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m2babaey
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Join Date: Apr 2007
Posts: 24

different differential gains

01/01/2008 10:16 AM

Hi

My simulation results in a differential gain of about 1.2k when doing a DC sweep on Vd where as AC simulation results in a gain of about 13k.

I include the waveform plots here and the circuit ; can you suggest any reason for this problem?

This is the result of DC sweep

And this is the result of AC sweep:

the circuit:

Thank you in advance for your help
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Electroman
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#1

Re: different differential gains

01/01/2008 3:11 PM

Well its easy enough to see the dual differential pair followed by a couple of buffers, but what those thingies are at rhe left top middle bit where the voltage inputs are is a mystery to me.

As for the x10 gain difference between dc and ac gain that could be due to the CAD software you are using... I've never got on with any decent analogue software packages...

I've tried quite a few of them but they just aren't up to some of the simple tasks I require... So maybe this is another software curse...

There again I can't see clearly the values of the components and I don't know what model you used for the software results... so I really couldn't comment in any detail except to say that from experience, I don't trust software CAD packages...

John.

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Electroman
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#2

Re: different differential gains

01/01/2008 3:14 PM

Ohhh hold on, are they capacitors decoupling the supply rails or are they voltage sources acting as the supply?

That could be why the ac and dc gains are so different?

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ronald
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#3

Re: different differential gains

01/02/2008 7:52 AM

Hi friend, It's been many years since I've dealt with this type of stuff; but could it be that your circuit is acting as a filter for removing AC component thru attenuation or as the above contributor said via the Decoupling Capacitors? Also I cannot see clearly your ckt because I don't have good software to see these diagrams.

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m2babaey
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#4

Re: different differential gains

01/02/2008 11:38 AM

Thanks for your replies
the problem solved by removing E sources (those things are at the left top middle bit ) and connecting Vd directly to the inputs
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U V
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#5

Re: different differential gains

01/03/2008 12:00 AM

Not to sure about the answer, but the poster seems happy.

I was more Impressed that the question was much better than most.

But what is an "E sources"? ( I didn't recognize it )

Thank you

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ronald
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#6
In reply to #5

Re: different differential gains

01/03/2008 12:22 AM

The poster's diagram has two (2) sources a VDD and an AC or E source. I think what he meant is that he obviated the E (AC) source and only used the Vdd source...?

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U V
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#7
In reply to #6

Re: different differential gains

01/03/2008 12:43 AM

Thanks

Brad

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m2babaey
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#8

Re: different differential gains

01/03/2008 9:50 AM

Hi

I meant this by E source:


It has 2 inputs from Vd ( differential voltage) and VCM ( common mode voltage) and a gain that i set to be 0.5
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Electroman
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#9
In reply to #8

Re: different differential gains

01/03/2008 9:55 AM

Ahh yes that would cause a problem...

Also don't forget your circuit doesn't have any capacitors for bypass at ac voltages?

I know some analogue CAD packages will take that to mean the gain will be different at ac and at dc...

John.

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ronald
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Technical Fields - Technical Writing - Validation Specialist-Medical Devices United States - Member - From the Big Apple! Hobbies - Model Rocketry - Rockets should go nuclear! Fans of Old Computers - PDP 11 - Studied computers at W.H.Taft H.S., Bronx, NY. Popular Science - Cosmology - Radio Science Observing

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#10
In reply to #9

Re: different differential gains

01/03/2008 10:25 AM

Now I can see! I totally agree....

Ron

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Electroman (3); m2babaey (2); ronald (3); U V (2)

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