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Old 21st January 2008, 03:01 AM   #1
jethdub is offline jethdub  United Kingdom
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Default Opamp output resistor..

I posted this in another thread but my question was probably a bit below the general level of the discussion so I'm reposting here in hope of some reply.

I'm aware of the need to protect chips such as the TL072 from capacititive loading, but just how close to the output pin does the low value R need to be?
Often find myself laying out a circuit and getting nervous about moving the resistor millimetres away.
Also, if a blocking cap is to be used, is there a problem to place it after the load resistor rather than before, as this often allows closer placement of the resistor to the output? or is it more crucial that the cap itself is close to the output pin?
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Old 21st January 2008, 05:47 AM   #2
jcx is offline jcx  United States
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older (slower) general purpose unity gain compensated op amps usually aren't too sensitive to Cload

the TL072 datasheet has at least one plot with 100 pF load so that much is likely fine - several feet of pcb trace unless your'e doing something strange

the basic idea is that the resistive output impedance of the output stage forms a low pass filter with load C, when the R*C filter corner frequency approaches the op amp loop gain intercept frequency the excess phase shift destabilizes the amplifier' feedback

op amp open loop output resistance can be a guessing game 100-200 Ohms might be a conservative guess for the TL072

the load isolating R should be of similar magnitude to the op amp open loop output R if your'e operating near the destabilizing load C range

note tha higher closed loop gain moves the gain intercept down, meaning more load C can be tolerated than in unity gain
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Old 21st January 2008, 04:07 PM   #3
BWRX is offline BWRX  United States
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Default Re: Opamp output resistor..

Quote:
Originally posted by jethdub
I'm aware of the need to protect chips such as the TL072 from capacitive loading, but just how close to the output pin does the low value R need to be?
Ideally, you want to place the resistor as close to the output pin as possible.


Quote:
Originally posted by jethdub
Also, if a blocking cap is to be used, is there a problem to place it after the load resistor rather than before, as this often allows closer placement of the resistor to the output? or is it more crucial that the cap itself is close to the output pin?
Edit: You still want the output series resistor as close to the output pin as possible. The DC blocking capacitor can be anywhere between the output series resistor and the load.
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Old 21st January 2008, 04:46 PM   #4
jethdub is offline jethdub  United Kingdom
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Many thanks all.. that's a bit clearer now..
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