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Design >> RF Design >> S- to ABCD-Parameters
https://designers-guide.org/forum/YaBB.pl?num=1332237850

Message started by DDC on Mar 20^th, 2012, 3:04am

Title: S- to ABCD-Parameters
Post by DDC on Mar 20^th, 2012, 3:04am

Hello,

I designed an amplifier which is not matched to 50 Ohm at the output, because the input impedance of the following stage ist highohmic. I want to measure the voltage gain of the stage when there is a 550 Ohm load. I made an ac simulation with a 550 Ohm port at the output, so I can get the graph for the ac gain. In reality I can't measure without 50 Ohm at the output. So I decided to place a series resistance of 500 Ohm to the output before connecting to 50 Ohm. So my load is equal and I can get the voltage with a simple divider. Now I want to calculate the voltage gain from my s-parameters. The problem is, that the calculated voltage gain and the simulated ac gain are not equal. I used the A-parameter for calculation with Av = 1/A = (2*S21*√(Z01*Z02)) / ((Z01+S11*Z01)*(1-S22)+S12*S21*Z01). Maybe somebody can describe my mistake. Thanks for response.

Title: Re: S- to ABCD-Parameters
Post by rfcooltools.com on Mar 20^th, 2012, 8:45am

DDC,

Given your situation the simple way is to place a negative 500 ohm in series with your output and simulate s21 with 50 ohm ports. Or you can multiply your ABCD generated from your measured 50 ohm system then multiply ABCD with a negative series 500 Ohm then calculate s21 as 2/(A+B+C+D). The proper way is to measure your circuit without the 500 ohm then get 50 ohm s-parameters. Then simulate the AC gain with the intended 550 Ohm load.

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