Regen III: A Journey of BJT Regen

2020/11

Based on JFET version, I reckon this  regen gonna works well, plus, the schematic is cute. 

This is not because the gain is too high. Even I tear down tickle winding( series with 2N2222's collector), oscillation sill present.  This is a clue, there is another feedback way which is not wanted.  So this is the key problem. I try to heal that by remove the emitter bypass capacitor.  yeah, it does works, more or less.

Problems still there, just be partly eliminated.  I use a bad  1mH RFC, which provided another possible oscillation mode.  I summarized this 3 oscillator together as following. 

But why these guy's works

So, why JFET version works well? 

There are good reasons.  JFET is much better on the junction capacitance, J301 only 2.5 pF and MPF102 is a little bit worse, but much better than 2N2222 or 2N3904.  Even worse, I deploy a 3DG130C, probably have 100 pF or so capacitance, godness. 

Then, how about regen on QST(1957/10 page 30)? 

Well.  The capacitance Cbe is directory to ground, which eliminated  2 other feedback altogether .  It's a clever design.

What can I do about this Regen Monster

Base on knowledge and practice, I recon following configuration could work.  0.47uf  emitter bypass capacitor effectively kill the parasitic feedback. Left only one RED feedback path.

Guess what, Almost nothing changed.  The Regen Adjust does could control the oscillation, but if Regen Adjust capacitor big enough(>200pf), there would emerged group of oscillation, from 1 Mhz extended to 50Mhz.

I even short the emitter to ground, It won't help either. This indicate there is another feed-back path hide there ! After days trying and crying, I confirmed this.  but, Where?  Which?  How?

I had to work out that by ltspice.  It's powerful: all symptom is identical to actually circuits,  that's means it's not because the messed fly wire nor bad RFC. There does existing another feedback path. 

Another experiment day, there is nothing left to try. I wonder how the regen adjust control the feedback, and is the gain achieved via the impedance of RFC?  why not try to connect the regen adjust capacitor directly to collector?

Both experiment and Ltspice simulation confirm this did successfully eliminated the Regen Adjust problem, now it's won't pop out frequency everywhere in whole HF band anymore. How surprisingly!

Simulation confirm, while the regen adjust capacitance increase,  the oscillation stop, but continue increase this capacitor, a much more high frequency  oscillation excited.  And, this time, the tickler winding and the tapped winding(to base of BJT) form a new oscillation tank.(confirmed by damping those tank  by resistor)

Even though the oscillation control is work now, the gain is so poor while the oscillation is stop, no station at all.  High gain region is too narrow to tune on a station.

Insert R2 series with regen adjust capacitor C1 would smooth the control.   (R4 is nothing but simulation a imperfect RFC. )  RFC here is not make the gain increase, but decrease,  what is a surprise. 

Tried everything I could come up with: change quiescent current/change feedback tap position/ change tickle windings.  Nothing seems work..  no station at all,  but obviously the oscillation get controlled. 

Cross compare all kind regen receiver, they all use a very very low working point: normally less than 100uA.  My version is obviously too high, before i realized this, the quiescent current  was almost 1-5 mA.   By inserting a 2k resistor to emmit, and use 39k collector loading resistor, a uA level quiescent current achieved. 

Now, there is few station.  It get better a little bit,  but again, It's so hard to tune, only one or 2 stations received, totally ruined reputation of  Regen receiver.

There must be something  I missed.

Deeper Dive Regen Detector

What save me is a point contact diode.  I don't know this diode is needed at all. Because almost every regen schematic i founded,  this diode is not needed.

This diode save everything.  Lots of stations pop out,  regen control is decent and I feel it's right.  So the circuits now  became this one:

Then, everything explained.  Regen Receiver don't need a diode detector: this is not ture.  

Regen does need a detector. The trick is BJT/JFET bias.   Every regen don't use diode detector, all configured to almost cutoff states.  It's not correct to turn off the BJT   by increasing the collector/emitter resistor! The corrector way is increase base resistor, in my scenario, it may need 1 Mohm, even larger. 

Review Circuits Debug Process

There is no loss but gain. Painful debugging process teach me lots of thing instead frustrated me.

• BJT is such a monster, parasitic capacitance need be tamed well.  Best of handling that surely is Vackar oscillator.

• Figure out 3 possible parasitic feedback path.  Next time, I could elegantly eliminated  this topology .

• The most big misunderstanding about regen is detector.  Most of circuits is intended for making the BJT as detector, by increasing base bias resistor.

• A resistor series with regen control capacitor would smooth the regen control. [Not really]

• A exceptional low current configuration also contribute to smooth regen control. 

• Connected the Regen Adjust capacitor directly to the collector is much better than the top of tickler. This configuration eliminated the possible feedback routed through tickler winding.

• low current might need more tickler windings, and few more ticker windings seems make the regen control easier.

Modified Original design

Since  the problem is solved, I consider recovering most of original design. And I come up with this regen receiver finally. 

Design notes:

• Regen Adjust move to collector, this help killing all parasitic oscillation.

• Add diode as detector, it's inevitable to do adjust the quiescent current if amplifier do the detector work.

• eliminated emitter resistor, simply use a very large resistor bias the transistor.

• a very low quiescent current does help to smooth the regen control.

• regen adjust capacitor series with a resistor won't help at all.

Alignment notes: ( following this procedure to make it work )

1. The tuned tank tapped at 1/3 or 1/4 position, don't push it too either edge.  Too much lower tap make the tune too hard and regen control is sensitive[feel like].

