I would like to ask your opinion regarding NMOS test-bench as a varactor (voltage control capacitor). I am trying to find the properties of Vg,W for achieving a \$200fF\$ capacitor.

Here is my test-bench : enter image description here

Calculated \$C\$ using AC analysis, where

\$C = \frac{I}{(V*2*pi*f)}\$

\$I = NMOS-Gate\$ point


\$V = voltage\$ on gate net.

Is this correct?

question - we are looking for \$Cgs\$ here, so can I just take the value from Cadence "Results Browser"?

  • \$\begingroup\$ Don't forget that you need to add DC beside the AC voltage, for a proper .op point and .AC analysis. See this for something similar. \$\endgroup\$ May 13, 2020 at 20:36

1 Answer 1


The following is the equivalent circuit model of a FET. Disconsidering the parasitic inductances, if the drain and source ends are short citcuited the equivalent gate capacitance is given by:




The gate capacitance can be calculated by connecting a gate resistance much bigger than \$R_G\$ and sweeping the AC gate voltage according to the following e.g.:

  1. Sweep AC gate voltage
  2. Find the frequency at which the amplitude drops by 3dB
  3. Workout the gate capacitance

In this case, the gate capacitance is given by:

$$C_{G(CALC)}=\dfrac{1}{2\pi\cdot23.4963kHz\cdot1k\Omega}\approx 6.77nF$$

It can be double checked by inspecting the model details:

$$C_{G(MODEL)}=5.2nF + 1.6nF=6.8nF$$


  • \$\begingroup\$ I did not understand few things : 1. you did not sweep the AC voltage, it is constant 1V according to your simulation, so where does Vac of the gate plays a part ? 2. why you took the 3dB point ? 3. according to your answer, it is much easier to take the model Cgs+Cgd capacitance as Cg other then calculate from simulation - is that right ? \$\endgroup\$ May 14, 2020 at 6:15
  • \$\begingroup\$ 1. It is actually being swept with the instruction .ac dec 1000 100 1e6. The 1V is the DC bias. 2. You are right provided that you have such information in a model and parasitics are not accounted for. Furthermore, this is a very simple model. However, in your case, if you dont have such information at hand, you have to extract them by either simulation or calculation, or a combination of both. \$\endgroup\$
    – vtolentino
    May 14, 2020 at 6:53
  • \$\begingroup\$ While the AC voltage is 1 V, what is swept is the AC Frequency \$\endgroup\$
    – jp314
    May 29, 2022 at 23:56

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