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howto: Smith Chart for matching network

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Jun 7, 2005
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smith chart matching network

how do you guys use smith chart for your matching network? I'd thought you rotate clockwise you add inductor in series, and counterclockwise you add Capacitor in series. So how did T. Lee came up with the idea to put Ls at the gate of his Common source of his LNA? I don't understand how did he used Smith Chart to come up with that idea. Could some one please help me understand?

Also, why does the Pav = (1/8)*(Vs^2/Rs). Where did that 1/8 came from? I'd thought it was only 1/2 for avg power? thanks

smith chart l matching network

it is easy and y should use the software to test several times and then y can do it ,good luck

matching network with smith chart

You need to use the special chart with two colors to indicate the admittance and the impedance values.

There are several free programs for this. One is many years old and given for free by Motorola. It is probably somewhere on the web. Perhaps some board member can give you a link to it.

so you guys are saying that we can not start with smith chart for hand analysis. So I have to use a program for that then deciding whether to add L/C in series or parallel?

Added after 2 minutes:

btw, smith chart is only for passive devices only right? I can't use it to match impedance for R?

See the following link

(the the document is named transmissionline2.pdf)


Smith Chart is used to represent impedance, admittance and so on ...
it doesn't matter if you are looking at a passive or an active device.

Thanks guy. I just went over the Cheng EMC book and understood Smith Chart. What I don't get the most is how in the world can you get your starting point on the smith chart? Do you just put your 50ohm input to any transmission/ckt. Then using a simulator like Cadence to tell you where is the starting point? Because once I know where is my starting point, I can go from there.

Also, does perfect match has to be exactly on the origin? Or can it be anywhere along imag = 0 line?

You have to first decide on how many sections the matching network needs. The more sections increase the cost but make the tolerances less important.

in general you need two reactive elements or transmission lines to match two impedances. For this case you have several binary choices, series or parallel first element. inductor or capacitor first element and capacitor or inductor second element.

The impedance ratios will determine which choice to take. On all of the above there is about 1/4 of the chart impedance values that will not work no matter what values you select.

In general putting the first element in shunt will propel you to a lower impedance and in series will propel you to a higher impedance.

right, I understand that. How to select my passive element or even using a transmission line, i.e. how to navigate around the polar chart. But what I don't understand is

1. Does a perfect match have to be exactly on the origin point? Or can it be anywhere along imag = 0 and anywhere on real axis line?

You normalize the impedances so that the center of the chart is the real part of the impedance you want to end up with. You then select the point where the imaginary part of the impedance is where you want to end up with.

For most RF-microwave situations you want to end up at the center of the chart which is 50 +j0.


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