I am using a RF amplifier (model: Minicircuits LZY-22+) to drive 6.5 MHz oscillation in a coil. I am using an antenna tuner (model: MFJ-969) to impedance-match the output of the amplifier to the coil. Up to some power (~40 dBm), I can get pretty good matching with almost no reflection (SWR < 1.1). But as I try to increase power beyond 40 dBm, SWR goes up and it's hard to keep it below the value of 2 while I try to optimize the matching again. I noticed that this transition seems to be close to the 1dB compression point of the amplifier.
Q1: Does the nonlinearity of the amplifier near 1 dB point actually mean that the impedance is changing, and hence I have to re-optimize?
Q2: Why would the SWR show an increasing lower bound near the 1 dB point, even after trying to re-optimize the tuner?
Best Answer
Yes, the impedance changes. Impedance matching is a concept based on the fact that the system behaves linearly. That means doubling the input signal will double all internal signals and the output signal. Also any reflections will double.
Up until the 1 dB compression point the amplifier behaves in a linear (enough) way so that the impedance matching works as expected.
However at the 1 dB compression point the output signal of the amplifier is 1 dB lower than what it should have been if it was an ideal amplifier. That output signal being lower than expected is non linear behavior and that harms the impedance matching.
You could try and tune for that specific situation (the 1 dB compression point) but even if you find such a point it will mean that at lower power signals, the impedance matching will be less optimal. You'll need to compromise in that respect, good small signal matching or best you can get large signal matching or a compromise between those two.
I am unsure why this is but I think it is a consequence of the non-linear behavior I explained above.