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ARTIFICIAL THERMAL BIAS COMPENSATION METHOD FOR RF TRANSISTORS

IP.com Disclosure Number: IPCOM000008556D
Original Publication Date: 1998-Mar-01
Included in the Prior Art Database: 2002-Jun-24
Document File: 2 page(s) / 92K

Publishing Venue

Motorola

Related People

Ron Lipton: AUTHOR [+2]

Abstract

Tracking the Icq (quiescent collector current) of a class AB amplifier when the thermal tracking device is in a non-ideal location or when the track- ing device is not located on the same plane as the device to be tracked. Ideally one would like the tracking device to be located in direct contact with the device to be tracked, to prevent thermal runaway and maintain Idq regulation, but this is usually not practical. This non-ideal location usually results from manufacturing considerations.

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MOTOROLA Technical Developments

ARTIFICIAL THERMAL BIAS COMPENSATION METHOD FOR RF TRANSISTORS

by Ron Lipton and Rodney Hagen

PROBLEM

  Tracking the Icq (quiescent collector current) of a class AB amplifier when the thermal tracking device is in a non-ideal location or when the track- ing device is not located on the same plane as the device to be tracked. Ideally one would like the tracking device to be located in direct contact with the device to be tracked, to prevent thermal runaway and maintain Idq regulation, but this is usually not practical. This non-ideal location usually results from manufacturing considerations.

SOLUTION

  To use a supplemental "heater" that is located in the proximity of the tracking device. This could be a resister or heating element whose sole purpose is to "thermally bias" the tracking device or it could be the same resistor that is used to "electrically bias" the tracking device. The later approach is depicted in the diagram below.

variations in heat dictated mainly by its drive level. This in turn appropriately adjusts the bias via the tracking device (the diode) to prevent thermal runaway and maintain Idq regulation. It should be noted that the heater is not a substitute heat source but rather a supplemental heat source. If the heater was used alone without the influence of the RF device, thermal runaway would likely result.

CIRCUIT OPERATION

  As the RF device heats up due to ambient rise or increased RF drive level the base emitter diode of the RF device will tend to lower its potential difference. If the base bias circuits potential stays the same, it will cause an increase in Idq due to the increased flow of base current. The base current increases due to the voltage at the bias circuit being greater than the voltage across the base emitter junction of the RF device. This causes an even greater collector current whi...