Signal Digger Regen Headphone Version
Signal Digger regenerative radio circuit - headphone version.
The PNP transistors Q1 and Q2 form a cross coupled differential pair feeding negative resistance into a portion of L1.
R2 can be adjusted to just cancel out the losses in L1 and C2 giving the resonant circuit a very high Q (very high voltage gain at resonance and selectivity.)
Q3 and Q4 are a current mirror at DC. Assuming Q3 and Q4 are approximately matched (same type and batch) then Q4 should be correctly biased to match R5. AM signals are demodulated by the non-linear behavior of Q4 to voltage source signals. CW and SSB can be demodulated by adjusting the Q multiplier to just cause L1 C1 to oscillate.
C4 filters out RF and is perhaps a little too large. However with this type of receiver it is important to stop RF getting into the audio amplifier chain.
Q5 and Q6 form a high gain headphone amplifier that relies on the difference between a typical base emitter drop of 0.6v and the typical collector emitter voltage of 0.2v.
R7 could be omitted but gives better gain and an improved impedance match to the AM demodulation circuit.
C7 further filters out RF and could be increased in value. The headphones could be replaced by a 270 ohm resistor if you want to connect an external amplifier.
Use a very low internal resistance power supply, such as a lithium cell or from a voltage regulator chip.
Circuit with more AM detection gain. C6 is not needed at higher frequencies (ie. 15 MHz) but may be needed at lower frequencies (ie. 1 MHz) to keep RF out of the following audio chain. R3 effects the voltage across R4 through Q3, Q4 and Q5. If you can make R3 1meg then you get a bit of extra gain, it just depends on the gain of the transistors.