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Old 28th June 2008, 03:57 PM   #71
EUVL is offline EUVL  Europe
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If my head is functioning properly today, then your first circuit should work without this common mode current problem :


The only change I would perhaps make is to refer R12,13 (100k) to Gnd, and cascode the JFETs. It is a bit difficult to explain, but it is to do with even harmonics of the input JFETs when driven by a differential signal, which would result in common mode currents flowing to Gnd, whereas the odd harmonic content of this current will cancel each other as they are of opposite polarity.

In the circuit by Circlomanen e.g., this JFET common mode current (at say 2x signal frequency) flows through the 22R resistor (& the 2200u cap) towards ground, then through the 100R output load resistor of the opposite leg, and then back to the power supply. This in turn creates a negative common-mode voltage at both outputs which is fed back to the JFET gate via the feedback resistor, ending up as positive feedback.

The solution would then be to connect the 22R resistors to each other (without the caps) and lift them from Gnd. This way, the JFET current is forced to flow through the feedback resistors and the load resistors before completing the loop to the power supply, and thus any positive common mode current will lead to an increase in JFET source voltage (i.e. returning back to negative feedback).

There should still be a very small common mode current at 2x signal frequency, but it should not be more than a few mA.

I don't have DSL at home, so I am not downloading LT Spice this weekend to prove my hypothesis. But I think this matches what you showed in the JFET and MOSFET currents.

Attached my new proposal.

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File Type: zip circlotron f5 v2.zip (5.1 KB, 152 views)
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Old 28th June 2008, 06:44 PM   #72
KSTR is offline KSTR  Germany
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Hi Patrick,

I see you went through the same mind-boggling "in-the-head evaluation" horrors...

To cut things short, I found that the major contributor to the effect is the high and varying Cgd of the MOSFETs, that is, when I disable Cgd in the MOSFET models, et voíla, all is fine. The JFETs contribute to a far lesser amount. I virtually went through dozens of different hookups (including cascoding etc) but the most effective way to block the capacitive current flow in the JFET drain resistors is to buffer them with a follower in the way I've shown. The way feedback is arranged seems not to be important to the effect, meaning that your last proposal (which was my initial one) doesn't fix the problem, according to my findings.

How important this issue is in real life is still another matter since we don't expect a 10kHz or 20kHz full scale signal anyway (the tweeters will thank it). OTOH those two followers are an easy fix...

Finally, compared to the F5 the performance doesn't seem to be any superior, only added complexity (not least the supplies). The F5 is really hard to beat and that's why Admiral Nelson is the Master.
I like this quote of him very much:
"Like the products of other young designers, my first commercial product had everything but the kitchen sink in it. Now I strive to be like Picasso, who could draw a woman with a single pencil stroke and create a masterpiece."

- Klaus
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Old 26th April 2011, 10:34 AM   #73
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Location: D-55629 Schwarzerden
Originally Posted by Tom2 View Post
Your link sends me to main page.
I looked through the web site and found it?
Is this it?
Hopefully the attached schematic works.
Thank you for this information. Maybe this is an appropriate solution for my question about
Conversion of Hiraga's "Le Monstre" to a Circlotron
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