# Simulating NMOS as capacitor example

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 : Calculated $$\C\$$ using AC analysis, where

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

$$\I = NMOS-Gate\$$ point

and

$$\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"?

• 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. May 13, 2020 at 20:36

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:

$$C_{G}=C_{GS}+C_{GD}$$ source

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:

$$C_{G(MODEL)}=5.2nF + 1.6nF=6.8nF$$ • 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 ? May 14, 2020 at 6:15
• 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. May 14, 2020 at 6:53
• While the AC voltage is 1 V, what is swept is the AC Frequency May 29, 2022 at 23:56