AARegen  - Regenerative Radio Receiver

AARegen

Q1 and Q2 form a differential pair amplifier with simplified biasing. Relying on the transistor collector emitter saturation voltage being a little lower (0.2 V) than the base emitter conduction voltage (0.65 V).

The price for that is somewhat reduced gain from the transistors and higher than normal collector base capacitance. However, since the differential amplifier is coupled to the tuned circuit (C3 T1 primary) via an impedance match (T1 secondary) any problems are mitigated. 

A signal is induced in the tuned circuit via a loose antenna connection. The signal is sent into the base of Q2 and amplifed by the differential pair. The output of the differential pair appears at the collector of Q1 and is coupled back into the tuned circuit as positive feedback. 

The aim is to provide just enough feedback to cancel out the losses in the tuned circuit (to get the best Q and selectivity) without prompting the circuit to oscillate. This is achieved by controlling the gain of the differential pair using R1 and R2.

Q3, Q4 and Q5 form a highly sensitive AM detector circuit which you very loosely couple to T1 via L2.

If you over couple to T1 you will be unable to smoothly control the amount of positive feedback (regeneration) because of strays (signal leakage back).

Q5 acts as a simple RF amplifier.

Q3 and Q4 form a current mirror at DC via the RFC L1.

 At RF the amplified signal from Q5 is forced into the base of Q4 which acts as an AM detector.

Since the exact current through Q4 is not known you can use a low value of resistor for R4 (1k), or you can measure the current an choose R4 such that it has about 1 volt across it. The LED D1 provides a useful indicator of the current flowing through Q4 but can be omitted .