blocking cap value question

Hey all:

I have a TL071 on a single supply in a noninverting amp configuration, and I'm trying to figure out some values for blocking caps. The voltage divider is 2 22K resistors, and this is connected via a 10Meg resistor to the + input. So for that cap there, do I use 10Meg as the value of R for

1/2*pi*f*R, or would I use 22K? My output is connected to the next stage via a 100ohm resistor. Do I use that as the R value in the equation, or would I use 47K (which is the input Z of the next stage)? My Ri is 3.9K, so from this I figured I needed about 10-15uF, but i'm not sure about the other 2.

Thanks

Reply to
tempus fugit
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I don't fully understand your post. Are you floating the + input of the opamp at half the supply voltage with a voltage divider of two 22k resistors? If so, the load on the capacitor is 11k ohms. Generally, one uses a capacitive reactance of one-tenth the load (so you want Xc = 1 k, or less, at the lowest frequency of operation).

I am confused about the 10 M resistor.

Reply to
Charles Schuler

If the OP can access a.b.s.e. perhaps [s]he can afford us a schematic; barring that, an ascii schematic would work.

That way we'd have something more solid to work with.

Cheers

PeteS

Reply to
PeteS

Your post is a bit confusing, what low frequency response do you need, and what are you ampilifing? Blocking caps will define the -3dB points, if you have a high impedance input, say 10M, then the cap can be quite low, but without knowing the lowest frequency, it is impossible to calculate

martin

Reply to
martin griffith

and

stage

would

from

  1. > >

one

or

Good plan Pete. I now realize that my origianl post is a bit confusing. Check it out on ABSE.

Thanks

Reply to
tempus fugit

use something in the neighborhood of 0.1 * 22k = Xr. etc... for that application, it's close enough.

--
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Reply to
Jamie

"tempus fugit"

** Get rid of the damn, stupid 500K pot and you dont need one at the input.
** The load seen by a cap 47 K ohms.

Use a 0.22 uF film cap.

........ Phil

Reply to
Phil Allison

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