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MOS as feedback resistor, how to calculate its resistance

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Hello all,

I use MOS as a feedback resistor.
I would like to ask how to calculate the resistance by simulation in Cadence spectre?
For high linear MOS resistor, how should be the dimension for the transistor?
Longest length?

Thanks in advance.
 

Re: MOS as feedback resistor, how to calculate its resistanc

Hi
For example, to measure resistance of MOS operating in triode region you can use the following circuit and dc analysis (sweep "vdc")

Small signal resistance for W/L = 5u/1u, 10u/2u and 15u/3u is the following

As you can see really resistance changes less for MOS with higher length.
Regards, pavel.
 
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    rficdesigner

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Re: MOS as feedback resistor, how to calculate its resistanc

Thanks you pavel_adameyko for your help.

Added after 27 minutes:

Hi pavel_adameyko,

In my case, I want the MOS working as variable resistor, and Vgs is control voltage. So how to calculate its resistance as a function of Vgs.

Thanks you.
 

Re: MOS as feedback resistor, how to calculate its resistanc

Hi, rficdesigner!
To calculate MOS resistance vs. Vg you can perform parametric sweep with parameter Vg.
The test bench circuit doesn't differ from the one in my previous post.

The resistance for Vg changing from 1 to 3 V is plotted on the following figure
expression in calculator - value(1/deriv(IS("/N0/S")),0,?histoDisplay nil ,?noOfHistoBins 1 )

As you can see for Vg<1.7 V resistance is extremely high - transistor is in closed state. Then it begins to drop and finally equal to near 1K for Vg=3V (Vgs = 3-1 =2V).
Regards.
 

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