blocking cap value question

[tempus fugit]

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

 

[Charles Schuler]

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.

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.

 

[PeteS]

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.

If the OP can access a.b.s.e. perhaps 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

 

[martin griffith]

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

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

 

[tempus fugit]

2.

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.

If the OP can access a.b.s.e. perhaps 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

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

Thanks

 

[Jamie]

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

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

 

[Phil Allison]

"tempus fugit"

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.

** Get rid of the damn, stupid 500K pot and you dont need one at the
input.

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)?

** The load seen by a cap 47 K ohms.

Use a 0.22 uF film cap.




......... Phil