Here's an old TRB post from DrM. Sorry about the poor formatting.
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Throttle transistor regen control substituting throttle cap.
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DrM
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Location: The Netherlands
Throttle transistor regen control substituting throttle cap.
Post by DrM » Sat Apr 28, 2012 9:29 am
In a classic Armstrong regen a throttle cap is used for controlling the regen level. But you will need another tuning capacitor which is rather expensive. This capacitor can be substituted by a series network of a capacitor and a throttle potentiometer for regen control. But the potmeter is in the RF-feedback path which can give troubles. I have found an alternative. I use a transistor which acts as a variable throttle resistor by changing the base bias. The advantage is that the potmeter for controlling the regeneration is out of the RF feedback path.
In these two schematics the regen control is done by P1, T2 and C8. Potmeter P2 sets the operating point of the FET for an optimized regen control without any backlash.
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qrp-gaijin
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Post by qrp-gaijin » Sat Apr 28, 2012 10:06 am
Excellent idea, one which I have also been wondering about for a long time (I thought to use a JFET-as-resistor instead of a BJT though).
Question: how large is the frequency shift with regeneration adjustment using this scheme?
I also wonder if putting resistance in series with the throttle will reduce tank Q. I asked on regenrx, but didn't get a clear yes-or-no answer. I have however successfully simulated regenerative circuits in QUCS, so maybe that would be one way to find out.
P.S. Another way to use a pot to control regeneration, with the pot outside of the RF path, is to use a varactor throttle. This worked well for me using a 500pF varactor.
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XtalHWJ
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Post by XtalHWJ » Sat Apr 28, 2012 7:30 pm
Just a thought....
R1 and C3 the "grid leak" circuit really isn't necessary for use on JFETs. The bias is established by P2.
Rich
Homebrew Radio ex-Silicon Valley
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Ham-er
Posts: 1931
Joined: Sun Dec 02, 2007 7:50 am
cool
Post by Ham-er » Sat Apr 28, 2012 10:49 pm
Good work DrM,
If we think about it, a tranistor ot tube or ANY active device, IS a variable resistance.
Qrp-Gaijin, Yes one could use a j-fet, or even a triode TUBE.
XtalHWJ, It is true that the "gate leak" components do not control the BIAS/Operating Point. They CAN however be usefull. In circuits like this these "gate leak" components make an RC filter.
Keeping the mechanical backlash and hand capacitance away from the component/s that are controlling the RF feedback is a good thing.
73
kb0lxy
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XtalHWJ
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Post by XtalHWJ » Sun Apr 29, 2012 6:55 am
I had some correspondence with Charles Kitchin about this a few years ago. He's written a lot of articles about regen receivers.
He agreed that the "gate leak" was a carry-over from old tube designs. I eliminated them in my last MPF102 regen with no evident difference.
Rich
Homebrew Radio ex-Silicon Valley
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qrp-gaijin
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Post by qrp-gaijin » Sun Apr 29, 2012 9:35 am
XtalHWJ wrote:I had some correspondence with Charles Kitchin about this a few years ago. He's written a lot of articles about regen receivers.
He agreed that the "gate leak" was a carry-over from old tube designs.
He made some comments to this regard right here on TheRadioBoard:
viewtopic.php?p=24825#24825
I also eliminated them with no apparent problems.
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DrM
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Post by DrM » Fri Jun 29, 2012 3:21 pm
Controlling the regeneration with a potmeter in the RF feedback path of a regenerative receiver. Is this method of controlling the regeneration suitable for the higher shortwave frequencies? The regen control potmeter must be properly shielded to avoid any hand capacitance effect.
Last edited by DrM on Wed Aug 01, 2012 7:26 pm, edited 1 time in total.
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KR1S
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Post by KR1S » Fri Jun 29, 2012 4:21 pm
DrM wrote:Controlling the regeneration with a potmeter in the RF feedback path of a regenerative receiver. Is this method of controlling the regeneration suitable for the higher shortwave frequencies? The regen control potmeter must be properly shielded to avoid any hand capacitance effect.[/i]
Someone did it with a photoresistor, so there's no rf on the control wires. You also could mount the pot near the detector and use a shaft extension. Grounding the shaft to the panel through a bushing should eliminate Theremin effects.
73,
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millwood
Post by millwood » Sat Jun 30, 2012 5:18 pm
I have to say that I don't know when having a pot there is a problem.
With that said, I think the issue with a transistor as a variable resistor is there sharp turning on / off (a little bit of Vb changes -> a lot of Rce changes at the kneeing point). Plus you already using a jfet so why not a jfet instead.
dual-gate mosfets actually would be a nice device to use here.
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golfguru
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Post by golfguru » Sat Jun 30, 2012 11:59 pm
Might be of interest:
http://homepage.eircom.net/~ei9gq/regen.html
......................
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etherman55
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Re: Throttle transistor regen control substituting throttle cap.
Post by etherman55 » Fri Jul 03, 2020 9:56 pm
DrM wrote: ↑
Sat Apr 28, 2012 9:29 am
In a classic Armstrong regen a throttle cap is used for controlling the regen level. But you will need another tuning capacitor which is rather expensive. This capacitor can be substituted by a series network of a capacitor and a throttle potentiometer for regen control. But the potmeter is in the RF-feedback path which can give troubles. I have found an alternative. I use a transistor which acts as a variable throttle resistor by changing the base bias. The advantage is that the potmeter for controlling the regeneration is out of the RF feedback path.
In these two schematics the regen control is done by P1, T2 and C8. Potmeter P2 sets the operating point of the FET for an optimized regen control without any backlash.
I know it was posted a "few" years ago, but I could not see a DC path to the collector of Q2, as it is blocked by C8, I was just wondering how does this circuit work?
Regards.
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mblack
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Re: Throttle transistor regen control substituting throttle cap.
Post by mblack » Fri Jul 03, 2020 11:22 pm
I remember an article in QST in 1972 about synthesizing large values of variable capacitors. If I remember properly it used a bipolar in a similar circuit.
I don't have the issue handy, likely tye September or Oct issue.
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gramlow
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Re: Throttle transistor regen control substituting throttle cap.
Post by gramlow » Sat Jul 04, 2020 12:01 am
Excellent memory! See "Synthesis of a Varicap," by Edmund S. Cromartie. QST, September 1972.
73,
George
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Norm_N_Tam
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Re: Throttle transistor regen control substituting throttle cap.
Post by Norm_N_Tam » Sat Jul 04, 2020 12:22 am
etherman55 wrote: ↑
Fri Jul 03, 2020 9:56 pm
I know it was posted a "few" years ago, but I could not see a DC path to the collector of Q2, as it is blocked by C8, I was just wondering how does this circuit work?
Regards.
But no one has still answered how the bipolar transistor is receiving power at it's collector. Since it's a silicon transistor, I'm wondering about that myself. From what I've seen, only germanium bipolar transistors can work reasonably well with no apparent power being applied to their collectors.
Norman,
Over 40 years
as KA1GUK
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KR1S
Site Admin
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Re: Throttle transistor regen control substituting throttle cap.
Post by KR1S » Sat Jul 04, 2020 12:42 am
gramlow wrote: ↑
Sat Jul 04, 2020 12:01 am
Excellent memory! See "Synthesis of a Varicap," by Edmund S. Cromartie. QST, September 1972.
cromartie.png
73,
Jim
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