OpAmp as a follower, with a resistor from out to -in instead of a wire

Bricolo · 5th April 2005, 12:09 PM

Hi,

I'm working on a project, and I need an opamp used as a follower, but for practical reasons, instead of having a wire from output to -in, there will be a resistor (but no resistor from -in to ground)

Is this worse than a direct connection? Can this cause problems? Is there an optimal value (impedance matching between -in and +in?) ?

richie00boy · 5th April 2005, 12:37 PM

In a critical application to minimise DC offset due to input bias currents it's actually good practice to put a resistor there that is equal to the impedance seen by the non-inverting input.

jcx · 5th April 2005, 02:42 PM

feedback R in unity gain can be destablizing if the R value is too high and adds phase shift to the feedback path due to op amp input Cin * Rfdbk

depends on the op amp, a few hundred Ohms is usually ok, much higher R and you may need to add a small C in parallel with the R

Bricolo · 5th April 2005, 02:49 PM

isn't adding a small cap there always a good idea?

00940 · 7th April 2005, 02:30 PM

if it's just for "practical" reasons, why not using a 0 ohms resistor ?

Bricolo · 7th April 2005, 05:38 PM

the "practical reason" isn't routing the connection

Nicke · 7th April 2005, 06:52 PM

Quote:

Originally posted by Bricolo
Hi,

I'm working on a project, and I need an opamp used as a follower, but for practical reasons, instead of having a wire from output to -in, there will be a resistor (but no resistor from -in to ground)

Is there an optimal value (impedance matching between -in and +in?) ?

Check the data sheet for AD797,that opamp needs a 100ohms resistor in the feedback when used as a follower

Quote:

Originally posted by Bricolo
isn't adding a small cap there always a good idea?

Not with an current feedback opamp

5th April 2005, 12:09 PM	#1
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	OpAmp as a follower, with a resistor from out to -in instead of a wire Hi, I'm working on a project, and I need an opamp used as a follower, but for practical reasons, instead of having a wire from output to -in, there will be a resistor (but no resistor from -in to ground) Is this worse than a direct connection? Can this cause problems? Is there an optimal value (impedance matching between -in and +in?) ? __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

5th April 2005, 12:37 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	In a critical application to minimise DC offset due to input bias currents it's actually good practice to put a resistor there that is equal to the impedance seen by the non-inverting input. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

5th April 2005, 02:49 PM	#4
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	isn't adding a small cap there always a good idea? __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease

7th April 2005, 02:30 PM	#5
00940 diyAudio Member Join Date: Sep 2003 Location: Paris	if it's just for "practical" reasons, why not using a 0 ohms resistor ? __________________ Ben.

7th April 2005, 05:38 PM	#6
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	the "practical reason" isn't routing the connection __________________ Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease


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5th April 2005, 02:42 PM	#3
jcx diyAudio Member Join Date: Feb 2003 Location: ..	feedback R in unity gain can be destablizing if the R value is too high and adds phase shift to the feedback path due to op amp input Cin * Rfdbk depends on the op amp, a few hundred Ohms is usually ok, much higher R and you may need to add a small C in parallel with the R

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