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Old 31st July 2002, 07:28 AM   #1
mirlo is offline mirlo  United States
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Default dc offset correction pot

The following circuit gets used a lot in amp input stages.

I don't like it. It may well adjust the DC offset, but it has the nasty side effect of simultaneously unbalancing the gain between the two sides of the diffpair. It also might wreak havoc if the wiper gets lifted from the resistive element.

On the other hand, it is simple.

It may work better with JFETs. Am I right to assume that DC offset in JFET amplifiers might result from differences in their physical characteristics such that the side with higher Beta or K factor or whatever you call the transconductance parameter, also have lower Vgs, so that when you adjust the pot to equalize the inputs again you actually improve the small signal balance as well?

What are the most elegant (fewest components, least effect on signal, etc) alternatives?
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Old 31st July 2002, 09:43 AM   #2
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Default DC trim pot

I don't know about FET's, but with BJT's it's quite simple to make a circuit that does not need any DC adjustment (That's if there's global feedback)

I have seen some Designs that uses a pot instead of the resistor in the Collector of the transistor that feeds the VAS stage.

It's also an option to have a POT to control the current throug one half of the inputstage (when there are two diff amps)

thats what i have seen, but since i believe in feedback, I have not used a DC trimmer in the design that i work on at the moment.

An Other design that don't use global feedback can be found at

http://www.lcaudio.dk/millennium.PDF (a really nice amp !!)

\Jens
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Old 31st July 2002, 07:33 PM   #3
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Actually it does not unbalanced the gain pair - they
see the same impedance regardless of setting.

However, I never use it because you can figure that
someday the wiper will get dirty and open up, and then
things will get interesting.

We always want to design for the day when the wiper
opens, and that is why on Vbe multipliers for bias
circuits we put the pot on B-E and set it up so that when
the wiper opens, the bias goes down.
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Old 31st July 2002, 08:15 PM   #4
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Default Pots

Just for the record: even multiturn pots will eventually get dirty?

I thought their being supposedly air-tight would prevent that. How much am I wrong?


Carlos
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Old 31st July 2002, 10:49 PM   #5
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They won't last forever. They might outlast the rest
of the circuit....

Most of the production applications of the above circuit
I have seen will put some resistors from either end of the pot to the wiper so that the diff pair will always see something.
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Old 31st July 2002, 11:22 PM   #6
mirlo is offline mirlo  United States
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Lightbulb same impedance !

Thank you Nelson Pass for pointing out the "obvious", right under my nose.

Something about my weird visual way of looking at circuits makes me look at that drawing and see the pot's wiper as a determined voltage, even though it is connected to a high-Z current source!

Sometimes I need people to make me see the things right in front of me!
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