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AC Analysis - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Post subject: ac analysis Posted: Tue Oct 25, 2005 6:09 pm
I am running a simple ac analysis (1GHz to 5GHz) for single NMOS
transistor. The schematic is set up as shown in the link

Please look at the schematic to see what I am saying. I am undergradute and I don't know how to put the question: I guess what I'm asking is that the way I have set up the schematic (inductor as an ac block, meaning approx. open circuit) wouldn't it stop my dc Vgs to get to the NMOS gate because I'm running my simulation from 1GHz to 5GHz and at that high frequency inductor will be like an open. I don't understand how ac analysis work. Does the dc bias get fixed before ac analysis is run?

Thank you


Post subject: Posted: Wed Oct 26, 2005 7:09 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany

In general: AC analysis is done at a given bias point which is set acoording to the voltages and currents in your circuit. In this example, this would be the quiescent operating point of the NMOS device. The AC analysis is being run in the frequency range you defined.

Referring to your circuit: I don't think that the simulation you are running, namely AC analysis is good for this freuqncy range, because you are missing an important parameter which PSpice and PSpice-wise simulators don't have: The Quality Factor Q. The lack of this parameter prevents an accurate modeling of the inductors and capacitos. You should use an RF simulator like ADS, MWO etc, which have high-frequency models for inductors and capacitors. Companies like Coilcraft even provide you such models that are specifically compatible for these tools.

For modelling your active device, you will need a spice model or S2P
(S-Paramaters), which is available from different manufacturers. If you want to model a general transistor, then find a device with similar paramters to yours.

Hope this helps.

Best regards,

- IR


Another Guest
Post subject: SPICE AC analysisPosted: Wed Oct 26, 2005 12:01 pm
To answer your question explicitly: Yes, SPICE does a DC analysis to determine the operating conditions of the nonlinear devices before doing an AC analysis.

Many versions of SPICE have the ability to specify device Q, by the way. But it is important as IR said.

Finally, remember that AC analysis is small-signal analysis - it may not necessarily look like it, but it is, and that carries with it some fairly severe limitations.

Good Luck!

Posted  11/12/2012

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