Class A 40W into 8 Ohms Amplifier

Posted on Feb 4, 2013

Both parts of the circuit are biased with the constant current sources as shown, and the signal from the collector of the PNP is followed by the Darlington class A output stage whose idle current is controlled by the bias circuit. The transistors Q1 and Q2 form the traditional differential pair but there are some twists added to the feedback and input networks. The feedback circuit formed by R3 R4 R5 C2 C4 is used to bootstrap the input impedance to a nominal value of 40k Ohms while providing a low impedance path for the input bias currents. This results in a high input impedance with low offset voltage. The capacitor C4 creates a high frequency input to the negative feedback circuit which rolls off the high frequency gain of the amplifier.

 Class A 40W into 8 Ohms Amplifier
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C4 frequency compensates the amplifier by creating internal feedback which allows the front end of the amplifier to work at satisfying the high frequency loop requirements by itself, ignoring the phase effects of the output stage and providing a high degree of stability for the system. As a form of feedforward technique, it does not impair the slew capabilities as lag compensation would, and comes into play at around 200kHz. The Pass A40 Power Amplifier The Audio Amateur 4/78 by NELSON PASS FLATTERED BY THE opportunity to publish a project circuit, the designer is often beset by seemingly contradictory considerations. On the one hand, it is tempting to design a complex circuit as a demonstration of technical prowess; an amplifier with large numbers of esoteric components performing obscure functions. Such an amplifier might be a smorgasbord of electronic technique, featuring class A operation, cascoding, constant current sources, current mirrors, and extra-loop error correction. It would be fascinating to build and perhaps would also sound good. On the other hand, complexity is not a good end in itself and a much simpler circuit would suit the needs of the amateur more ideally for low cost, high reliability, and easy construction. Simplicity can often yield sonic benefits, inasmuch as the fewer the number of components in a signal path, the simpler the open loop transfer curve of the amplifier. The importance of a...

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