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Browse Prior Art Database

Push Pull Power Amplifier

IP.com Disclosure Number: IPCOM000080106D
Original Publication Date: 1973-Oct-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 36K

Publishing Venue

IBM

Related People

Commander, RD: AUTHOR

Abstract

In a Class B push-pull transistor amplifier, the possibility of cross-over distortion necessitates the output transistors to be operated with a small standing bias, whose value represents a compromise between quiescent power dissipation and the tolerable level of crossover distortion in the amplifier. The circuit shown defines the bias using negative feedback, thereby avoiding the need for potentiometer adjustment and reducing liability to drift.

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Push Pull Power Amplifier

In a Class B push-pull transistor amplifier, the possibility of cross-over distortion necessitates the output transistors to be operated with a small standing bias, whose value represents a compromise between quiescent power dissipation and the tolerable level of crossover distortion in the amplifier. The circuit shown defines the bias using negative feedback, thereby avoiding the need for potentiometer adjustment and reducing liability to drift.

In the figure T1 and T2 are the output transistors, T1 being driven directly from an operational amplifier while T2 is driven via a feedback circuit, which sets the bias for the output stage. The amplifier voltage gain is controlled in conventional manner by negative feedback via R3.

In operation, T1 base is driven directly from the operational amplifier output and R4 senses the resulting collector current in T1.

D1 limits the voltage across R4 so that heavy load currents on positive output signal swings do not cause T1 to saturate. On negative signal swings T1 will tend to turn off. However, when R4 voltage drops below about 500 mV (the voltage across R5, established by potential divider R5, R6 with the values illustrated), T3 will start to be turned on in grounded-base mode. T4 compensates for T3 Vbe. T3 collector current turns on current mirror T5, T6 (otherwise biased off by R10) which provides base current to turn on T2. Resistors R7 and R8 arrange that the current mirror has a magnificatio...