Designer’s Guide Community

Pages: 1

how to plot gate capacitance versus gate voltage (Read 5680 times)

mixed_signal Senior Member Offline Posts: 183	how to plot gate capacitance versus gate voltage 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?
Back to top	IP Logged

Geoffrey_Coram Senior Fellow Offline Posts: 1999 Massachusetts, USA	Re: how to plot gate capacitance versus gate voltage Reply #1 - Oct 12^th, 2011, 10:05am You can do a nested sweep -- run an ac analysis with a SWEEP.
Back to top	If at first you do succeed, STOP, raise your standards, and stop wasting your time. IP Logged

raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: how to plot gate capacitance versus gate voltage Reply #2 - 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.
Back to top	IP Logged

mixed_signal Senior Member Offline Posts: 183	Re: how to plot gate capacitance versus gate voltage Reply #3 - 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.
Back to top	IP Logged

raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: how to plot gate capacitance versus gate voltage Reply #4 - 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.
Back to top	IP Logged

Geoffrey_Coram

Senior Fellow

Offline

Posts: 1999
Massachusetts, USA

Re: how to plot gate capacitance versus gate voltage
Reply #5 - 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.

If at first you do succeed, STOP, raise your standards, and stop wasting your time.

IP Logged

mixed_signal Senior Member Offline Posts: 183	Re: how to plot gate capacitance versus gate voltage Reply #6 - Oct 19^th, 2011, 1:28am Thank you so much!
Back to top	IP Logged

Pages: 1

‹ Previous topic | Next topic ›