Home > Community > Forums > RF Design > Plotting Vth

Email

* Required Fields

Recipients email * (separate multiple addresses with commas)

Your name *

Your email *

Message *

Contact Us

* Required Fields
First Name *

Last Name *

Email *

Company / Institution *

Comments: *

 Plotting Vth 

Last post Wed, Mar 7 2012 10:08 PM by Andrew Beckett. 4 replies.
Started by Shahnaf 04 Mar 2012 11:39 PM. Topic has 4 replies and 3723 views
Page 1 of 1 (5 items)
Sort Posts:
  • Sun, Mar 4 2012 11:39 PM

    • Shahnaf
    • Not Ranked
    • Joined on Mon, Aug 29 2011
    • Posts 14
    • Points 265
    Plotting Vth Reply
    Hi, I am designing rectifier for very low power applications using gate cross coupled architecture of MOSFETs. So, My input is ac voltage which is around 500mV and less. In order to simulate my design, I am trying to look around the ways to lower the Vth. To do so, I want to check how Vth is varying w.r.t Width of NMOS, PMOS and Length of NMOS, PMOS. How should I plot Vth vs L or W in case of ac input? Suppose, i give some biasing to gate voltage by using some voltage source, at that time, how should I plot Vth? Please help me out as I am very confused. Thanks, Shahnaz.
    • Post Points: 20
  • Tue, Mar 6 2012 3:45 AM

    Re: Plotting Vth Reply

    Not sure what analysis you're doing, but if you want to plot the Vth from anything other than a DC operating point, you'll need to get the simulator to save the vth. You can do this with:

    save M1:vth

    or

    save M1:oppoint

    where M1 is the name of the transistor. This would need to go into the spectre netlist - if you're using spectre from the Analog Design Environment, you can create a file called (say) "mysave.scs" with the above contents, and then include it by referencing it from Setup->Model Libraries.

    Having done that, you can access the vth for whichever analysis you're using by using the Results Browser in ADE.

    Regards,

    Andrew.

    • Post Points: 20
  • Tue, Mar 6 2012 5:17 PM

    • Shahnaf
    • Not Ranked
    • Joined on Mon, Aug 29 2011
    • Posts 14
    • Points 265
    Re: Plotting Vth Reply

    Thanks Andrew.

    I am doing transient analysis for the rectifier for very low power applications.

    I want to lower the Vth of the MOS and so I want to know Vth for different aspect ratio's.

    I will try the method suggested by you once I am in lab.

    If no success, I will post further details.

     

    Thanks & regards,

    Shahnaz. 

    • Post Points: 5
  • Wed, Mar 7 2012 12:42 AM

    • Shahnaf
    • Not Ranked
    • Joined on Mon, Aug 29 2011
    • Posts 14
    • Points 265
    Re: Plotting Vth Reply

    Andrew,

    I tried the method suggested by you. I have used four MOSes in my circuit.

    So, I included all four MOSes in .scs file to save the Vth. 

    Here are some of the observations and queries:

     1. When I do Transient analysis,  I opened the results browser -> tran-tran . I found only two MOSes in that list. But when I click Vth, it plotted Vth vs time.

    2. I want to plot between Vth and W or L or Vth and aspect ratio (W/L) and so I chose ac analysis by sweeping variable 'w' (width of Tr). To my great surpise , I found none of the transistor list in Results Browser -> ac-ac.

    Do I need to do anthing more in order to plot Vth vs W, Vth vs L and Vth vs W/L?

    If so, what should be done?

    Thanks in advance. 

     

    • Post Points: 20
  • Wed, Mar 7 2012 10:08 PM

    Re: Plotting Vth Reply

    In response to your points:

    1. The chances are that you either got the names wrong, or the transistors which are missing are subckt models and so the devices are not at the level you thought they were - you will need to investigate (hard for me to tell without seeing the data)
    2. I wouldn't expect to see operating point data in an ac simulation result. That's because the operating point data doesn't vary in an ac sweep normally - the ac analysis is a small-signal analysis around an operating point, and it's the operating point which will vary. You'd have to do a dc operating point or a dc sweep to see the effect.

    Regards,

    Andrew.

    • Post Points: 5
Page 1 of 1 (5 items)
Sort Posts:
Started by Shahnaf at 04 Mar 2012 11:39 PM. Topic has 4 replies.