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An All NPN AB Biased Power Output Stage

IP.com Disclosure Number: IPCOM000089800D
Original Publication Date: 1977-Dec-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Killorn, RL: AUTHOR

Abstract

A class AB power amplifier utilizes a minimum of current sensing elements, in the high current paths. This is achieved by using the base-emitter junctions of pull-down devices to sense current. This approach is substantially temperature stable, and with the addition of a low value resistor (R4) and a diode (D1) a very wide variation of component tolerances can be allowed.

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An All NPN AB Biased Power Output Stage

A class AB power amplifier utilizes a minimum of current sensing elements, in the high current paths. This is achieved by using the base-emitter junctions of pull-down devices to sense current. This approach is substantially temperature stable, and with the addition of a low value resistor (R4) and a diode (D1) a very wide variation of component tolerances can be allowed.

In operation of the amplifier, the current of transistor Q2 is set by the base-to- emitter voltage of Q1. NPN transistors Q4 and Q5 operate at very low currents. The quiescent current of transistor Q3 is very close to that in Q2.

In order to provide sufficient base drive to NPN transistor Q3, its emitter must be more negative than the emitter of Q5. The current of Q6 is equal to that of Q3 plus the drop across resistor R2. As a result of the voltage drop across resistor R3, current flows in both output devices of the "pull down" Darlington circuit. Paired transistors Q7, Q8 and Q9 and Q10 form "pull up" and "pull down" power Darlington circuits, respectively.

Resistors R1, R2 and R3 are all of equal value. Diode D1 and resistor R4 serve to provide thermal stability. Resistors R5 and R6 set the gain relationship between V(IN) and V(OUT).

By virtue of the minimal use of voltage dropping current sensing elements in the output stage, the maximum swing realized at low load is substantially the same as that obtained at high load.

A feature of the circuit is nois...