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Op-amp as voltage follower settling time

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
 

barry

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

Easy peasy

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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.
 

Chinmaye

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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?
 

KlausST

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Hi,

Show your circuit with resistor values.

Klaus
 

Chinmaye

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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
 

KlausST

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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
 

FvM

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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.
 

Chinmaye

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

KlausST

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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
 

barry

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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.
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.
 

Easy peasy

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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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