0
\$\begingroup\$

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

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"?

\$\endgroup\$
1
  • \$\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

1
\$\begingroup\$

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:

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

model

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:

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

circuit_2

\$\endgroup\$
3
  • \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.