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From: Winfield Hill
Subject: Re: An interesting little prob.
Date: 30 Oct 2002 05:02:17 -0800
Organization: Rowland Institute
References: <firstname.lastname@example.org> <email@example.com>
X-Newsreader: Direct Read News 4.00
> Spehro Pefhany wrote:
>> Tony, something as simple as this would do?
>> | |
>> | | \ |
>> VDAC-------- | --|- \ |----x
>> 0-5V | | -----x----| |
>> x---|+ / | |----x
>> | | / | |
>> | | 0V
>> [499K] | | \
>> | x-----------------|+ \
>> 0V | --- high for
>> x-------------|- / CV mode
>> | | /
> Yes, I'm leaning towards something along those lines.
> A schottky diode in series with the drain-lead is probably a
> prudent safety measure in this app. I can put up with a
> non-zero Vmin for the sake of some protection.
>> (any required compensation is left for later consideration..)
I've considered such op-amp-controlled clamp circuits many times
before, and implemented several variants. One issue I often ponder
is clamping speed. Imagine the "load" on the current-souce suddenly
opens. If the clamp isn't very fast, there will be a substantial
voltage overshoot. In Spef's circuit, before the clamping action
is called for the opamp has swung far the other way, and therefore
in the open-load scenario, after the clamping-voltage is reached it
has to swing back before voltage-clamp action can occur. During the
slewing process the output is free to overshoot the desired clamp
voltage. :-( Compensation issues exacerbate this effect by forcing
slower slew rates, especially if a FET is used in common-source mode.
One of my solutions has been to use BJT emitter followers rather than
FETs, because only a very small opamp swing is necessary to handle
clamp action and high currents. A few extra parts can insure that
the opamp output voltage sits near where you need it. If the BJT's
base-voltage is prebiased by running the transistor at a modest
current, clamping action can be even faster.
I've modified Spef's drawing to show the idea, adding a crude 50mA
current source to bias the BJT. Tony's Schottky diode is included,
performing a critical role.
. _____ -> 50mA
. V+ --|__|__|-----,
. | | | |
. | | \ PNP V | |
. VDAC-------- | --|+ \ |/ | |
. 0-5V | | ------| | |
. x---|- / |\ | |
. | | / \ | |
. | | | |
. [49k9] 0V | | | \
. | | '--|+ \
. 0V | | --- high for
. '----------|- / CV mode
. | /
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