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Old 11th April 2009, 02:21 PM   #1
bigwill is offline bigwill  United Kingdom
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Question Virtual chokes

Does any know of any good and simple schematics to simulate a choke for an anode load? I guess it needs to present a low impedance at DC and "infinite" impedance at AC, with the DC voltage set to about half the rail voltage to give some space to "swing"

I have some IRF820 mosfets and various bits which might work for this but I am open to circuits using other devices

Any help would be appreciated!
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Old 11th April 2009, 02:37 PM   #2
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Well, the simple answer is, I think, to do a search for DN2540 in this forum. Also try to include cascode in your search: it is a very simple circuit that consists of two DN2540's, three resistors and some wire + solder. The same circuit can be made with the 10M45S or 10M90S...
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Old 11th April 2009, 02:42 PM   #3
bigwill is offline bigwill  United Kingdom
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Quote:
Originally posted by ErikdeBest
Well, the simple answer is, I think, to do a search for DN2540 in this forum. Also try to include cascode in your search: it is a very simple circuit that consists of two DN2540's, three resistors and some wire + solder. The same circuit can be made with the 10M45S or 10M90S...

Thanks for the reply Though not quite what I'm looking for. What I was looking for specifically is not quite the same as a CCS. It's similar, but it gets its "programmed current" from the load, much like a choke. I want to be able to use a cathode CCS to set and adjust the current. If you adjust the current with an anode CCS with a fixed cathode resistor / diode string, the anode voltage changes until you readjust the cathode load.

I'd like it so both can be set independently
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Old 11th April 2009, 02:49 PM   #4
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Oh well

So you want a gyrator! Wavebourn has demonstrated a working schematic. A recent thread discussing it is here

Anti-Triode SEPP, how to do best?

post 9 shows the gyrator, and afterwards there is lot of ideas exchange about it!

Erik
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Old 11th April 2009, 03:42 PM   #5
bigwill is offline bigwill  United Kingdom
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Quote:
Originally posted by ErikdeBest
Oh well

So you want a gyrator! Wavebourn has demonstrated a working schematic. A recent thread discussing it is here

Anti-Triode SEPP, how to do best?

post 9 shows the gyrator, and afterwards there is lot of ideas exchange about it!

Erik

Hmm, I've had a look at this, but I can't seem to figure out how it works!

Edit: Ok, I get it, but doesn't the signal just pass straight through the "feedback" resistors, so that the dynamic resistance is basically the value of the resistor instead of something very high?
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Old 11th April 2009, 04:27 PM   #6
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Look at the transistor closest to the rail. It forms a current source at AC, with the reference voltage being a divided-down version of the difference between rail and plate. At AC, that's a constant voltage because of the bypass cap. At DC, that voltage varies so the transistor's drain current varies to compensate.

Low impedance at DC, high impedance at AC, with the turnover frequency being the time constant formed by the bypass cap. The result is something that looks like an inductor.
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