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gain vs. input common-mode voltage

 
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SwordFish



Joined: 27 Apr 2002
Posts: 102
Helped: 3
Location: Europe
Post22 Jul 2004 14:35   gain vs. input common-mode voltage
Hi,

My design is a opamp.
I need to know if the gain (differential gain) depend upon input common-mode voltage. Something like "Av vs Vcm"

Is there a way to make such a plot ?

For simulation I use hspice. Maybe you could give a "xxx.sp" file .

Thanks
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batdin



Joined: 03 Jun 2004
Posts: 181
Helped: 3
Location: Bulgaria
Post22 Jul 2004 16:32   Re: gain vs. input common-mode voltage
If the emitter current source is OK,the gain should depend very little on common mode voltage.
You can find out the dependence by using three voltage sources.
V0 1 0 DC 0V (Vcommon mode)
V1 2 1 AC 0.1V (V diff/2)
V2 1 3 AC 0.1V (V diff/2)

nodes 2,3 are differential amp inputs.

By sweeping V0 you can see how this affects the AC gain.
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SwordFish



Joined: 27 Apr 2002
Posts: 102
Helped: 3
Location: Europe
Post22 Jul 2004 16:56   Re: gain vs. input common-mode voltage
Yes, but the gain is available on AC analisys and common-mode voltage is on DC analisys. Also AC use a freq on X axis and DC use Volts as X axis.

Thanks.
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Humungus



Joined: 10 Jul 2001
Posts: 417
Helped: 25
Post23 Jul 2004 8:21   Re: gain vs. input common-mode voltage
In HSPICE make and an AC analysis using the common mode voltage as sweeping parameter (go to the AC analysis section in the HSPICE manual) You can make AC analyses while changing DC bias. You'll get a family of AC results with the common mode voltage as parameter.
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batdin



Joined: 03 Jun 2004
Posts: 181
Helped: 3
Location: Bulgaria
Post23 Jul 2004 8:40   Re: gain vs. input common-mode voltage
Try with parameter sweep
First you must define V0 value as a parameter :

.PARAM V0_value - 0
V0 1 0 {V0_value}
.STEP PARAM V0_value -5 5 .2

P.S. It should work with pspice,I don't know if hspice is equivalent.
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SwordFish



Joined: 27 Apr 2002
Posts: 102
Helped: 3
Location: Europe
Post23 Jul 2004 14:35   Re: gain vs. input common-mode voltage
What I am looking for is something like the figure 5 (page '8') from this pdf file.

Sorry, but you need login in to view this attachment
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lmlee



Joined: 25 Jul 2004
Posts: 27
Helped: 1
Post25 Jul 2004 4:16   Re: gain vs. input common-mode voltage
common mode voltage is to make sure the circuit can operate well.
in that range , the gain does not change very much
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sunking



Joined: 25 May 2004
Posts: 914
Helped: 46
Post25 Jul 2004 6:49   Re: gain vs. input common-mode voltage
yes. the gain depend on the circuit you design.
if the circuit of input stage is not in triode, the av does not change very much
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