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**posted on**

- Jim Thompson

November 16, 2016, 5:29 pm

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

Any ideas?

Thanks!

...Jim Thompson

*<http://www.analog-innovations.com/SED/SmoothClampQuestion.pdf>*Any ideas?

Thanks!

...Jim Thompson

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| James E.Thompson | mens |

| Analog Innovations | et |

| James E.Thompson | mens |

| Analog Innovations | et |

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Re: Smooth Clamp Question

On Wed, 16 Nov 2016 09:44:27 -0800 (PST), George Herold

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 :-(

...Jim Thompson

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 :-(

...Jim Thompson

--

| James E.Thompson | mens |

| Analog Innovations | et |

| James E.Thompson | mens |

| Analog Innovations | et |

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Re: Smooth Clamp Question

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

Tim Wescott

Control systems, embedded software and circuit design

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Re: Smooth Clamp Question

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:

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

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

Dr Philip C D Hobbs

Principal Consultant

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Re: Smooth Clamp Question

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

--

Dr Philip C D Hobbs

Principal Consultant

Dr Philip C D Hobbs

Principal Consultant

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Re: Smooth Clamp Question

On 11/16/2016 01:28 PM, George Herold wrote:

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

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

Dr Philip C D Hobbs

Principal Consultant

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Re: Smooth Clamp Question

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

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

I'm pretending to be a leftist... you're supposed to guess the outcome

...Jim Thompson

I'm pretending to be a leftist... you're supposed to guess the outcome

...Jim Thompson

--

| James E.Thompson | mens |

| Analog Innovations | et |

| James E.Thompson | mens |

| Analog Innovations | et |

We've slightly trimmed the long signature. Click to see the full one.

Re: Smooth Clamp Question

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

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...

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

derivatives.

...Jim Thompson

--

| James E.Thompson | mens |

| Analog Innovations | et |

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

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

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"

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.

I've solved the diode parameter variation... it's downright trivial...

measure the diode drop and subtract it...

but "soft", when gain is low does exactly as you say... sloppy

precision.

...Jim Thompson

--

| James E.Thompson | mens |

| Analog Innovations | et |

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.

I've solved the diode parameter variation... it's downright trivial...

measure the diode drop and subtract it...

but "soft", when gain is low does exactly as you say... sloppy

precision.

...Jim Thompson

| James E.Thompson | mens |

| Analog Innovations | et |

We've slightly trimmed the long signature. Click to see the full one.

Re: Smooth Clamp Question

On Wed, 16 Nov 2016 23:07:44 -0600, "Tim Williams"

Almost...

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 |

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 |

We've slightly trimmed the long signature. Click to see the full one.

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