[SOLVED] How to Plot transconductance Vs Vin(diffrential) in OTA

[shreza]

HI every body
I have Big problem. I Have an OTA with 20pF Capacitor on output. my ota is used in a gm-C filter. input of ota is differential ( applied into Bulk . (bulk Driven Technique)) and has a single output.
how can i plot gm/vid (in HSPICE) to see linearity of ota.
i have read much solutions about it , in this site. i did approximately all of them.
unfortunately none of them didn't Work to me.
Can you Explain that step by step in the accuracy Way?
absolutely it will be useful for all of the people since accuracy information about it is very very rare in the internet.
THANKS IN ADVANCE

 

[LvW]

shreza, the transconductane is a small-signal parameter - therefore, its variations cannot be simulated sinply by increasing the amplitudes in small-signal ac simulations.
However, I see two alternatives for revealing the non-linearities:

1.) Use different dc values at the input (different operational points) and superimpose (add) an ac signal source of 1V. As a result, the ac output current gives you the transconductance for the different operation points.
More than that, for PSpice (I don`t know the capabilities of other programs) you can command a so-calles "performance analysis" which allows you to display the simulation results (transconductance) over the various operating points (dc values at the input). For a linear system, this should result in a horizontal line (transconductance independent on dc biasing).

2.) You can perform a TRAN analysis - for example using a triangel waveform at the input for different amplitudes - and evaluate the form of the output cuirrent. This gives you an impression of the non-linearities of the device, but no direct information about the transconductance (and its constance).

 

[shreza]

Thank you So Much Dear LvW For Your Elaborate Reply .
I Did What you Said in Part1 on HSPICE.(Actually I'm Working on HSPICE) . I saw the linearity of Transconductance, but in (gm versus frequency). Usually in The papers Show That in (gm/Vin) as i Attached below.
I don't know , how can i get gm/vin while i have ac input . it is questioned a lot in this site. Solutions are different.
actually i want to plot something like attached figure.
Thanks Dear

 

[LvW]

Yes - you are right. I forgot to mention that you can/should perform an ac analysis for one single frequency only - but for different dc bias voltages (positive and negative).
A performance analysis (available in HSpice) will give you a display as you have shown.

 

[shreza]

unfortunately I Can't Get That figure with performance analysis on the HSPICE.
How can I Transform (gm vs frequency ) to (gm vs V(in+ - vin-) ) By Performance analysis??!
Thanks for your frequent help LvW
By the Way , i'm Using Cosmos Cosmos scop

 

[LvW]

As I have mentioned already, transconductance is a small-signal parameter and, thus, NOT dependent on the amplitude. The only alternative is to modify the bias point at the input (using a small dc voltage) and superimpose the ac signal.
As another alternative, you can perform a DC analysis - output dc current as a function of a variable dc input voltage.
This gives the transfer characteristic Iout=f(Vin). The slope of this curve is the transconductance.
Your simulation program certainly will be able to display the slope (differential quotient d(Iout)/d(Vin)) - as a function of the input voltage.
I suppose, that`s what you need.