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Class B Amplifier

IP.com Disclosure Number: IPCOM000081234D
Original Publication Date: 1974-Apr-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 2 page(s) / 28K

Publishing Venue

IBM

Related People

Cole, TG: AUTHOR

Abstract

A circuit configuration for a class B power amplifier is shown in the figure. When the amplifier is connected across the supply as shown, base current for PNP transistor T2 will be drawn through resistor 5 and current through PNP transistor T1 and PNP transistor T2 will increase. The arrangement is such that when the voltage across points A and B reaches 1 volt, then PNP transistor T3 starts to conduct. The collector current of transistor T3 reduces the base drive to transistor T2 and will continue to do so until a stable state is reached. The quiescent current is set by choosing suitable values for resistors 1, 2, 3 and 4 and the Vbe of transistor T3. Diodes D1 and D2 are not conducting at this time.

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Class B Amplifier

A circuit configuration for a class B power amplifier is shown in the figure. When the amplifier is connected across the supply as shown, base current for PNP transistor T2 will be drawn through resistor 5 and current through PNP transistor T1 and PNP transistor T2 will increase. The arrangement is such that when the voltage across points A and B reaches 1 volt, then PNP transistor T3 starts to conduct. The collector current of transistor T3 reduces the base drive to transistor T2 and will continue to do so until a stable state is reached. The quiescent current is set by choosing suitable values for resistors 1, 2, 3 and 4 and the Vbe of transistor T3. Diodes D1 and D2 are not conducting at this time.

Input signals are applied to terminal 6 connected to the base of transistor T1. If the input voltage EIN goes positive, current will enter load resistor 7 initially through resistor 1 and then through diode D1. The feedback from transistor T3 to transistor T2 will ensure that the potential across points A and B remains at 1 volt. A current is thus maintained through resistor 2 and transistor T2, since the diode D1 can be selected to pass the full-load current with a forward drop of less than 1 volt. If the input voltage goes negative then transistor T1 will try to turn off. This in turn reduces the potential across the points A and B and consequently the emitter current of transistor T2 is increased, so that the output voltage across the load 7 fol...