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https://designers-guide.org/forum/YaBB.pl Simulators >> Circuit Simulators >> How to find Early Voltage (VA) of nmos/pmos transistor..? https://designers-guide.org/forum/YaBB.pl?num=1321368356 Message started by vinaydreddys on Nov 15th, 2011, 6:45am |
Title: How to find Early Voltage (VA) of nmos/pmos transistor..? Post by vinaydreddys on Nov 15th, 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 15th, 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 17th, 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 17th, 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 17th, 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 17th, 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 17th, 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 17th, 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 18th, 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 18th, 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 18th, 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 23rd, 2011, 4:37pm vinaydreddys wrote on Nov 17th, 2011, 5:38pm:
Output resistance is VA/IDS. It is also dVDS/dIDS, that is, it is the inverse of the slope of the I vs Vds curve. Thus VA = IDS * dVDS/dIDS |
Title: Re: How to find Early Voltage (VA) of nmos/pmos transistor..? Post by weber8722 on Nov 24th, 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 |
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