Op-amp as voltage follower settling time

[Chinmaye]

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]

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

Could you please elaborate on this?

--- Updated Nov 28, 2020 ---

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]

Hi,

Show your circuit with resistor values.

Klaus

 

[Chinmaye]

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]

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]

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]

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

 

[KlausST]

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

 

[danadakk]

https://www.ti.com/lit/ds/symlink/ths4561.pdf?ts=1606573199612&ref_url=https%253A%252F%252Fwww.ti.com%252Famplifier-circuit%252Ffully-differential%252Foverview.html

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.

 

[Easy peasy]

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