PA output matching

[junhao]

I'd like to confirm a theory. It's about PA output matching, after getting PA's Zopt, should I translate from 50 ohm to Zopt or Zopt*(conjugation of Zopt). THANK YOU.

 

[mtwieg]

If not doing conjugate match can things go wrong if effect of feed transmission line impedance is taken in account, if transmission line have a decent length relative wavelength.
To make thing simpler, assume no reactance in below example:

View attachment 171687

It is a bit worst case with for example a 20 mm long PCB transmission line at 2.4GHz, but 50% loss can be bad enough for its power economy and battery weight if it is a battery powered wearable of some kind compeating with concurrent to have smallest IoT thing and longest working time and best RF coverage for a single battery charge, such as a inear headset.

In your graphic, the source impedance seen from the load looking back towards the PA is very different. But the power transferred from the PA to the load is not affected by the transmission line.