rf dmos models

What is the difference between using a RF transistor model in your circuit to normal transistor model..
LIke in what way RF transistors are different from normal transistor models that we use in simulators like cadence...

yeah , usually foundries provide two sets of transistor models , the normal model which is used for low frequancy analog blocks and DC bias networks , and the RF model

there is a huge difference between the two models , the RF model is a normal model , but includes some additional network like substarte network , drain and source diodes and capacitances , gate resistance , gate induced noise effects ,
also the RF model is heavily dependent of the layout and fingering and gate contacts

khouly

Thanks khouly for this nice response.
I need to know if the the usage of these transistors is the same. I mean for exemple If I have to design a PLL I'd better use normal transistors if my requirement in terms of speed is satisfyed since normal transistor and according to your reply are less complicated to implement on silicon. If the requirement in term of speed are not satisfyed I have to use RF transistors.
Am I rigth ? If not please correct.

the PLL is a mixed signal

so in the CP and PFD u and the bandgap reffence , u can use the normal model

but in the VCO and its buffer and the first stages in the prescaeler it is better to use the RF devices , coz this part is working in very high frequency and u need to be as accurate as possible to represent the phase noise and any loading effect on the VCO

khouly

how about the layout of the transistor itself?

does the RF transistor layout differs from the normal transistor layout?

s, can i use the model of the RF model in high frequency but at layout, i put the layout of the normal transistor?

and does this affect LVS stage after extraction?

thanx
mohamedabouzied

it's not advisable, as the model of the rf mos has been modelled together with the layout. different way of layout induces different values of parasitic cap which results in great differences at high freq.

I think the difference is parasitic. the RF model is more close to the MOS in fact.

The MOS model frequency is specified in the documents of the models. IF your working frequency is equal to normal model frequency please use normal model otherwise go for RF model.
Please let me know if you need any further clarifications.

I am trying to desing a double balanced mixer .... what model file should i use....?

it will depend on the frequency
but i think u need to use the RF models

khouly