Spice question - small signal gain.

A

alphacat

New Member

• Jul 1, 2009

• #1

Hello.

I've drawn an CS amplifier as can be seen in the scheme.
I'm not sure what should I fill in these two fields in order to receive the small signal DC gain of this amplifier.
I'd love to receive help regarding that,

Thank you very much.

 

Attachments

• small signal.JPG

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M

millwood

Banned

• Jul 1, 2009

• #2

the first box is the source of the dc "signal / disturbance". what it is depends on what you want to do. For example, this circuit is configured as a common source amplifier. so you want to know how a change in Vgs is going to produce a (bigger) change on the CCS. so the first box should be V1.

the 2nd box is essentially the output you desire from the circuit. it could be voltage on R1(V(r1)), or current on R1 (I(r1)).

 

A

alphacat

New Member

• Jul 1, 2009

• #3

Hey,

I tried this out, and wrote:
V1 in the upper box.
R1(I(r1)) in the lower box (I also tried writing I(R1), and R1(I(R1))).

I received this error:
*Analysis directives:

.TF R1(I(r1)) V_V1
----$
ERROR -- Must be independent source (I or V)
.PROBE V(alias(*)) I(alias(*)) W(alias(*)) D(alias(*)) NOISE(alias(*))
.INC "..\SCHEMATIC1.net"

Do you have an idea what is the problem?

Thanks.

 

M

millwood

Banned

• Jul 1, 2009

• #4

sure.

here is what I did in a sample circuit.

my signal source for this analysis is V13 and my output is voltage on the load R43.

 

Attachments

• small signal analysis.PNG

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M

millwood

Banned

• Jul 1, 2009

• #5

here is the analysis output:

since my output is going through a transformer, there is no DC content so the gain is 0.

 

Attachments

• small signal analysis 2.PNG

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A

alphacat

New Member

• Jul 1, 2009

• #6

Thank you very much
It worked out great.

I followed exactly what you have written there - calling the netlist 'Vout', placing a voltage prob on the netlist - otherwise it didnt work.

What would you write if you wanted the Output Variable to be the load (R1) current?

Here is what I wrote, and I received the same error message as before.

 

Attachments

• small signal2.JPG

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D

Dr.Ehsan

New Member

• Mar 30, 2013

• #7

wow, really helpful topic