ABSTRACT
Power amplifiers are measured, the input is normally driven from a low impedance signal generator. The lowest value available is almost invariably used because, it minimises the Johnson noise from the source resistance; it minimises level changes due to loading by the amplifier input impedance. The amplifier under test was a conventional Blameless design with an EF output stage comprising a single pair of sustained-beta bipolar power transistors. The amplifier is very linear with a low source impedance, and it might well be questioned as to why the input currents drawn are distorted if the output is beautifully distortion-free. The reason is of course that global negative feedback constrains the output to be linear because this is where the negative feedback (NFB) is taken from but the internal signals of the amplifier are whatever is required to keep the output linear.
