load pull amplifier
I’m a bit stuck in the concept of load pull…
I’m using ADS to model and tune a amplifier, my questions are:
(regardless of stability)
1- by loadpull, we simulate the effect of different loads on the performance of the system. Imagine I picked the load ZL which leads the amp to have the best performance.
As I assume I should put this as the output load, but what I observe in the examples is that starting from this point, a matching network to 50ohm is put in the output…
I agree to match to 50ohm, but the unclear part for me is that when we match to 50ohm, the amp sees 50ohm as the output so the performance is not what I expected??
2- why normaly left half of smith chart is used to simulate arbitrary load? why I have not seen the right portion in any example?
3- when I find a desirable load for fundamental harmonic (ZL)
I think in the ideal case for 2nd and 3rd harmonic, would be to get the max reflection coefficient (=1) (or Zout=inf???)
and most probably it would not be the case with that very ZL that I picked.
so what would be the approach here?
Answer to your Q2: This is chosen based on experience. If you want, you can simulate whole of the smith chart and the load will either come on the upper left or the upper right of the smith chart depending on the device chosen.
I am not clear about other questions.
1) ignore matching the output to 50 ohms... just match to the ZL that gives best performance. Not that ZL for max efficiency is not the same as ZL for max power. It is a compromise.
3) There are papers on harmonic terminations... I am no expert. But it is correct that high impedance is required. At first harmonic, you might want a short circuit. This could be done with an open stub close to the FET that is quarterwave at 2F0. Nothing that you add to the matching network after a short circuit will change the termination impedance at 2F0. At F0, it will look like a capacitive stub that you can work into the output matching network, and get to your load-pull match. The power amp will perform much better if you take care of that second harmonic. Dealing with the third harmonic is probably a lot harder and will have diminishing returns. Like "milking mice" as my old boss used to say...
I hope this helps
1) If your RF system uses 50 ohm, then, of course, your amplifier output should be 50 ohm. But your transistor drain should see impedance ZL. ZL can be an impedance regarding design requirements such as the highest efficiency, max output power, etc.
2) You can simulate any part of smith chart, you can set it at schematic. ADS examples usually use reflection value to define the sweep are for load-pull simulations. Therefore, you need to calculate reflection if you use impedance data as reference. There are lots of online calculators from impedance to reflection or other way around.
3) Terminations of the harmonics are important. But you have to be careful. If you have a transistor model including package parasitics, bonding wires, etc., you may need a specific impedance value instead of short or open to maximize the performance. There are many studies about harmonics terminations. If you are a beginner, I recommend books at first.
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