Calculating Input Impedance - RF Cafe Forums
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: Smith Chart.
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
Sep 02, 2005 7:25 pm
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
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 figure.
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.
on this site: http://rfcafe.com/references/electrical/smith.htm