Load Pull
Dear BB,
1- I am used to characterize all devices I use in PA designs using a pair of manual tuners manufactured by Focus.
2- I only had a reasonable agreement between measured and simulated load pull for devices (either bipolar or FETs) biased in Class A.
3- The conclusion I came in terms of device characterization for power amplifier design is to use a test fixture that mimics the attachment you are going to use in the real PA including the real bias circuit, with all bypass, etc...
4- Instead of to perform a full load pull measurement that is very demand, I adjust the tuners to get the highest output power with the best input return loss for the desired operating point. These are the optimum source and load impedances for power.
Around this point I take four additional points de-tuning the output tuner so that I get 0.2dB less power (respect to the maximum power) The Impedances corresponding to these 04 points belong to a -0.2dB isopower contour. This procedure is repeated for some points more in the passband of interest.
4- Based in the optimum impedance points I get the input and output networks to provide the right impedance transform.
5- So I plot on the Smith Chart the optimum impedances, the -0.2dB isopower contours and the response of the network. This way you have a perfect idea about how much the output power of the PA will change.
Note: 0.2dB is used because that is normally my target spec for power fluctuation over the passband.
6- The result I have been getting with this approach is 100%. The most of the times all considerations made at design time are there in the real circuit,so that I don't have to perform any kind of tuning.
Best regards,
NandoPG