Help me on Differential Amplifier

[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

 

[Mad I.D.]

differential amplifier

Could you show the plot? Thanks.

 

[tyanata]

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)

 

[LvW]

........................
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.

 

[millwood]

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?

 

[LvW]

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.

 

[cdbular]

Schematics and simulation screeshot would be useful

 

[ramesh441]

Thanks for your replies

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

 

[FvM]

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.

 

[tyanata]

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.

 

[ramesh441]

If I place AC source in between of inputs

The output is not we expected

 

[LvW]

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.

 

[tyanata]

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

 

[LvW]

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.

 

[FvM]

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.