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ac analysis - RF Cafe Forums
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Below are the old forum threads, including responses to the original posts.
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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?
Post subject: Posted:
Wed Oct 26, 2005 7:09 am
Joined: Mon Jun
27, 2005 2:02 pm
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
(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.
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.
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