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Simulators >> Circuit Simulators >> How to find Early Voltage (VA) of nmos/pmos transistor..?
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Message started by vinaydreddys on Nov 15^th, 2011, 6:45am

Title: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by vinaydreddys on Nov 15^th, 2011, 6:45am

Hi,
I am trying to design an opamp using gm/id methodology described in

MILLER OTA DESIGN USING A DESIGN METHODOLOGY BASED ON THE
GM/ID AND EARLY-VOLTAGE CHARACTERISTICS: DESIGN
CONSIDERATIONS AND EXPERIMENTAL RESULTS by Fernando Paixão Cortes, Sergio Bampi.

I dont know how to find the Early Voltage (VA) of nmos/pmos transistors...

Can someone please help me...?

Thanks a lot in advance.... :)

Vinay

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by loose-electron on Nov 15^th, 2011, 5:37pm

Early VOltage is more of a bipolar term than CMOS, most call it "conductance" in the CMOS devices

see attached the Va is the early voltage

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by weber8722 on Nov 17^th, 2011, 7:56am

Well-done by loose-electron. For analysis, just calculate the Rout by the slope and multiply by Id.

Look up that Early voltage is not that good for many CMOS processes (like few Volts only). Use large L for large Vea and making good current sources. Even better use cascodes or even active cascodes.

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by RobG on Nov 17^th, 2011, 11:24am

I'll only add that the OP should be aware that the real (or simulated) curves don't always extrapolate back to a single "Va" point because the slope is also a function of Vds and Vgs-Vt because of hot carriers and other issues.

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by vinaydreddys on Nov 17^th, 2011, 5:23pm

Thank you loose-electron. You are right, early voltage is more of a BJT term.

Following are few sentences from the paper I mentioned earlier.

“In addition to the discussed design method(gm/id methodology), we included an additional optimization in the design phase: choosing the transistor lengths (L) to minimize power. In order to accomplish that, we consider another important characteristic related with the gm/ID curve: the relationship between gm/ID versus VA (the Early Voltage parameter). This relationship is considered as a fundamental device
technology relation that determines the minimum allowable transistor lengths. Once the values of the gm/ID ratio and VA parameter (that is directly related with the transistor output resistance, and consequently with the stage DC gain) are chosen, the L of the transistors can be determined in our methodology.

Considering both characteristics, the design procedure consists in two basic phases. First, the W/L of each transistor is found through the gm/ID vs. ID/(W/L) curve. Then, the transistor lengths (L) are found considering the gm/ID vs. VA curve.”

The authors say, "one can find the transistor length from the graph of gm/id vs Va".

I am looking for a procedure or method that helps me plot gm/id vs va in cadence tool.

Thanks a lot... :)
vinay

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by vinaydreddys on Nov 17^th, 2011, 5:38pm

Thank you Weber8722.

Can you please let me know how to find "Rout by the slope".

If early voltage is not good for CMOS processes(few volts), then why is that it is used for calculating transistor length in short-channels?
(gm/id methodology is for short-channels).

Can you please help me understand the same...?

Thanks a lot.. :)
Vinay

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by vinaydreddys on Nov 17^th, 2011, 5:51pm

Thank you RobG.

Can you please explain how to extrapolate the curves to find va..?
How is it done using cadence tool.

Thanks a lot.. :)
Vinay

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by Forum Administrator on Nov 17^th, 2011, 5:57pm

If you wanted help with a simulation tool, you should not have put this topic in a design forum.

Please read and follow the forum guidelines. By not doing so you have wasted the time of the people trying to help you.

-Ken

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by loose-electron on Nov 18^th, 2011, 9:38am

The bias curves shown above can be done as a set of DC sweeps on a transistor.

The graphical results shown can then be used to find the Early Voltage.

Print it on a piece of paper and use a ruler.

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by vinaydreddys on Nov 18^th, 2011, 8:03pm

Hi Ken,

I am sorry.

My sincere apologies for the inconvenience caused.

Vinay

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by vinaydreddys on Nov 18^th, 2011, 8:05pm

Thank you loose-electron.

Vinay

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by RobG on Nov 23^rd, 2011, 4:37pm

vinaydreddys wrote on Nov 17^th, 2011, 5:38pm:

Output resistance is VA/I_DS. It is also dV_DS/dI_DS, that is, it is the inverse of the slope of the I vs Vds curve. Thus VA = I_DS * dV_DS/dI_DS

Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..?
Post by weber8722 on Nov 24^th, 2011, 7:34am

Hi Rob,

the concept of Early voltage is not soo bad for CMOS too ;) :)! Of course Vear is not a constant, but depends on dcOP and length, etc. However, it is still a good criteria, easier to check than the output resistance or so.

To get Id curve vs Vds use in Cadence IS("..."). Use the calculator expression deriv(IS("...")) to get the slope, i.e. dId/dVds=1/Rout.

Vear=Id*Rout

Bye Stephan