Smooth Clamp Question

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Trying to smooth the abruptness of a classic OpAmp clamp circuit...

<http://www.analog-innovations.com/SED/SmoothClampQuestion.pdf>

Any ideas?

Thanks!
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Smooth Clamp Question
On Wednesday, November 16, 2016 at 12:29:13 PM UTC-5, Jim Thompson wrote:
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Would resistance in the feedback path cause the diode to turn on slower?  
A few diodes in series?
An LED?  

(lotsa ideas, few good ones.)  

George H.  
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Re: Smooth Clamp Question
On Wed, 16 Nov 2016 09:44:27 -0800 (PST), George Herold

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Only if you restrict the gain of the OpAmp, then the final VCLAMP
accuracy suffers... that's why I posted the question... I'm somewhat
stymied :-(
  
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                                        ...Jim Thompson
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| Analog Innovations                               |     et      |
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Re: Smooth Clamp Question
On Wed, 16 Nov 2016 10:29:01 -0700, Jim Thompson wrote:

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How accurate a clamping voltage?  How smooth a corner?  How many extra  
parts?  How cleanly does it need to come off of the clamped voltage?

I thought you were a past master at piecewise-linear circuits with diodes  
and op-amps -- is there none of those you can use?

Triodes are supposed to have nice warm distortion characteristics -- what  
about using an op-amp with a 6J5 output stage to provide a smooth clamp.  
If you need to clamp top & bottom, invert that and feed another one.  Or,  
give us enough information that it's less tempting to meet your  
specifications with something absurd.

I don't think it'll be easy to get a really accurate clamping voltage  
while getting a smooth transition to that clamping voltage, at least not  
without using lots of parts.  But I'm much better at throwing lots of  
parts at a problem than finding a really elegant solution, so I won't be  
surprised to be surprised.

--  
Tim Wescott
Control systems, embedded software and circuit design
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Re: Smooth Clamp Question
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You might take a look at the circuitry used in some of the classic
analog function generators.  They make sorta-sine-waves by generating
a square wave, integrating it into a triangle wave, and then running
it through a soft-clamping network which involves the use of multiple
resistor-diode current suckers in parallel.

Re: Smooth Clamp Question
On 11/16/2016 12:29 PM, Jim Thompson wrote:
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The simplest method is probably to reduce the effective open loop gain.

At the price of a bit of noise and inaccuracy, you can do the usual
gain-reducing trick for running decompensated op amps at unity gain,
inside the loop.

Resistor from Vout to SJ, resistor from SJ to +Vclamp, resistor from SJ
to output.

Cheers

Phil Hobbs
--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Smooth Clamp Question
On 11/16/2016 12:54 PM, Phil Hobbs wrote:
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Another approach would be to drop Vclamp a bit and use a diff pair to
increase it again smoothly when Vin crosses Vclamp.

Cheers

Phil Hobbs


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Dr Philip C D Hobbs
Principal Consultant
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Re: Smooth Clamp Question
On Wednesday, November 16, 2016 at 1:01:11 PM UTC-5, Phil Hobbs wrote:
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Oh how about two clamps, first one reduces the gain by 1/2 or something,
and the second one clamps.  
Like the sqrt circuit Spehro mocked up here for me.  
(Then you get two sharp corners.)  

George H.  
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Re: Smooth Clamp Question
On 11/16/2016 01:28 PM, George Herold wrote:
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Or use two low gain amps controlling a 4-diode sampling bridge that
clamps Vout to Vclamp.  That'll give nice smooth transitions.

Cheers

Phil Hobbs


--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Smooth Clamp Question
On Wednesday, 16 November 2016 17:29:13 UTC, Jim Thompson  wrote:
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Use a geranium diode. Or perhaps germanium + R // silicon.
Make input impedance low.
If it were for fun I might suggest copper oxide.
Look at guitar fx, it's a classic question there.


NT

Re: Smooth Clamp Question
On Wednesday, 16 November 2016 18:47:14 UTC, tabbyp wrote:
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Or take it further with 4 diodes... silicon, schottky, germanium and HV silicon, 3 of those 4 with their own series R.


NT

Re: Smooth Clamp Question
On Wednesday, November 16, 2016 at 9:29:13 AM UTC-8, Jim Thompson wrote:
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Either limit the op amp slew rate, or limit the abrupt turnon of the diode.
So, putting an inductor in series with the diode would do... something nearly appropriate.
So would  using a low-power, low-slew op amp.  The slew also goes with
the forward voltage of the diode, so multiple diodes in series (or an  
LED) might be a productive approach.

That all presumes you are OK with time-domain trickery, which doesn't accomplish
a low-speed characteristic curve correction.

Re: Smooth Clamp Question
wrote:

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Doesn't work too well at DC ;-)
        
                                        ...Jim Thompson
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| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Smooth Clamp Question
On Wednesday, 16 November 2016 22:18:55 UTC, Jim Thompson  wrote:
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No. If you told us more...


NT

Re: Smooth Clamp Question
On Wed, 16 Nov 2016 16:44:44 -0800 (PST), snipped-for-privacy@gmail.com wrote:

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I'm pretending to be a leftist... you're supposed to guess the outcome
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                                        ...Jim Thompson
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| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Smooth Clamp Question
On Thursday, 17 November 2016 01:36:03 UTC, Jim Thompson  wrote:
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You could always do the lightbulb & resistor trick. But with little info who knows if the resulting set of limitations suit you, probably not.


NT

Re: Smooth Clamp Question
On Thu, 17 Nov 2016 10:23:24 -0800 (PST), snipped-for-privacy@gmail.com wrote:

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You missed my post replying to Tim Williams that this is for a
simulation model.

After I sat down and actually did the math properly, I came up with...

<
http://www.analog-innovations.com/SED/HardClampSoftCorner_2016-11-17_11-30-36.png


which gives me the independent degrees of freedom I need, slope,
transition width, and clamping voltage... end use if for an OpAmp
behavioral model... gain _and_ swing limit control... with _smooth_
derivatives.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Smooth Clamp Question
IRL, or /in silico/?  (I'd think the latter is a long-solved problem, given  
your toolkit of TANH's and whatnot, but checking just in case.)

The IRL manifestation is due to Vf knee divided by loop gain.  Of course, Vf  
is temp sensitive, so you can't make the knee itself stable without adding  
many dimensions of compensation hackery (i.e., errors with respect to  
Vf/Vth, N, Is, and leakage).

Is that what you're after?

But then, Vclamp precision suffers, when gain is low.  Are you looking to  
avoid that, too?

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: Smooth Clamp Question
On Wed, 16 Nov 2016 23:07:44 -0600, "Tim Williams"

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In simulator-land ;-)

My TANH/COSH stuff is working great for merging to slopes, but I can't
find a "soft" equivalent for clamping a node that is current driven.

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I've solved the diode parameter variation... it's downright trivial...
measure the diode drop and subtract it...

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but "soft", when gain is low does exactly as you say... sloppy
precision.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Smooth Clamp Question
On Wed, 16 Nov 2016 23:07:44 -0600, "Tim Williams"

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Almost...

<
http://www.analog-innovations.com/SED/HardClampSoftCorner_2016-11-17_10-06-02.png


Finally sat down and did the math on paper rather than groping with
the simulator :-[

Still not quite right, slope at origin should be RG, so back to
revisiting the math ;-)
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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