How do you evaluate output impedance in load pull simulation ?
Show me equations you use.
By output impedance, I mean the impedance that the amplifier needs to see for optimum performance (power ,gain, PAE etc).
Let me re-phrase my question. Why is there a difference between the imaginary part of the optimum impedance from loadpull and the conjugate of the imaginary part of the impedance?
And how do I measure the output impedance seen looking into a PA?
1. I can use sp simulation in SpectreRF and look at ZM2 (of course I have ensured that the input port has the correct impedance along with the proper matching network). But at large signals, checking output impedance is not fully valid.
2. I can use large signal sp simulation. At low output powers (where the amplifier is more in 'small signal'), the large signal s-parameter results and the small signal s-parameters are very close.
My amplifier is a differential structure with feedforward cancellation (I check this by checking s12).
The way I understand is that the capacitance at the output of the amplifier needs to be tuned out. Which means that reactance of the optimum impedance from the loadpull simulations should be close to the complex conjugate of the reactance part of the output impedance of the amplifier.
So what am I missing here?