How to model the non-linearity when MOS is used as a resistor?

[deepa]

if a mos is to used as a 100K resisitor,,
Its Vs =Vo+vx
Vd=Vo-vx
Vg=Vbias,values of Vo=0.5v,vx is an ac signal with utmost 0.2v amplitude,,wat should be the value of Vbias if the nonlinear part of the current in the resistor is utmost 5%..
in this how to model the nonliearity?

 

[butterfish]

Re: MOS as a resistor

1> why not use a nwell or poly or pplus resistor for 100k?
2> if mos used for resistor, it should work in linear region, gate usually pull to VDD or GND, why need a Vbias??

 

[deepa]

Re: MOS as a resistor

to approriately use it at a particular value of resistance,,else it ll be some random value

 

[ericnguyen]

MOS as a resistor

if a mos is to be used as a resistor, I has got to work in its triode region, not in saturation region. Because when working in the triode region, the linearity is there. once you increase the Vds, your current increases.

 

[dog1357]

MOS as a resistor

Someyimes the MOS is used in a feedback way and it determined the transimpedance Gain of a TIA circuit block.

In this case, any model or formula to follow to determine the value of the resisitor?

 

[butterfish]

Re: MOS as a resistor

to approriately use it at a particular value of resistance,,else it ll be some random value

*** I'm not sure about your application, could you upload some figures?

 

[amriths04]

Re: MOS as a resistor

though a mos is used as an amplifier in the saturation region, you can also use it as a resistor(between drain and source).

R out=Va/Id
Id=.5*kn*(w/l)(Vov)^2.

and Va is the early voltage.
you can bias the transistor appropriately for the necessary value of Id and knowing it's Va you may proceed.

 

[deepa]

Re: MOS as a resistor

this is the MOS that needs to be biased as a resistor,,n have to determine the W/L value too ,,
what ll be the effects of non linearity in this n how to model it [/img]

 

[butterfish]

Re: MOS as a resistor

do you means, you want a voltage controlled resistor?

if you just want a fixed 100K, why not use a passitive resistor for good linearity??
100K is a comfortable value for passitive resistor.

 

[paladinzlp]

MOS as a resistor

When you regard mos as resistor, the mos is on the linear region on common.

 

[Vabzter]

Re: MOS as a resistor

To find out the value of W/L u can use this formula:
R = 1 / (un*Cox*(W/L)*Vov) ..Vov = overdrive
Refer Johns and Martin..
They have used Mos as a resistor in Opamp compensation..
I hope this helps
BR
Vabzter