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Simulators >> Circuit Simulators >> how to plot gate capacitance versus gate voltage
https://designers-guide.org/forum/YaBB.pl?num=1318386515

Message started by mixed_signal on Oct 11^th, 2011, 7:28pm

Title: how to plot gate capacitance versus gate voltage
Post by mixed_signal on Oct 11^th, 2011, 7:28pm

HI, I want to plot gate capacitance versus gate voltage of a mos . i am using hspice. How can i do it?

Title: Re: how to plot gate capacitance versus gate voltage
Post by Geoffrey_Coram on Oct 12^th, 2011, 10:05am

You can do a nested sweep -- run an ac analysis with a SWEEP.

Title: Re: how to plot gate capacitance versus gate voltage
Post by raja.cedt on Oct 13^th, 2011, 9:31am

hello,
do ac simulations and find the img(current) through Vgs and find the cap....sweep this for some range of gate voltage

Thanks,
Raj.

Title: Re: how to plot gate capacitance versus gate voltage
Post by mixed_signal on Oct 15^th, 2011, 3:12am

Thank you geoffrey and raja! But what do u mean by img(current). How can I evaluate capacitance from that? i know

i=C(dV/dt)

What kind of AC input shall i use? Is it a ramp so that i know dV/dt.

I m a novice. Pls explain in detail.

Title: Re: how to plot gate capacitance versus gate voltage
Post by raja.cedt on Oct 15^th, 2011, 9:15am

hello,
don't use i=cdv/dt....

1. Do .ac simulation, then measure the current through vgs, in that find imaginary current. That's the capacitance.

Thanks,
Raj.

Title: Re: how to plot gate capacitance versus gate voltage
Post by Geoffrey_Coram on Oct 17^th, 2011, 7:35am

It's basic Spice theory -- ac analysis linearizes the circuit, and the current is I = (G + sC) V, where V is the ac magnitude of your source, s is the Laplace frequency -- usually s = jω -- and I is the current you measure in your source.
v1 1 0 3.3 ac 1
is a Spice voltage source from node '1' to ground, dc voltage is 3.3, ac magnitude is 1. Note that the current you measure is ω times the capacitance.

You should try this with a plain linear capacitor and verify this.

Title: Re: how to plot gate capacitance versus gate voltage
Post by mixed_signal on Oct 19^th, 2011, 1:28am

Thank you so much!