What is difference between ron and 1/gds?

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peterlau1984

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ron gds

Dear all,

I found the statement in cadence document that "ron" is on-resistance, while "gds" is common-source output impedance. Both of them are operating point parameters. What is the difference between ron and 1/gds?

Furthermore, we usually caculate with the parameter "ro" during design, which is equivalent with "ro", "ron" or "1/gds"?

Thanks!
 
cadence ron

sorry, it is my typing error. gds is common source output conductance.

I found the statement in cadence document that "ron" is on-resistance, while "gds" is common-source output conductance. Both of them are operating point parameters. What is the difference between ron and 1/gds?

Furthermore, we usually caculate with the parameter "ro" during design, which is equivalent with "ro", "ron" or "1/gds"?

Thanks!
 

mos gds gm

In my available literature (Gray/Mayer, Razavi), ro is the usual term for output impedance of common source amplifier. 1/gds is just the same, to my opinion. ron, in contrast is the impedance in unsaturated operation (using the transistor as a switch, @ Vds = 0). It's surely much lower than r0, cause the values are representing completely different operation points.
 
cadence ron or 1/gds

I agree with FvM "ron" is the impedence when mos is in linear region and "ro"="1/gds" is the impedence when mos is in saturation...
 
what is a difference gds gds2

How about if transistor works in subthreshold region? How much would be the output resistance of the trarnsitor?

Thanks.
 

1 / gds

a.zar said:
How about if transistor works in subthreshold region? How much would be the output resistance of the trarnsitor?

Thanks.
Click to expand...

ro(subthreshold) = n*Vth/Id with slope factor n = 1.5 (+/- 0.3) depending on Vds
 
gds=1/ro

Thanks erikl,

So assuming Vth is thermal voltage, are you saying that the ro is 1/gm? Is there any reference for this equation? Is n=1+Cdepletion/Cox? How does n depend on Vds?

Thanks.
 

common source gain gm gds

a.zar said:
Thanks erikl,

So assuming Vth is thermal voltage,
Click to expand...
Correct
a.zar said:
are you saying that the ro is 1/gm?
Click to expand...
Yes
a.zar said:
Is there any reference for this equation?
Click to expand...
/1/ Behzad Razavi: "Design of Analog CMOS Integrated Circuits", McGraw-Hill Int.Ed. 2001 Chap. 2.3 p. 27/28 . The slope factor "n" here is printed as (grec) "zeta"
/2/ Christian C. Enz, Eric A. Vittoz: "Charge-based MOS Transistor Modeling", Wiley 2006 Chap. 5.1.1.3 Approximation in weak inversion, eq. (5.20)
a.zar said:
Is n=1+Cdepletion/Cox?
Click to expand...
To a first approximation, yes; Exact: n = 1 + (Gamma(b) / 2*sqrt(Psi(s))) /2/ eq. (3.34)
Gamma(b) being the substrate modulation factor /2/ eq. (3.30) ,
Psi(s) is the surface potential /2/ Fig. 2.2
a.zar said:
How does n depend on Vds?
Click to expand...
S. /2/ Chap. 3.5.1 eq. (3.31)..(3.37) and Fig. 3.7 & 3.8 (p. 21 ff)

Cheers, erikl
 
what is gds transistor parameters

Thanks a lot.

Added after 2 hours 52 minutes:

Hi again erikl, just another quick question: With rout=1/gm, what would be the gain of the transistor? Is the equation gain=gm.rout still valid?
Thanks.
 

difference between gds and gdsii

for me , i think, if the device is working in deep linear range, the Ron=1/gds, but if working in saturature range, you can not say that, you can see small signal model, the r0=1/gds

 

ron equation of cmos

a.zar said:
Thanks a lot.

Added after 2 hours 52 minutes:

Hi again erikl, just another quick question: With rout=1/gm, what would be the gain of the transistor? Is the equation gain=gm.rout still valid?
Thanks.
Click to expand...
If rout=1/gm then gm.rout=1 . gain=gm*Rload
 

cadence transistor gds ron

some times Ron is used for the triode case, and 1/gds is also called ro, but depends on the text book convention. sometimes, instead of ro, they just use rds as well
 

gds pmos formula

What cadence call ron is actually the On impdeance of the transistor.
Say when you need it as a switch, then you should make it work it in triode reqgion. There the dc output impedance is "ron".
Whereas the 1/gds is the small signal output impedance of the transistor and should be seen when the device is in the saturation region.
 
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    Nadida

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Re: gds pmos formula



I have simulated this for a NMOS, where ro=(1/gds) and ron is from device parameter, clearly both are not same!
 

Re: gds pmos formula

sudip said:
... ro=(1/gds) and ron is from device parameter, clearly both are not same!
Click to expand...
I think ro=(1/gds) is the small signal (differential) output impedance, whereas ron is the DC output resistance, as sanjeeb stated above.
 
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    anhnha

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