AC and DC gain of amplifier circuits.
I once purchased from a “Tandy Store” (Radio Shack) an audio and video amplifier combined box. Using the audio amplifier then through an transistorised audio, I found that the audio sounded to the ear as a distorted audio. Testing with an audio signal tone and an oscilloscope, I found that the audio test tone was indeed quite distorted. While analysing the circuit design, I removed the emitter by-pass capacitor from the emitter bias resistor, and behold the audio tone reproduction then was quite excellent in the quality of sound.
Realising that the emitter by-pass capacitor was by normal convention to set the A.C. signal gain of the common emitter amplifier, I tried various capacitance values. While the AC signal gain altered, I could not get around the signal distortion found using the oscilloscope. By removing the by-pass capacitor altogether, the D.C. bias point gain was sufficient for the amplifiers signal gain requirements.
The test tone was in anyway an A.C. signal, coupled to the amplifier by coupling capacitors. As this was the case, the A.C. signal was amplified as an A.C. would be, but amplified around the D.C. bias point of the amplifier, the A.C. signal gain coupling through the emitter capacitor altered the D.C. bias point of the amplifier, and so amplified test tone signal to the circuit output was distorted.
This discovery is very important, as would indicate that there is really no such thing as the A.C. signal gain in an amplifier circuit, just a D.C. bias point circuit gain to an input A.C. signal. Any circuit frequency response components cannot be related to the D.C. Bias point as a companion component. That is to say that any frequency response design of a circuit must has the frequency selection components as external to the amplifiers on circuitry components. The frequency response components must not be allowed to affect the D.C bias points of the amplifier circuit.
In an Radio Frequency amplifier, say a pre stage or final stage amplifier, there can only be present D.C. bias components. Any A.C. signal components will only alter the D.C. bias point and cause a distortion figure of the amplifier and the subsequent inter modulation signal performances. The effect for an R.F. amplifier at any point within a receiver or transmitter would be profound, the same would be for active component mixers. This would include any audio amplifier circuits as well, be it in the receiver or the transmitter audio stages.