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Design >> RF Design >> Need some help explain the curves of Vg and id
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Message started by baab on Jan 26^th, 2014, 5:35am

Title: Need some help explain the curves of Vg and id
Post by baab on Jan 26^th, 2014, 5:35am

Hi,

I am simulating rfnmos1v in TSMC 0.13 with the set up as follows:

Generally, I swept Io and plot ig, id, Vg, Io accodring to Io.
Could anyone help me explain the theory behind the curves of Vg and id.
I can't explain why they have the shape like that.
As in the picture,ig = 0 all the time. In the initial stage, id = Io. However, after Io > 0.3A, id almost saturated and not increase much.I am wondering why id isn't equal to Io after that time.
BTW, I drew the small signal model but that doesn't make sense at all as the circuit has no independent source, the only dependent current source here is gm*Vgs, and I can't calculate anything.

Here is the circuit for simulation:

Title: Re: Need some help explain the curves of Vg and id
Post by baab on Jan 26^th, 2014, 5:39am

Here is the set up:

Title: Re: Need some help explain the curves of Vg and id
Post by baab on Jan 26^th, 2014, 5:44am

Here is the result:

Title: Re: Need some help explain the curves of Vg and id
Post by aaron_do on Jan 26^th, 2014, 4:38pm

Look at your y-axis. You are using a 1-V transistor. Looks like you're pumping way too much current through the device.

Aaron

Title: Re: Need some help explain the curves of Vg and id
Post by baab on Jan 26^th, 2014, 7:46pm

Thank you.
You are right. I am using 1V transistor.
In that case, the transistor is breakdown?

Title: Re: Need some help explain the curves of Vg and id
Post by aaron_do on Jan 26^th, 2014, 10:04pm

The transistor would definitely breakdown in practice, but the transistor model doesn't extend into that region. I'm not sure what is happening with the model at that point. The equations could be meaningless, or maybe some diode has turned on...not really sure.

regards,
Aaron