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.