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
subject: Z-matchPosted: Fri Oct 21, 2005 8:25 pm
In two words: Smith Chart.
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 reference quickly.
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
Location: Hampshire UK
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!
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
Best of all, on this very site, look in the software section. There are free Smith Chart
matching tools in there to get you started.
Post subject: Smith ChartPosted:
Tue Dec 13, 2005 2:31 pm
Any software you guys suggest?
Post subject: Z
inPosted: Wed Dec 14, 2005 6:39 pm
Software recommendation: QuickSmith.
Link on this site: http://rfcafe.com/references/electrical/smith.htm