Stats

  • Locked Locked
  • Replies 4
  • Subscribers 125
  • Views 18544
  • Members are here 0
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

From ideal switch to CMOS transitor switch

Jaikrishnan

I am currently doing a simulation using an ideal switch from the analoglib  with an open resistance of 1Tohm and closed resistance of 1ohm. Also there is a supply (vpulse) given to the switch to define the on time and off time. 

Rise time: 1ns, Fall time: 1ns, pulse width: 166ns, period: 10us

I want to convert it to a CMOS transistor based switch. I know that the transistor should be operated in linear region to behave as a switch. That means Uds < Ugs-Ut

But what other factors should I need to consider to exactly replicate the behavour given by the ideal switch (of course it will no longer be an ideal switch when it is implemented using transistor)

How about the width and length of the transsitor used (if it is an NMOS or PMOS based transistor). 

How should I come up with how much Ugs I need to supply?

Parents

  • Hello Jaikrishnan,

    An ideal switch has zero on resistance and infinite off resistance, but of course we do not live in an ideal world.

    To understand the switch performance, you should simulate your switch for the worst case conditions. The worst case condition is achieved: 

    1. By making VDS=0 and then the VGS=Vclk-VIN 

    2. Making the off resistance very large.

    These conditions will allow you to choose the transistor W/L ratio.

    I would start with a simulation of NMOS switch by modeling the NFET as a resistor and see how it is transferring charges to the Capacitor. If you are satisfied with the performance, you can use the resistance to model your NFET size. Ron=1/Knp.W/L(Vgs-Vth).

    There are some excellent Analog Textbooks on how to go about doing this. I would refer to CMOS Analog Circuit design by Phillip Allen and Douglas Holberg chapter 4. https://www.abebooks.com/9780199937424/CMOS-Analog-Circuit-Design-Phillip-0199937427/plp
    I hope that helps. 

  • in reply to wgtkan

    Thank you wgtkan. I will refer those any try the simulations

Reply Children
No Data

Community Guidelines

The Cadence Design Communities support Cadence users and technologists interacting to exchange ideas, news, technical information, and best practices to solve problems and get the most from Cadence technology. The community is open to everyone, and to provide the most value, we require participants to follow our Community Guidelines that facilitate a quality exchange of ideas and information. By accessing, contributing, using or downloading any materials from the site, you agree to be bound by the full Community Guidelines.