2. make enough windings for tickler to ensure the highest frequency could reliably  start oscillating.

3. change 1.5M resistor to 2M or 1M,  find a great work point you think it is.

Some Thoughts

What could I expect from a regen receiver?

• does it  could drive a loudspeaker to a comfortable volume? 

Coped with a LM386, get gain 30dB, this is possible, but only for strong signal. Most of regen receiver without a high enough gain, say It is sensitive only when listen it with a earphone.

To listen with loudspeaker, a additional gain stage is needed,  a extra audio stage, or a RF stage before detector.

• does it could pull much more stations from a long wire antenna ?

Not really.  For example, I had a 15 meter long wire antenna for HF. This antenna injected too much strong HF stations to the receiver, even the regen theoretically improve the Q,  I,e the selectivity. the whole receiver still been severely overloaded.

• does  regen receiver much more sensitive than a TRF?

It depends.  For simple enough TRF, regen is much better. I could not make a simple TRF beats the regen so far. 

• How Regen receiver like? 

I tuning the regen receiver follow a procedure.  I always tune up, than tuned down:

1)  Use regen control make the receiver just begin oscillating,  you gonna hear a soft pop noise. then tweak it a little back. 

2) Now tune up a little bit, if there is a station, we gonna hear it. 

3) tweak the regen control back more, make the oscillating stop, then tweak the main tune, lock the station.

repeated 1) - 3)

Driven loudspeaker

One Regen Stage following by a 30 dB audio stage does not enough for driven loudspeaker to a comfortable volume. I then attached another RF stage before detector.

Well, It's works.  It could driven the  Loudspeaker now. for long story short, here is a summary feeling about this regen receiver configuration:

• both RF amplifier bias badly. 

• Second stage severely loaded the first stage.  Second stage input impedance roughly change from 1.5k to 300 ohm, while tune up from 500 khz to 1.6Mhz.  And reality could be worse than estimate.

• because loading effect, regen control is much worse than regen stage  works alone, the major defect is that: while on top of RF band, the regen adjust need more capacitance to cease the oscillation, that's totally insane. (but let's dig into it to find what made this, is it because the awful wiring?) 

• For bright side, just after the regen stage ceased the oscillation, there is a longer high gain region than before.  

Soften Regen Control

What actually happen is disappointed. With a loudspeaker, It's much less sensitive than the earphone. This demand the regen control to be 'soft' --  needs a wider high gain region before bursting into oscillation. In this 'soft high gain' region,  we could located the station, earphone is far more sensitive than loudspeaker, means this soft gain region is very large).  Only if we could hear something, we could know how tune the regen.  

With the loudspeaker, the "soft high gain region" is too narrow,  which gonna missing every stations, except we try hard to wiggle the regen adjust knob, that's experience is too bad.

The loudspeaker prototype  is  coupled to regen stage via a large capacitor, around 4700pf or so. This arrangement might not a bad thing.  Second stages gonna loading the regen receiver, if it's  burst into oscillation, the loading effect might became significant tending to cease the oscillation, the cause list here might be wrong,  except the fact told me so. 

And plus another loading source:  the tap which connected to base,  also contribute to make a soft regen control. Tapped from bottom, ie, 1/5 or so, I get a "harder" control, but connected into 1/3 -- 1/2 tap, the regen is much soft.  With this change, the tickler winding need to increased, and the top band then get harder to start oscillation.

I decide it's the loading might make  the regen control soft: more energy loss in the oscillator, more energy needed  pump into the tickler, loss is everywhere but ergney pump  control  by only regen adjust.

Perfermacy summary for capacitive couple to regen:

• Delivery comfortable volume to loudspeaker.

• Sensitivity is acceptable,  lots of stations. 

• Regen control is soft, comfortable. top band need connected the tap to 1/3.

• Difference transistor need different bias, 1.5M might not enough for 2N2222 since beta is high. (Bias method is lazy.), the extremely low steady current is another key factor to ensure a smooth regen control. Try less than 50uA on first round !!

• Bias and tap position and tickler need to be adjusted to balance soft control vs top band gain.

Following  Rule learned

Ok, put aside whether the theory is right or not,  that provide me a idea,  I could make the second stage input impedance lesser and get high gain simultaneously: just bypass the emitter degeneration resistor. 130 ohm resistor with a 330k bias,  there is about 1mA quiescent. (last setup get 2 mA or so),  if beta is about 100, input impedance about  2k(low frequency), this is half than before,  and get the extra gain.

 note: the 3dg6 is a low hfe transistor,  for 2N2222, change 330k to  560k or even larger to get 1mA quiescent current.

This configuration really surprise me.

• Even regen is fully attenuate the feedback(lowest gain), It cloud drive loudspeaker comfortably for strong station. This make tuning is very easy and comfortable.

• Top band still lack of gain to starting oscillation.  This could be good thing for receive AM broadcasting stations.

• Tap point should located 1/5 or less to get highest selectivity.

• Second stage RF gain should be tamed, 1 mA quiescent current is comfortable.

• BTW, regen stage quiescent current about 50 uA or so.

This receiver now is totally OK for sensitivity, regen smooth,  easily tuning,  and these performance stay almost identical while replace  transistor. For second stage, it needs to lower quiescent current for high beta transistor.

The key to make easy tuning and smooth regen control:

• second stage provide extra 25~35 dB gain, make the loudspeaker is same sensitive as earphone.

• second stage is loading the regen stage. This is key to smooth the regen control, if use impedance matching to eliminate this loading effect,   regen control became sharp.