gm is not independent ofVGS IN linear region

[arya.jagadeesh]

gm is decreasing in simple ckt ..where i am applying vds=1 and changing gate voltage from 0 to 1.8 vt=.4..after entering triode equation says independent....but it is decreasing ...why

 

[goldsmith]

gm is decreasing in simple ckt ..where i am applying vds=1 and changing gate voltage from 0 to 1.8 vt=.4..after entering triode equation says independent....but it is decreasing ...why

Hi Arya
May i saw your just SIMPLE CKT ? how do you expect i know what is in your mind and expect help ? shall we be oracles here ?


Best Wishes
Goldsmith

 

[ftsolutions]

What is the MOSFET (or MOSFET model) you are using?
Gm does vary at low Vgs (linear mode) operation in quite a few MOSFETs, though it is something which is often not considered. Looking at the data sheet curves of some larger MOSFETs, such as IRF9640/IRF640 may show this that is can actually change negatively, before it starts to change positively.

 

[arya.jagadeesh]

Hi Arya
May i saw your just SIMPLE CKT ? how do you expect i know what is in your mind and expect help ? shall we be oracles here ?


Best Wishes
Goldsmith

for nmos transistor i am applying vg at gate...source grounded ...applying vd at drain...nothing else

 

[goldsmith]

for nmos transistor

What kind of nmos ? i hope you know it is important to know the part number which you've used because threshold and on state voltage and IDSS and ... of every mosfet is different in compare with the other ones . WHEN YOU WANT ASK A QUESTION FIRST GIVE FULL INFORMATION OF YOUR QUESTION ! ok ?

 

[dick_freebird]

I have never seen an ID-VG curve that looks like constant-gm.
There's always a peak and it's right below VT. By positioning
yourself near the peak you can get close to constant gm, if
that's what you're after, but across a broad range and especially
in linear region, I would not expect anything close to constant.