a stable voltage reference to the negative input of the operational amplifier Ul. This results in a current flow through Rl and a voltage is generated across it. This voltage is fed to the positive input of Ul, which results in the output of Ul to follow it in polarity, but not in amplitude. Due to the voltage gain in Ul, which operates in an open loop mode, its output voltage excursions are much higher than those generated across Rl. Thus, if the current through Rl tends to increase for any reason, part df the output voltage of Ul fed to the amplifier gate...