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From: brianr@tekig5.PEN.TEK.COM (Brian E Rhodefer)
Newsgroups: sci.electronics
Subject: Re: Good Clamping Circuit
Message-ID: <1542@tekig7.MAP.TEK.COM>
Date: 11 Mar 91 02:20:26 GMT
References: <1991Mar06.181426.10500@cdsmn.mn.org> <1041@eplunix.UUCP> <1991Mar10.093752.18289@neon.Stanford.EDU>
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Reply-To: brianr@tekig5.PEN.TEK.COM (Brian E Rhodefer)
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In article <1991Mar10.093752.18289@neon.Stanford.EDU> zimmer@calvin.stanford.edu (Andrew Zimmerman) writes:

>I don't see how the above circuit solves the original posters question.

  (referring to Nico Garcia's posted circuit, not shown here)

>I believe that Vout = (V1+V2)/2.  (No unity gain)
>I don't see how it clamps, unless it makes use of the upper rail of the
>op-amp supply.  
>
>How about the following circuit instead?
>
>     --------R1----------R1--------
>     |            |  |\           |       a
>    ---           ---|  \         |     --|>|-- +5
>     -               |    \       |     |
>                     |      \_____|_____|_____  Vout
>V1------R1----|      |      /           |
>              |------|    /             |  /  z
>V2------R1----|      |  /               |--|<|---
>              |      |/                    /    |
>             "C"                               ---
>                                                -
>

Two problems here:

1) Trying to control the output of an opamp by shorting it to a voltage
   source (even through a diode) is a bit brutal.

2) The original poster wanted fairly high clamping accuracy:  9mV out of 5V.
   Variations in clamping current and operating temperature will prevent
   this circuit from meeting that goal.

I suggest that the following circuit be used to clamp the voltage
at the noninverting input of Andrew's circuit:


                   D1
         r------------|>|---------,
         |                        |
To Point |               D2       |
 "C" <---*---R2---*---------|<|---*
                  |  |\           |
                  ---|- \         |
                     |    \       |
                     |      \_____|
  +5V --R2----,      |      /
              *------|+   /
     r--R2----'      |  /
     |               |/
    ---
     -

R2 should be >> R1.  To compensate for the small current through D2
(which keeps this second opamp balanced even while it isn't clamping),
a resistor of 2*R2 should be connected from +5V to the inverting input
of the summing opamp.  The clamping opamp should be chosen or trimmed
to have an input offset voltage less than the allowable clamping error
of nine millivolts.

I tried sending this through email, but it probably didn't work, and
I hadn't gotten the unity gain topology right, anyway.

Brian Rhodefer