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The final-stage collector current of a push-pull complementary-symmetry amplifier is a half-wave rectified version of the signal being amplified. By biasing the amplifier in class C, and using a large amount of negative feedback, the rectification process can be made almost-perfectly linear. This provides the basis for a high-quality AM detector, or a high-frequency linear AC voltmeter. Both functions can also be realised simultaneously, with different time-constants, because there are two collectors from which signals can be extracted.
Hi-fi AM detector, by Robert Batey .
AM Pre-emphasis standards (NAB) USA.
Hi Q notch filter (NS LB-5 1969)
Ultrasonic receiver .
Racal RA17 C18 .
RA17 adapter, part 2 (demodulators).