Electronics > RF, Microwave, Ham Radio

S Parameters : Output Impedance and S22

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jamfletch:
Hi all,

I have a theory question with regards to measuring S22 of devices, e.g transistors etc.

When we measure S22, we're effectively looking at the impedance looking into port 2.

If I want to know the output impedance of a transistor, does the S22 actually give me the output impedance? I.e the impedance looking out from the transistor, to port 2?



 

szoftveres:
When you look out from the transistor, you'll see VNA port 2, which is 50ohms (assuming standard, ideal VNA).

glenenglish:
S22 gives you the impedance looking into that port of the device.
S21, S12 equally important as these will influence input/output isolation.
How you might load the transistor for a specific application though is a completely separate matter.
S params are small signal params. You can measure hot S params, also.

pdenisowski:

--- Quote from: glenenglish on June 06, 2024, 08:35:41 am ---You can measure hot S params, also.

--- End quote ---

I was just about to say this:  A hot S22 measurement (i.e. one where drive power is being applied to the amplifier input) gives a much more accurate indication of amplifier output match during "normal" operation.  It's also a somewhat harder measurement to make :)

mag_therm:
The S parameters can be converted to Z11 and dB_of_S21 etc.
This model is of a discrete transistor antenna amplifier showing the conversions in the lower tabulation.
https://app.box.com/s/4xah5d2jegu7ps5ic8g2fv0a0mz9hhp6

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