Current mode switching has side effects that make it useful for many ham radio circuits.
For regenerative radio circuits the Tanh response curve gives smooth regeneration control.
The multiplier effect allows for mixer circuits and AM moduation circuits.
Well if you say 1 mA into the base emitter results in the base emitter junction having a resistance of 26 ohm, the 1uA gives a resistance of 26k and 1 nA 26meg. The input resistance of Q4 is then about 8 meg. Q3 is a short at RF so the tuned circuit sees a load of 8 meg in parallel with 4.7 meg.
Q3 only provides an exact current related voltage drop for biasing, and even without the bypassing capacitor has such a low resistance it can be ignored.
There is no point in using a collector load resistor for Q4, that would only throw away gain.
The sensitivity of any AM detector falls off with increased frequency. I have no measurements to say how well the circuit will operate at 20 Mhz, probably it will be fine.
Obviously with 3 nA going into the base of Q4 each electron counts and natural fluctuations in the exact number of electrons flowing at any particular time cause Gaussinan noise. You can hear some noise when you connect an audio amplifer. I would say the level is low enough not to be any problem, but loud enough to provide reassurance that the circuit is working.
You could replace R2 with headphones directly. If you want to use the LM386 audio chip some extra RF filtering is needed, expecially for the AM band where the LM386 retains a lot of gain up to 500KHz or more. A resistor capacitor low pass filter will do. For the AM band you might as well try connecting the AM detector circuit to the full tuned circuit. That would not be good for SSB though.
I redrew the diagram for more clarity:
Of course you could cheat and just attach a FET drain bend detector to the tuned circuit instead of Q3, Q4 and Q5 but that would be too easy.
When you click on the pin that takes you to pinterest and then you have to click on the pin again to get to the actual web-site.
I am narrowing in on exactly what regen I want:
The turns ratio between the primary of T1 and the secondary is about 1:5 to 1:10
The tap on the secondary should be say 1/4 the way up to 1/2 the way up.
I will fix on this circuit because it seems to perform very well. I need to take a rest from any design kind of thing but I will try this circuit at a number of differnt frequency ranges, check if it is very repeatable ect.