Simple(?) potmeter + opamp puzzle

Well, this is a very neat solution, and it works like a charm! Thanks!

Richard Rasker

You are quite welcome. I like "puzzles" ;-)

But watch the currents. You'll probably want to "pad" the emitter end of the pot to prevent excessive currents. ...Jim Thompson

Summing junction.

Sure. You are asking for a wide-range variable gain, after all. This one has an exponential gain characteristic, though, i.e. the gain in dB per degree of shaft rotation is constant, which is what I thought you were after.

Cheers

Phil Hobbs

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Exactly

... better to change the value of the fixed Rs.

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With the right op-amp you can get the output with a single supply to be

y=3Dmx+c

where y is the output voltage, x the input voltage andm and c can be pos or neg constants.

Hardy

For "non-inverting", m=>1, failing the OP's stated requirements. ...Jim Thompson

WHOO! That's brilliant! It seems a little sad that it's just used to avoid using a negative supply, though. Gain (assuming some range-limiting fixed resistors are added) is of the form Av = (1-X)/X where the 'dB' knob solution would be Av = a + exp(b*X) where X is the fractional setting of the pot (in the [0, 1] range).

Both forms can hit the three gain targets at equispaced settings on the knob.

Probably THAT2181 as an exponential-gain element and something QUITE other than a digital potentiometer would suit as an inelegant but flexible solution. I kinda .. hate... digital potentiometers.

Back from holidays, so I hope it's not too late...

Like that one?

|\ -----------|+\ | >----------+-------- .---|-/ | | |/ | | | | R1 /| | | ___ /+|-' +----|___|-+-< | | | \-|--+---. .-. | \| | .-. | | | ___ | | | R2| | '-|___|-' | |R2 '-' R1 '-' | ___ | '--------|___|---------' A | GND

I've not worked out the resistors values but you should be able to do that :-)

And mind the stability issues too.

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Dunno if someone already suggested this almost trivial circuit or not- gain is linear adjust with pot rotation- closed loop operation throughout- wide range- and noninverting single supply operation all the way:

Please view in a fixed-width font such as Courier.

. . . . |\ . Vi >------|+\ . | >---. . ---|-/ | . | |/ | . | _ | . | /| | . +----[Rv]--' . | / . | Vi . |- -- x Rv + . + | Rs . | . Vi [Rs] . | . - | . --- . . . . Vi produces linear wide range voltage across Rv . . Ground reference this voltage with diff amp: . . . . . . Vi . Vi >-----+----------[R]--+-----[R]--+--> -- x Rv . | | | Rs . | |\ | | . '---|+\ | |\ | . | >-. '--|-\ | . ---|-/ | | >----' . | |/ | .--|+/ . | _ | | |/ . | /| | | . +--[Rv]--+-[R]--+ . | / | . | | . [Rs] [R] . | | . | | . --- --- . . . .

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If you want exponential adjust then interchange Rs and Rv...

Ah, stuffing a unity-and-higher gain signal in a differential amp with the input also at the negative input -- another interesting solution that's actually so simple it's strange that no-one ( including me) came up with it before. This is definitely one for the records! Thanks!

Richard Rasker

... And we have another prize winner! Sheesh, so there's me, shoving potmeters around in all the worn-out familiar patterns in opamp circuits, but never a moment thinking outside the box (or into double loops, like this one ;-) -- but here people have come up with several very nice and (at least to me) novel solutions within mere days. I really love this group :-) Thanks!

Indeed that could get a bit dicey here. I'm sure to try and build it in the worst possible configuration (breadboard without ground plane) before committing it to PCB. Then again, small negative feedback caps solve most of these problems (except in the case of Jim Thompson's current mirror solution, which behaves a bit, um, "differently" in this respect).

Cheers,

Richard Rasker

See! I told you that Bloggs was smarter than Larkin ;-) ...Jim Thompson