The Regenerative Effect (Radio)

Regenerative radios are a type of radio receiver that uses positive feedback to amplify weak signals. This makes them very sensitive and selective, making them ideal for receiving long-distance and weak signals.

The basic regenerative radio circuit consists of a tuned LC resonator, a sustaining amplifier, and a feedback coil. The feedback coil couples the output of the amplifier back to the input, creating a positive feedback loop.

The regenerative effect helps cancel out losses in an LC circuit due to resistance, the eddy effect, "iron losses" etc.

The limit to the amount of loss cancellation possible is suggested to related to the phase noise of the sustaining amplifier.

Also important are phase shifts caused by the sustaining amplifier, which should tend toward 0 or 180 degrees at the design frequency.

The final requirement of the sustaining amplifier is that its gain falls with increasing input strength otherwise the circuit will suddenly "pop" into oscillation as regeneration is increased and it will be impossible to reach the point of maximum loss cancellation.

Obviously increasing gain with signal strength means increasing positive feedback with signal strength leading to the "pop."

You can use a differential pair with a tanh gain curve as the sustaining amplifier, find a way to restrict gain with increased loading with signal strength, or cause the level of bias to reduce with signal strength as a method of restraint. 

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