Op-amp as voltage follower settling time

Chinmaye

Full Member level 2
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

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

Easy peasy

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

Full Member level 2
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

Super Moderator
Staff member
Hi,

Show your circuit with resistor values.

Klaus

Chinmaye

Full Member level 2

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

Super Moderator
Staff member
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

Super Moderator
Staff member
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

Full Member level 2
Thank you for the reply. How does one check for the settling time of such Op-Amp?

KlausST

Super Moderator
Staff member
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

Page 21 discusses step response testing.

Regards, Dana.

barry

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.