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