• Skip to main content
  • Skip to search
  • Skip to footer
Cadence Home
  • This search text may be transcribed, used, stored, or accessed by our third-party service providers per our Cookie Policy and Privacy Policy.

  1. Community Forums
  2. Custom IC Design
  3. Why gm is not zero even when iDS is a constant DC current...

Stats

  • Locked Locked
  • Replies 9
  • Subscribers 126
  • Views 17877
  • 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

Why gm is not zero even when iDS is a constant DC current?

BaaB
BaaB over 9 years ago

I am trying to plot transconductance gm of M0 in the picture below. However, what I am supprised here is why gm is not zero when iDS is a constant DC current (1uA).

gm = diDS/dvGS

So according to this, gm should be zero when iDS is a constant.

As seen from the picture below, gm is not zero.

Could anyone explain why and how gm is calculated by the simulator?

Thank you.

  • Cancel
Parents
  • Andrew Beckett
    Andrew Beckett over 9 years ago

    I had a quick look (it's the weekend so I didn't have time to spend too long looking at it) - there isn't much on exactly how gm is formulated. In the operating point parameters section of bsim4, for example, it references equation 12-32 (in the chapter on common mosfet equations) which just shows the derivative equation. Precisely how it's computed is probably dependent upon the model implementation, but it's equivalent to the derivative. You could potentially download the bsim4 source code (if it's bsim4) from Berkeley and take a look (I've looked at the code in the past, but I'll leave this as an exercise for you - I can't promise that the code is identical to our implementation because it's probably been optimised). I'm really not sure it matters that much though. The point is, it gives you the number that you can use in your calculations.

    • Cancel
    • Vote Up 0 Vote Down
    • Cancel
Reply
  • Andrew Beckett
    Andrew Beckett over 9 years ago

    I had a quick look (it's the weekend so I didn't have time to spend too long looking at it) - there isn't much on exactly how gm is formulated. In the operating point parameters section of bsim4, for example, it references equation 12-32 (in the chapter on common mosfet equations) which just shows the derivative equation. Precisely how it's computed is probably dependent upon the model implementation, but it's equivalent to the derivative. You could potentially download the bsim4 source code (if it's bsim4) from Berkeley and take a look (I've looked at the code in the past, but I'll leave this as an exercise for you - I can't promise that the code is identical to our implementation because it's probably been optimised). I'm really not sure it matters that much though. The point is, it gives you the number that you can use in your calculations.

    • Cancel
    • Vote Up 0 Vote Down
    • Cancel
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.

© 2025 Cadence Design Systems, Inc. All Rights Reserved.

  • Terms of Use
  • Privacy
  • Cookie Policy
  • US Trademarks
  • Do Not Sell or Share My Personal Information