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OpAmp as a follower, with a resistor from out to -in instead of a wire
OpAmp as a follower, with a resistor from out to -in instead of a wire
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Old 5th April 2005, 12:09 PM   #1
Bricolo is offline Bricolo  France
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Default 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?) ?
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Old 5th April 2005, 12:37 PM   #2
richie00boy is offline richie00boy  United Kingdom
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OpAmp as a follower, with a resistor from out to -in instead of a wire
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.
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Old 5th April 2005, 02:42 PM   #3
jcx is online now jcx  United States
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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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Old 5th April 2005, 02:49 PM   #4
Bricolo is offline Bricolo  France
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isn't adding a small cap there always a good idea?
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Old 7th April 2005, 02:30 PM   #5
00940 is online now 00940  Belgium
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if it's just for "practical" reasons, why not using a 0 ohms resistor ?
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Old 7th April 2005, 05:38 PM   #6
Bricolo is offline Bricolo  France
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the "practical reason" isn't routing the connection
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Old 7th April 2005, 06:52 PM   #7
Nicke is offline Nicke  Sweden
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Default Re: OpAmp as a follower, with a resistor from out to -in instead of a wire

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
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