OP AMPS in ac circuits ?

[mykaitch]

Okay, I've had enough of Google. Suppose I have a single supply op amp with a pot div biasing the no inv input.
Now the usual R from o/p to inv input.
The 'other' resistor to ov via a largish cap. What does this achieve ?
I thought the idea was to remove a dc at the o/p but now I think the o/p will be same as biasV....


...but then the non inve input has 0v so the op will be zero as the opamp tries to make the pos input the same ????

Yeah I should know. But I don't.

Thanks

[DOOLEY]

hello myKaitch,

right oh,
here goes,
seen as no-one else has replied...........
LOL.

as I understand it,
the setup you have in mind is pictured below :



search online for " op-amp basic rules " OR " how op-amps work "

you will find a list of basic rules, think there 5 of e'm...........


one of the rules is :
an op-amp will do what ever it can by using the output to make the -input the same as the +input.

ie: view it as the +input is the dominant input.

in your circuit,
gain is determined by R1 & R2,
and the low pass cut off frequency is determined by R1 & C1.

sooooooo,
basically,
when you alter the input pot,
the output increases inline with the gain setup by R1 & R2,
BUT,
C1 quickly catches up,
and once both -input and +input are once again the same the output RETURNS TO WHATEVER VOLTAGE THE +INPUT IS.

so,
if +input is 4v, then output, in this configuration will be 4v ( once it's "settled" )

hopes ive explained this ok for you ??
(if not, many "guru's" will be on my tail.............)
LOL.

OH,
one final note,
some op-amps will go into phase inversion if input drops below a certain voltage,
( where the output immediately goes high, rail out )

IE: a TL072 inverts below about 1.7v input,
TL062 inverts at about 0.7v,

in this configuration best to use an op-amp that "senses to ground"
IE: LM358 and many others...................

phew,
hopes that helps,
that's my good deed done for the day then...............

[Carl-NC]

If the inv side has a series R-C to ground, then the DC gain is 1 and the output voltage will equal the non-inv bias voltage.

Edit: Dooley posted as I was typing, his pic is what I assumed. For DC bias, R1 & C1 do nothing.

[rickb]

https://courses.cit.cornell.edu/bion...ngleSupply.pdf

https://www.ieee.li/pdf/essay/single...amp_design.pdf

https://www.google.com/search?num=50...84.mV6S7Kocsgw

Biasing the input will offset the output, no way around it. Use caps to ac couple to the next stage.

The first 4-5 pages of the first pdf will make things clear.

Rick

EDIT: So we all jumped on this at the same time. No such thing as to much information

[mykaitch]

Thanks y'all. I hardly ever use this config. Can see a use in a +- supply. It does what I figured in the circuit I nabbed. In the circuit I have there are three DC coupled stages--it is a compressor.