Hi
I'm looking for some info about how to use the voltage controlled current source vccs of the cadence analoglib as a voltage controlled capacitor. Is there a way to use it with equations such as C=C0/sqrt(Vctrl) ?
I looked in the Analoglib Reference Guide but there are no information about that.
thanks

Hi Timmy,

in general it should be possible also in an analog circuit simulation program to model such a behaviour.
Question: Should it be (1) a real device which can be built or (2) only a behavioural model for simulation purposes ?
To (1): At first, an active capacitance multiplier using an opamp has come into my mind. In such a case, one of the resistors determining the multiplication factor has to be controlled by an external voltage.
However, an additional complication is the required square root relationship.  
To (2): Something like a voltage controlled current source with a frequency dependent current should be realizable. However, I think it gives only a grounded capacitor. Is that a problem ?

I am trying to help you. I recommend that you read http://www.designers-guide.org/Modeling/varactors.pdf and then use Verilog-A. You will likely save yourself a lot of trouble if you do.

-Ken

Wow, you guys really have a stubborn streak!  Ken was trying to help you, and that document about modeling varactors also gives some hints on bad formulations that can cause convergence problems.  Verilog-A is a language for writing what you want for simulation, I don't know what you think it is ...

Anyway, your model needs to be written in terms of charge (see the document for why!), and would look like this:

Code:

`include "discipline.h"
module timmy(t, b);
  inout t, b;
  electrical t,b;
  parameter real C0 = 1p from (0:inf);
  real charge;

  analog begin
    charge = 2 * C0 * sqrt(V(t,b));
    I(t, b) <+ ddt( charge );
  end
endmodule 

Note that capacitance (for a (nonlinear) capacitor is dQ/dV, so this model has the dependence you asked for.

Wasn't that simple?

Thanks also for the file buddypoor, but I expect to simulate my model with spectre in Cadence, so I think that to model it with Verilog A is a better solution because I can't find informations about the vccs in Cadence. Thanks for bothering anyway.