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Power Amplifier Design Guidelines

Power Amplifier Design Guidelines

I am amazed at the number of amplifier designers who have, for one reason or another, failed to take some of the well known basics and pitfalls of amp design into consideration during the design phase. While some of these errors (whether of judgement or through ignorance is uncertain) are of no great consequence, others can lead to the slow but sure or instantaneous destruction of an amplifier’s output devices.
This amp uses the following…

long tailed pair input stage

single stage bootstrapped Class-A driver

complementary compound pair output stage

RC Zobel network (it hates inductors)

no current mirrors or sources (other than the bootstrap)


It is stable with all conventional loads, capable of 80W into 8 Ohms, and simple to build. Using only commonly available parts, it is also very inexpensive.

Protection From Radio Frequency Interference

A favourite pastime of many designers is to connect a small capacitor as shown in Figure 3 directly to the base of the input transistor. This is supposed to prevent detection (rectification) of radio frequency signals picked up by the input leads. Well, to a certain degree this is true, as the Resistor-Capacitor (RC) combination forms a low pass filter, which will reduce the amount of RF applied to the input. As shown this has a 3dB frequency of 159kHz.

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