Electronics > RF, Microwave, Ham Radio

S Parameters : Output Impedance and S22

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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?


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

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.


--- 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 :)

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.


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