Discussion in 'The Projects Forum' started by mik3ca, Oct 13, 2008.

1. ### mik3ca Thread Starter Active Member

Feb 11, 2007
189
0

They all seem to have one thing in common. the coupling resistor is at least double of the quench frequency resistor. In this circuit, the quench resistor (R3) is 2.7K and the coupling resistor is 5.6K (R4).

Now I tried ratios myself. I tried making R3 and R4 in my circuit equal values, and that did not seem to work well. (the stations I picked up were replaced with noise). I also tried 15K for R3 and 36K for R4, and it worked better. (I can receive stations, but some are not coming in clearly).

Does anyone know the mathematics behind R3 and R4, or can someone explain to my why R4 must be double of R3 (instead of 1 1/2 times, or 3 times)?

thanks.

Apr 5, 2008
15,536
2,302
Hello,

R3 is for setting the current in the FET.
R4 is taken much higher to have the a good load.

Greetings,
Bertus

3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
R3 is critical to the operation of your FET. It manages current through Q2 for best performance. R4 limits current to the volume control of the 386 so it is capable of a larger range before losing gain or clipping.

You might find that R3 can be optimized somewhat but the 2.7 K value is probably pretty close to good.

What you are finding is why superregens lost out to superhets. You might want to read up on some FET theory, or at least go over the data sheet for the MPF102.

4. ### Audioguru New Member

Dec 20, 2007
9,411
895
Hi again Mike,
You know what I think about a lousy and simple super-regen radio circuit.

The MPF102 FET has a 15 to 1 range of how much current it conducts with a certain gate voltage so R3 in the circuit should be changed to match the spec's of the particular FET. If R3 has a value too low for the FET then the FET is saturated. If R3 has a value that is too high for the FET then the FET is cutoff.