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Op-amp as voltage follower settling time

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Chinmaye

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Hello,
I have designed a 2 stage differential input and differential output op-amp with gain close to 45dB and phase margin of 60 degrees. I am trying to connect it as voltage follower to check its response for a step input. I am connecting a large resistor between the (+)input and (-)output and vice versa. With VDD = 0.9V and VSS = -0.9V, and differential input 200mV, I am expecting the same differential output voltage. However, I get an amplified output voltage (Almost -0.8V to +0.8V). I checked that the op-amp is not getting saturated. What am I missing? I want to check what time the output takes to settle.
TIA
 

I think you need a series resistor on each of the the inputs.
 

what do you mean large resistor? between the (+) input and the (-) output ? for a unity gain follower the gain must be one - having an unknown but apparently large f/b resistor along with unknown input ( source ) resistances is clearly not a gain of 1, but larger ...
If you had posted an accurate circuit diagram sketch the answer would be obvious.
 

I think you need a series resistor on each of the the inputs.
Could you please elaborate on this?
--- Updated ---

what do you mean large resistor? between the (+) input and the (-) output ? for a unity gain follower the gain must be one - having an unknown but apparently large f/b resistor along with unknown input ( source ) resistances is clearly not a gain of 1, but larger ...
If you had posted an accurate circuit diagram sketch the answer would be obvious.
I cannot short the input to output directly in case of differential input and differential output amplifier right? Any other method to determine the settling time of opamp?
 

Hi,

Show your circuit with resistor values.

Klaus
 

1606554738955.png

Is This configuration correct to measure the settling time of op-amp. How does one measure the settling time of the opamp in such a confi
 

Hi,

Usually the gain of fully differential amplifiers is determined by the (two identical) input resistors Rg and the (two identical) feedback resistors Rf. A = Rf/Rg.
Without Rg (as in your schematic) gain can not be determined.

Klaus
 

    Chinmaye

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I don't see how to operate a fully differential amplifier as voltage follower, I suppose it's not feasible. Try an inverting G=1 configuration.
 

Thank you for the reply. How does one check for the settling time of such Op-Amp?
 

Hi,

Some options:
* I did an internet search. It seems there are a lot of documents around. Didn´t go deeply through them.
* read a differential OPAMP datasheet. Usually the settling time is specified. And usually they tell "how" they determine settling time.
* or straight forward: Step input. See how long the ouput takes to be stable (within xx% error)

Klaus
 

I don't see how to operate a fully differential amplifier as voltage follower, I suppose it's not feasible. Try an inverting G=1 configuration.
If you assume by ”voltage follower” the op means an amplifier with a gain of +1, this is most certainly achievable with a diff amp.
 
The OP needs to do more reading before posting - all the information is out there - posting the actual op-amp name would help too ... Klaus's post is quite straightforward & helpful too
 

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