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Old 25th April 2013, 02:38 PM   #38561
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Almost everything is easy, once you come to understand it, or even think it possible.
33 years ago, I had to show Dr. R.G. Meyer a simplified version of this ckt, in order for him make a better low noise balanced input preamplifier. He, too, was caught up in the 'limitations' of the OP AMP possibilities at the time. It was used in a cryo experiment at LBL.
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Old 25th April 2013, 03:09 PM   #38562
godfrey is online now godfrey  South Africa
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@ elektroj
Here's a "bare bones" version of the circuit. The closed loop gain is set by R3, R14 and R15.

Vout1 = Vin1*(r14+R3)/R3 - Vin2*R14/R3
Vout2 = Vin2*(r15+R3)/R3 - Vin1*R15/R3

So with the values given:

Vout1 = 11*Vin1 - 10*Vin2
Vout2 = 11*Vin2 - 10*Vin1

Making sense now?
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Last edited by godfrey; 25th April 2013 at 03:13 PM.
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Old 25th April 2013, 03:17 PM   #38563
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I would like see concep of local cascodes. New to this, among other things.
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Old 25th April 2013, 04:35 PM   #38564
godfrey is online now godfrey  South Africa
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Quote:
Originally Posted by buzzforb View Post
local cascodes
You mean on the input JFETs, something like this?

I'd expect lower input capacitance, but possible stability problems. Haven't a clue about the effects on open loop gain and linearity. Anybody else can shed some light?
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Old 25th April 2013, 06:05 PM   #38565
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Wonderful, Godfrey! By the way, what schematic drawing program are you using? I like it, as mine is too complex.
You have SHOWN why there is a difference in the two outputs, WHEN only a single ended drive is used.
Someday, I hope to show how the late Jim B. 'solved' this problem, one that I could only correct with relays.
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Old 25th April 2013, 06:26 PM   #38566
bcarso is offline bcarso  United States
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Quote:
Originally Posted by godfrey View Post
You mean on the input JFETs, something like this?

I'd expect lower input capacitance, but possible stability problems. Haven't a clue about the effects on open loop gain and linearity. Anybody else can shed some light?
The Csanky bootstrapped cascodes in a circuit do have a negative input impedance at high frequencies, but I've found this to be usually easy to compensate with some lumped C at the gate to common. However the reduction in input capacitance modulation with voltage swing is very advantageous for distortion reduction at high frequencies.

Sorry to be slow in responding on the formulae, and thanks to all for the work while I was sleeping

I have done some simulations with the upper half circuit which may be worth "publishing", but I think the gist of circuit operation is emerging. Remember that this is a relatively high feedback design, despite some of the inputs being low impedance open-loop. However it is fast enough that distortion is not much changed at 10kHz compared to 1kHz, and this is without cascoding. More at 11.
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Old 25th April 2013, 06:48 PM   #38567
godfrey is online now godfrey  South Africa
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Quote:
Originally Posted by john curl View Post
By the way, what schematic drawing program are you using?
Hi John.

It's the free version of SIMetrix SIMPLIS. I find it a lot easier to use than LTSpice. I'm not sure if there's a MAC version though.

Quote:
Originally Posted by john curl View Post
Someday, I hope to show how the late Jim B. 'solved' this problem, one that I could only correct with relays.
Looking forward to that!

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Originally Posted by bcarso View Post
The Csanky bootstrapped cascodes in a circuit do have a negative input impedance at high frequencies, but I've found this to be usually easy to compensate with some lumped C at the gate to common.
Thanks. I suspected something like that, but hadn't thought it all the way through.

Last edited by godfrey; 25th April 2013 at 07:01 PM.
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Old 25th April 2013, 07:37 PM   #38568
bcarso is offline bcarso  United States
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Quote:
Originally Posted by godfrey View Post


Thanks. I suspected something like that, but hadn't thought it all the way through.
Walt Jung did the historical research about Csanky and corresponded with him. A lot of material was uncovered after I discovered that the base current recovery topology most still attribute to Baxandall and Shallow was in fact invented by Frank Boxall, who passed away recently.

All of these circuits with such techniques are susceptible to the mechanism to some extent. In this case it is modified by the closed loop configuration a good deal. Remember of course (as I'm sure you know, but mentioning this for other readers who may not) that the added FET needs a higher pinchoff voltage than the input device to work well.

The added advantage of the Csanky circuit is to much reduce the signal-induced self heating of the input devices with voltage swing. When the overall circuit is driven single ended there is no longer means of inducing tracking dissipation shifts with signal.

Demian I think remarked, a while back in here, that the devil is in the details of getting this topology and related ones to do multiple duties: all combinations of single-ended/balanced inputs/outputs. And in particular the availability of a polarity reversal for those who hear the Wood Effect (no relation btw) requires that the gains be made the same. The higher the closed-loop gains are, the smaller the relative disparity between output magnitudes, but it would still be appreciable for the values shown.
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Old 25th April 2013, 07:52 PM   #38569
godfrey is online now godfrey  South Africa
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OK, I'm feeling ignoranter and ignoranter by the moment now.

I'd never heard of Csanky before, but a quick search turned up some of Walt's posts. Likewise, I had no idea the "Baxandall super-pair" was actually invented by Frank Boxall.

BTW, what's the Wood Effect? (sometimes Google don't help)
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Old 25th April 2013, 07:56 PM   #38570
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A real concern is the Gm of the respective devices in the self cascode. I tend to prefer to use normal cascodes or folded cascodes rather than the self-cascode. I think it is best to use a LOW Gm cascode part and a HIGH Gm part for the input part. This is subtle territory however, and I have not made extensive measurements of the difference.
The advantage of the 'normal' cascode is that you can increase the voltage across the input device, thereby lowering and linearizing the feedback capacitance across the input fet.
From my understanding, this 'self cascode' or as you call the Csanky cascode was first used as the FETRON, tube replacement jfet cascode (a very high voltage jfet was used as the cascode) fore replacement of the input tube for VTVM's back in the late 60's. I once made the same basic design for the GD back in 1973, to experiment with open loop subjective performance. They still preferred tubes, however. '-)

Last edited by john curl; 25th April 2013 at 08:07 PM.
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