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[Moved]: How to calculate the overall transconductance of an OTA circuit in cadence

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simplsoft

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Can anyone help me in plotting the overall transconductance of an OTA (Gm) circuit in cadence step by step.

Do I have to do the ac analsis or DC analysis?
 

Re: How to calculate the overall transconductance of an OTA circuit in cadence

simplsoft said:
Can anyone help me in plotting the overall transconductance of an OTA (Gm) circuit in cadence step by step.
Do I have to do the ac analsis or DC analysis?
Click to expand...

The transconductance gm is a small-signal parameter. Hence, we need a small-signal analysis - that means: AC analysis.
However, gm depens on the bias current (Iabc) and - of course - also on frequency.
Therefore, for answering your question, we need to know what you want: gm=f(f) or gm=f(Iabc)?
 

Re: How to calculate the overall transconductance of an OTA circuit in cadence

LvW said:
The transconductance gm is a small-signal parameter. Hence, we need a small-signal analysis - that means: AC analysis.
However, gm depens on the bias current (Iabc) and - of course - also on frequency.
Therefore, for answering your question, we need to know what you want: gm=f(f) or gm=f(Iabc)?
Click to expand...

I want to calculate the transconductance vs vid to check the linearity.I think I need gm f(Iabc). I want to calculate gm=Iout/(V+-V-) i.e. gm=Iout/Vid vs Vid.

I hope you can understand for your convinience I am attaching a graph. I want to plot this graph.

transconductance vs vid (2).jpg
 

Re: How to calculate the overall transconductance of an OTA circuit in cadence

simplsoft said:
I want to calculate the transconductance vs vid to check the linearity.I think I need gm f(Iabc). I want to calculate gm=Iout/(V+-V-) i.e. gm=Iout/Vid vs Vid.
I hope you can understand for your convinience I am attaching a graph. I want to plot this graph.
View attachment 134679
Click to expand...

Linearity can best be checked by plotting the output current Iout as a function of the differential input voltage Vd for a CONSTANT biasing with Iabc.
From this plot, the transconductance gm=d(Iout)/d(Vd) can be derived as the slope of the function.
 

Re: How to calculate the overall transconductance of an OTA circuit in cadence

How can I plot this graph gm vs vid in cadence.Please can you tell me step by step process of this plot.
 

Re: How to calculate the overall transconductance of an OTA circuit in cadence

simplsoft said:
How can I plot this graph gm vs vid in cadence.Please can you tell me step by step process of this plot.
Click to expand...

For my opinion, I have given you a corresponding advice in my former post:
1.) Perform a DC analysis and plot Iout=f(Vid) ,

2.) Create a second plot gm=d(Iout)/d(Vid).
I think, each simulator can plot the differential (the slope) for an existing plot.
 

Re: How to calculate the overall transconductance of an OTA circuit in cadence

I don't get why, if you're already simulating, you
feel a need to calculate by hand any attribute that
is simulation-results-based.
 

Re: How to calculate the overall transconductance of an OTA circuit in cadence

Perform DC analysis and take diffrentiation of that plot.This will give you transconductance which is constat over some differential volatage.This range will give you linearity and corresponding transconductance.

Am I right?Someone can review this.
 

Re: How to calculate the overall transconductance of an OTA circuit in cadence

Thanx for your reply. I want to know what dc value to I have to place at the output (vdc)? Can it be any value?
 

What is the purpose of the DC source at the output?
It has - most probably - the task to measure the output current. Some simulation programs require such a source for measuring currents.
Hence, it must be, of course, a zero Volt source (VDC=0)
 

Re: How to calculate the overall transconductance of an OTA circuit in cadence

When vid is an ac signal why do we have to do dc sweep? why not ac sweep?
 

Re: How to calculate the overall transconductance of an OTA circuit in cadence

simplsoft said:
Thanx for your reply. I want to know what dc value to I have to place at the output (vdc)? Can it be any value?
Click to expand...

what is your output dc level ?

You have to set the same dc voltage.
 

Re: How to calculate the overall transconductance of an OTA circuit in cadence

why do we have to do dc sweep? why not ac sweep vid is ac.
 

LvW said:
What is the purpose of the DC source at the output?
It has - most probably - the task to measure the output current. Some simulation programs require such a source for measuring currents.
Hence, it must be, of course, a zero Volt source (VDC=0)
Click to expand...

No. It wont be zero volt source. In order to have same conditions you have to set it to your output dc level.

- - - Updated - - -

simplsoft said:
why do we have to do dc sweep? why not ac sweep vid is ac.
Click to expand...

How can you find gm with ac sweep when your vid is constant and your frequency is changing ???

gm = delta (Id) / delta (vid)

think about it.
 

I want to plot this curve attached.

I am using cadence. I am a bit confused. vid is a ac signal. Vd is the dc bias. You are saying sweeP vid. When vid is an ac signal it means we have to do ac sweep. Am I right?



Ata_sa16 said:
No. It wont be zero volt source. In order to have same conditions you have to set it to your output dc level.

- - - Updated - - -



How can you find gm with ac sweep when your vid is constant and your frequency is changing ???

gm = delta (Id) / delta (vid)

think about it.
Click to expand...
 

Attachments

  • Transconductance vs Vid.jpg
    Transconductance vs Vid.jpg
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Last edited:

Vdc=0 would be suitable for an OTA with bipolar power supply. Nothing has be said yet about the test setup details.

Technically, gm versus vid can be measured either in AC analysis with parametric vid sweep or by differentiating Iout = f(vid) obtained in a DC sweep. Considering existing tool features, the latter is probably easier to achieve.

vid is a ac signal. Vd is the dc bias.
Click to expand...
Don't see this difference between vid and vd anywhere in your previous posts. LvW used vd instead vid in one post, without a particular intention as far as I see.

Simply consider that AC and DC components are superimposed when generating the OTA input voltage.
 

simplsoft said:
I want to plot this curve attached.

I am using cadence. I am a bit confused. vid is a ac signal. Vd is the dc bias. You are saying sweeP vid. When vid is an ac signal it means we have to do ac sweep. Am I right?
Click to expand...

If you want to obtain that curve. you have to do dc sweep as I explained on that figure not ac.

Vid is ac signal I know ofcourse.

Ac signal means It has frequency and it swing. So we are doing the same thing we give a sweep and change vid between -1.2 and 1.2 but at low frequency, so we are measuring the DC gm of OTA. :)
 

I got your point but I am not able to do this in cadence. Can anyone help?

- - - Updated - - -

Ok. You mean. I give a fix ac signal mine is 20mV. I gave Dc bias voltage 0.2 V. Instead of 0.2 I have to use -1.2 to 1.2 sweep value and do dc sweep? Am I right now
 

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