Noisy Regen

The Noisy Regen circuit as described by Alan Yates is a simple and easy to construct regenerative radio receiver. In addition it is a very repeatable circuit in contrast to other regen designs which are problematic to replicate. (See: www.vk2zay.net/article/128)

In the circuit the differential pair amplifier provides negative resistance (increase in voltage leads to decrease in current) to the LC tuning circuit. This cancels out the unavoidable losses in a real LC circuit, giving the LC circuit increased selectivity and Q (voltage magnification at resonance.)

Of course providing too much negative resistance will cause the LC circuit to oscillate.

Since only small signals are involved and there are no resistor collector loads, the circuit can be biased by direct connection to the negative rail. This is allowed because the collector emitter saturation voltage of a typical transistor is about 0.2 V, yet in operation there is about 0.575 V across the collector emitter junction. Therefore voltage swings of up to 0.375 V are possible at the collector if there is an inductive load present.

The disadvantages of biasing like that are increased collector base capacitance and lower transistor gain.

The repeatability of the noisy regen circuit is largely due to the highly defined gain curve of the differential pair.

The important point is that the gain systematically decreases with input signal strength. This results in less and less negative resistance with increasing signal strength. 

If the opposite were to occur and negative resistance were to increase with signal strength then a sudden strong input would cause the circuit to "pop" into oscillation and not go back.

Ideally the 2 transistors in the noisy regen circuit would be matched so that the current through each was identical to get the nice gain curve indicated, in practice it is sufficient that the transistor are of the same type and lot.

The main design issues with the noisy regen that make it "noisy" are transistor parameter interactions with the LC circuit and ineffective AM detection.

LC circuit operation is harmed by collector output impedance and capacitance (including varactor effects as the collector base reverse bias voltage moves around) and base input impedance and capacitance. 

The best way to reduce all these LC problems is with an impedance match.

AM Detection

The positive point about AM detection in the noisy regen is the low impedance take off point at the emitters of the 2 transistor. Any changes in AM detector input impedance will have a limited impact on the amount of negative resistance supplied by the differential pair. Avoiding instability.

Unfortunately there is very little asymmetric non-linearity in differential pair that would allow efficient AM detection.  Resulting in the need for high audio amplifier gain and a lot of the resulting noise of the circuit.

An AM detector with a relatively stable input impedance independently coupled to the LC circuit is one of the best options to get clean audio signal out.