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From: Fred Bloggs
User-Agent: Mozilla/5.0 (Windows; U; Win 9x 4.90; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0
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Subject: Re: An interesting little prob.
References: <email@example.com> <firstname.lastname@example.org>
Date: Wed, 30 Oct 2002 14:22:33 GMT
NNTP-Posting-Date: Wed, 30 Oct 2002 06:22:33 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Winfield Hill wrote:
> Tony wrote...
>>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,
LOL- very kind, the difference between night and day more than a
> adding a crude 50mA
> current source to bias the BJT. Tony's Schottky diode is included,
> performing a critical role.
> . _____ -> 50mA
> . V+ --|__|__|-----,
> . |
> . ,-----[150k]----x---x--|<|--x--->
> . | | | |
> . | | \ PNP V | |
> . VDAC-------- | --|+ \ |/ | |
> . 0-5V | | ------| | |
> . x---|- / |\ | |
> . | | / \ | |
> . | | | |
> . [49k9] 0V | | | \
> . | | '--|+ \
> . 0V | | --- high for
> . '----------|- / CV mode
> . | /
Right- you always make things look like a stand-alone app when it's an
ordinary emitter follower with precision opamp DC regulation. The
programmable current source over-voltage will drive the circuit as
common-base so that the impedance remains low at frequencies way beyond
anything the opamp can handle. I was wondering if it is worth hanging a
"broadbanding" resistor off the base and possibly allowing for
proportioning the "crude" current-source pre-bias to the programmable
current-or maybe just a BFC at the emitter to limit worst-case dV/dt.
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