calculating input impedance - RF Cafe Forums
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Post subject: calculating input impedance Posted: Fri Oct 21,
2005 3:42 pm
hi....i am new to this. i have a matching question,
sort of. let's say i have a 50 ohm source and it's going into a MOSFET.
i need an appropriate matching network between the source and mosfet.
my question is i need to calculate what is the impedance looking into
the gate or gate-drain of mosfet. i heard ADS has a design guide that
you can use to do this but i don't have ADS. how can i do it in general?
i am using cadence. of course, the input impedance will be complex and
i would need to transform it to 50-ohm real? also, if i need to match
the output to 50 ohm, how can i calculate impedance looking into the
drain or drain-source? thanx a lot
Post subject: Z-matchPosted: Fri Oct 21, 2005 8:25 pm
In two words:
There are free programs available on the web that
do a great job. I'm at a public computer right now, and can't get the
Also, how to use the Smith Chart is in many
books and articles.
Post subject: Posted: Wed Oct 26, 2005 9:14 pm
Joined: Fri Sep 02, 2005 7:25 pm
Kris - let me echo that .. Smith Chart!
There was a time
when RF guys used the Smith Charts to get their values directly, with
very large paper charts, and fine-line graphics for accuracy. Now, with
computer calculation of complex numbers so easy, we don't need the Smith
Chart for getting the answers graphically.
So why then do we
keep seeing them? We even make computers plot them!
The answer is
that we now have the best of both worlds. The PC can do the hard work
of plotting frequency sweeps and parameter sweeps of hundreds of runs,
and giving accurate numbers output, and yet we get the value of the
chart to visualise the routes to find a network that will match a device.
The Smith Chart mapping delivers a quite special ability to do this.
You quicky figure out that there can be an infinity of solutions, but
some are going to be very convenient. You can see the no-go areas for
stability. You can see where to aim for to trade gain for best noise
Best of all, on this very site, look in the software
section. There are free Smith Chart matching tools in there to get you
Post subject: Smith ChartPosted:
Tue Dec 13, 2005 2:31 pm
Any software you guys suggest?
Post subject: Z inPosted: Wed Dec 14, 2005
Software recommendation: QuickSmith.
Link on this site: