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Op-Amp output Resistor

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ballimo

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Hello,

Can anyone explain why a 100 Ohm resistor is added at the Op-amp output of the attached circuit?

This circuit take a variable input voltage between 0 and 3V (DAC) and output a higher voltage range to control a module.

Thanks,
Ballimo
 

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Are sure about the connection of the 10pF capacitor?
Normally, such an additional feedback capacitor is connected directly at the opamp output.
In this case, the additional resistor (RT=100 ohms) can stabilize the amplifier circuit against heavy capacitive load impedances CL.
The principle is as follows: The resistor RT=100 ohms forms - together with a load capacitance CL (not shown in the circuit) - a passive lowpass. The influence of the corresponding pole frequency can be (partly) compensated by a zero created by a highpass capacitor CF (10pF in your case). Both RC combinations together can be seen as a frequency-compensated voltage divider (known from the oscilloscope probe).
For this purpose, it is necessary to make k*CL(r,out+RT)=CF(r,out+R2) with r,out=opamp open-loop output resistance and k~1.3
 

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