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# Help me on Differential Amplifier

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#### ramesh441

differential amplifier symmetry

Hello everyone,

I have designed a Differential amplifier have perfect symmetry. But When I plot gain,
I am getting a shift of 100mv on one side, making it assymetrical.

Can any say me the reason from where I am getting this 100mv offset

Thanking you,

Ramesh

differential amplifier

Could you show the plot? Thanks.

My be rason is that you have applied AC source only to one of input pins. I think the rigt way is to apply AC source between both pins and to observe gain as follows: (Voutp-Voutn)/(Vinp-Vinn)

tyanata said:
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I think the rigt way is to apply AC source between both pins and to observe gain as follows: (Voutp-Voutn)/(Vinp-Vinn)

And I think, that this is NOT the right way. In this case there is no ground reference.
The circuit could not work at all.

I have designed a Differential amplifier have perfect symmetry. But When I plot gain,
I am getting a shift of 100mv on one side, making it assymetrical.

so your amp with "perfect symmetry" is "assymetrical"?

can you find a more contradicting statement?

ramesh441 said:
Hello everyone,
I have designed a Differential amplifier have perfect symmetry. But When I plot gain,
I am getting a shift of 100mv on one side, making it assymetrical.
Can any say me the reason from where I am getting this 100mv offset
Thanking you,
Ramesh

I suppose by saying "perfect symmetry" you refer to the circuitry (optical/visual impression) .
But it can only operate symmetrical if you inject a symmetrical signal. Did you ?
Provide us with your "unsymmetrical" result - and somebody will tell you the reason.

Schematics and simulation screeshot would be useful

My ckt is perfect symmetry and I am giving the snapshot of ckt and gain reponse.

Plz correct if there are any wrongs.

W/L=5.4u/1.8u; Vm=250m;frf=50M

Thanking you
Ramesh

I wonder, if your tools are capable of producing diagrams with readable annotation?

You may want to repeat the analysis with an appropriate step width. I guess, you are only discussing artefacts.

As I said AC source must be between inputs with DC=0, I have forgotten to mention that common mode has to be provided by DC source between INN and GND.

If I place AC source in between of inputs

The output is not we expected

tyanata said:
As I said AC source must be between inputs with DC=0, I have forgotten to mention that common mode has to be provided by DC source between INN and GND.

I only can recommend, NOT to follow these procedure. It is completely unrealistic.
Nobody would put intentionally a common mode voltage at one input and to place a floating ac source (which does not exist in practice) between both terminals.

Use instead one or two input voltages at the differential input according to realistic conditions.

I dont think that AC source is floating it is completely determined as DC level by Kirchoff's low

tyanata said:
I dont think that AC source is floating it is completely determined as DC level by Kirchoff's low

I am afraid you are not completely aware of the meaning of "floating".
A floating source is a source with two terminals which have a potential which is determined not by the source circuitry itself but by the surrounding network.

It's correct to state, that the input AC source of a differential amplifier must not float. Actually, it doesn't in the circuit in question, which has been correctly designated as perfectly symmetrical.

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