Operating Limits
Quality of a receiver is defined by its sensitivity and selectivity. For a single-tank TRF (tuned radio frequency) receiver without regenerative feedback, where Q is tank "quality" defined as, Z is reactive impedance, R is resistive loss. Signal voltage at tank is antenna voltage multiplied by Q.
Positive feedback compensates the energy loss caused by R, so we may express it as bringing in some negative R. Quality with feedback is . Regeneration rate is .
M depends on stability of amplification and feedback coefficient, because if R-Rneg is set less than Rneg fluctuation, it will easily overstep the oscillation margin. This problem can be partly solved by "grid leak" or any kind of automatic gain control, but the downside of this is surrendering control over receiver to noises and fadings of input signal, which is undesirable. Modern semiconductors may offer more stability than vacuum tubes of the 1920s, depending on other circuit parameters as well.
Actual numbers: To have 3 kHz bandwidth at 12 MHz (short waves travelling all around Earth) we need . A two-inch coil of thick silvered wire wound on a ceramic core may have Q up to 400, but let's suppose Q = 100. We need M = 40, which is attainable with good stable amplifier even without power stabilizing.
Read more about this topic: Regenerative Circuit
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