PA

D8

1N4002

The rated power of 225W into 4 ohms or 145W into 8 ohms is obtained with the Ferguson PF4363 transformer.

“taił”. By virtue of the diodes, Dl and D2, and the emitter-follower action of Q3, about 0.6 volts is applied to the associated 680fi emitter resistor. This sets the current through Q3 at around one milliamp and this is shared equally through Q1 and Q2.

The 22kf2 resistor in series with the collector of Q3 serves no purpose in the

normal course of events. It is a fail-safe component. If Q3 happened to fail and go short-circuit, the 22kfi resistor would limit the current to three milliamps and allow the amplifier to operate almost normally. Without the 22kfi resistor, a failure in Q3 would possibly cause almost all the semiconductors in the amplifier to be blown.

The reason why Q3 is there, instead of a simple resistor, is to act as a constant current source. By maintaining the total current through Q1 and Q2 very constant, it improves the power supply rejection ratio (PSRR) of the entire amplifier. The better the PSRR, the less likely is the amplifier to respond to variations in the power supply rails.

35

ELECTRONICS Australia, October, 1985