Matching rf amplifier: calculating complex impedance

oktoober 29th, 2013
by jaanus

I'm building a small 430 MHz  1 W RF amplifier to get some practical knowledge about RF design. Choosing components and making the PCBs was walk in the park. But suddenly my instructor said that I have to match my amplifier with next stage.  It is weird, because I thought that RF amplifier meant that output would be 50 Ω, but no, it is something totally random.

So, my amplifier TQP7M9105 has page with S parameters. S parameters are parameters that describe RF components working in a 50 Ω environment. In our case S22 is what we want. 22 mean that it is second port reflections to second port aka output information. S22 at 400 MHz is -2.73 dB 176.91 degrees. This is - connecting output directly to 50 Ω output you get attenuation of 2.73 dB and phase shift of 176.91.

To calculate complex output impedance I used scikit-rf python library, because I couldn't find any online calculators doing the same job. Input was fed from  Touchstone SnP Format file type, because this was the only way I could think of to convert dB form to rectangular form:

# MHz S DB R50
400 -2.73 176.91

And code itself was:

import skrf as rf
amp = rf.Network('amp.s1p')
print amp.z

So, my amplifier output impedance @400 MHz should be 7.799Ω+1.3158j. Quick check from mismatch calculator confirmed my results. Next I used PI-match impedance matching calculator to calculate necessary values. Got some answers, probably will use other methods to check if they are correct.

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One Response to “Matching rf amplifier: calculating complex impedance”

  1. MIMP, Advanced Design Studio, scikit-rf » Projects of Jaanus Kalde Says:

    [...] was searching for a program to match RF circuits. I already calculated input complex impedance and choose pi filter as [...]

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