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https://designers-guide.org/forum/YaBB.pl Simulators >> Circuit Simulators >> Simulation for finding cgs and cgd https://designers-guide.org/forum/YaBB.pl?num=1425864183 Message started by vlsi_design on Mar 8th, 2015, 6:23pm |
Title: Simulation for finding cgs and cgd Post by vlsi_design on Mar 8th, 2015, 6:23pm Hi, Is there any way to find cgs and cgd of transistor using simulations? I know that AC can be done to find imaginary part but that will give cgs+cgd=cgate. I also know about DCop point and captab. I am looking for either AC or S param sim that allows to seperate cgs and cgd |
Title: Re: Simulation for finding cgs and cgd Post by Geoffrey_Coram on Mar 13th, 2015, 11:06am If you put an ac voltage on drain and measure the gate current, this will be related to cgd. If you put the ac voltage on the source instead, this will give you cgs. If you put the ac voltage on the gate (still measuring the gate current), this is cgs+cgd (+cgb). |
Title: Re: Simulation for finding cgs and cgd Post by vlsi_design on Mar 21st, 2015, 7:40pm Hi Geoffrey_Coram, Thank you very much for the reply! I tried to simulate it but I find that Cgate < Cgs + Cgd. Actually it should be Cgate> Cgs + Cgd due to Cgb. I have attached the circuit and curves Also why these caps decrease with frequency? Is it due to inversion layer trap phenomenon as explained in device books? But that occurs at 100KHz onward and only when inversion layer is completely isolated. |
Title: Re: Simulation for finding cgs and cgd Post by vlsi_design on Mar 21st, 2015, 7:41pm Here is the simple circuit |
Title: Re: Simulation for finding cgs and cgd Post by Geoffrey_Coram on Mar 26th, 2015, 7:44am vlsi_design wrote on Mar 21st, 2015, 7:40pm:
Your plot shows Cg (M6, in green) above the other two (Cgs from M9 and Cgd from M12). Quote:
It's not the inversion layer trap phenomenon. I think it's just a voltage-divider effect with all the different capacitors and conductances (gm, gds -- but maybe also RD,RS, and RG). |
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