the series R of varactor is different between m=2 and f=2 ?

[walkingsun]

where is different, from physical point?

thank you!

 

[Fahmy]

If the varactor is based on Mosfet transistor then :
M=2 is equivalent to repeating the transistor two times , each instance have its own source/ drain . so we have two gates and four S/D regions.
F=2 is equivalent of having Transistor with two fingers . i.e we have one source and two drains.So we have only 3 S/D regions.

 

[walkingsun]

Re: the series R of varactor is different between m=2 and f=

If the varactor is based on Mosfet transistor then :
M=2 is equivalent to repeating the transistor two times , each instance have its own source/ drain . so we have two gates and four S/D regions.
F=2 is equivalent of having Transistor with two fingers . i.e we have one source and two drains.So we have only 3 S/D regions.

from the simulation, the equivalent res of nmos varactor with M=2 F=1 is smaller than the one of with M=1 F=2.

and, if the number of M*F is large, the difference of equivalent res is very obvious.

so, could i say that the more fingers mean smaller area, and reasonable multiplier means suitable resistance?

 

[Fahmy]

Re: the series R of varactor is different between m=2 and f=

from the simulation, the equivalent res of nmos varactor with M=2 F=1 is smaller than the one of with M=1 F=2.

and, if the number of M*F is large, the difference of equivalent res is very obvious.

so, could i say that the more fingers mean smaller area, and reasonable multiplier means suitable resistance?

of course as the number of fingers increases, the series resistance decreases but also the effective capacitance per unit area decreases , so it is a tradoff between large capacitance and small series resistance given the same area. It actuallly depends on the application and the required specs.
By the way, don't put M with a value more than 1 except when you are doing the same thing in the layout . M means that you have duplicate instances of the device each separated from the other (they are not merged together). The LVS tools usually don't differentiate between (M=1,F=2) and (M=2,F=1) cases , although they give different results , so take care.

 

[walkingsun]

Re: the series R of varactor is different between m=2 and f=

of course as the number of fingers increases, the series resistance decreases but also the effective capacitance per unit area decreases , so it is a tradoff between large capacitance and small series resistance given the same area. It actuallly depends on the application and the required specs.
By the way, don't put M with a value more than 1 except when you are doing the same thing in the layout . M means that you have duplicate instances of the device each separated from the other (they are not merged together). The LVS tools usually don't differentiate between (M=1,F=2) and (M=2,F=1) cases , although they give different results , so take care.

but the interesting thing is, by the simulation, i find that the equivalent series res of the m=1 f=100 and the m=100 f=1 is very very diff.

is it right?

 

[Fahmy]

Re: the series R of varactor is different between m=2 and f=

is it right?

I think it is right.