Comments for "Biasing JFETs, AGC IF and other fun"

1st April 2013 22:07

peter wrote ...

gday . you seem to like that expenceiv pcb bord , i use blank stuf with holes , its easer to cut or snap , anyway i made an if amp with bc549 s it was ossilateing till i put 15 k across each tank , it is in a sw recever i made , just to see if i could . it has an mpf 102 mixer 3.3k sorce R and a DCblocking cap osc conection it works but tuneing is too tuchy , i'll have to gear it down


28th March 2010 10:44

Doug wrote ...

Whoops! One mistake on that schematic. The 1 meg resistor is supposed to go to the junction of the 100pf and 68k resistor on the gate of the lower fet! Sorry! I should have proof read!

28th March 2010 10:12

Doug wrote ...

Alan, Ive included a link to the schematic of the detector/bfo. Sorry it took me so long to respond. I kind of forgot I posted here! I also built a 455KHZ SSB filter out of cheap ceramic resonators. Bandwidth is around 2.5khz. I will email the schematic as Im not sure how to post pictures here.

Link to schematic

22nd December 2009 16:50

Alan Yates wrote...


Do you have a circuit diagram of that?

The cascode "infinite impedance" detector I've seen in a few places but I am yet to try it. As you say it would be easily amendable to LO injection as a product detector.



21st December 2009 09:46

Doug wrote ...

I came up with a design much like this one using cascade JFETs to emulate mosfets. I used an amplified AGC system and applied it to the 2nd "gate" of my home made mosfets. I also used an infinite impedance detector which could be easily turned into a product detector. This is a VERY quiet IF system.

26th June 2008 18:54

Alan Yates wrote...


I've never had much luck with AGC in regens.

The regen-reflexive receiver has a bit of an AGC effect as its bias point is modulated by the recovered modulation envelope amplitude. It makes it much smoother to break into oscillation for detecting SSB, but it doesn't have the dynamic range needed for true AGC to compensate for variations in signal strength.

I tinkered with this circuit a bit a few days ago, adding a resistor in the emitter and controlling the device's bias point and gain more carefully. It improved the demodulated audio quality, but didn't improve the AGC range.

What you suggest sounds plausible in principle, you could use a dual-gate FET or a pair of JFETs in cascode to give you pretty good control, but there is no reason why it wouldn't work to some degree with just pulling the gate down. The regen control might be the source resistor with a Hartley oscillator topology, or the AGC line might bias the bottom of the regen pot and pull it below ground. I'll have to give it a try!



26th June 2008 04:12

Curtis Williams wrote ...

Hi, Alan.

I have run across your experimental circuit with AGC, and I would like know if this circuit will work in the tuned rf stage of a regenerative radio receiver. (Note: The AGC circuit is placed between the cold side of the antenna coil in the tuned RF stage at the radio's input and the output of the audio preamplifier stage of the radio.)

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