Very curious about optimum impedance from load-pull in PA de

Hi all,

I have some questions to ask about optimum load impedance in power amplifiers designs.

As can be seen from the attached figure, in order to obtain maximum output power, we need to use load-pull simulation to find an optimum load impedance (Zopt). Then, we design an impedance transformation network to transform the 50-ohm load to be Zopt.

With this matching network, the transistor will see Zopt at its drain, and will deliver maximum power to the 50-ohm load (assuming lossless matching network).

What I am wondering is that, what is the impedance seen by the 50-ohm load (Zx in the figure) ? I think that we cannot know the value of this impedance during the design process of the matching network, because we don't do conjugate matching at the output.

So, if Zx is very different from 50 ohm, there will be high reflection at the junction between the 50-ohm load and the matching network.

In the design process, how could we deal with Zx ?

I found that, if S22 is too high (order of -3 dB), K factor will be lower than 1.


Thank you for your reply in advance,

DYL

Re: Very curious about optimum impedance from load-pull in P

Zx should be 50 ohms. This is the reason using of the Impedance Transformation Network.

Hi vfone,

Thank you for your feedback. However, I am still wondering. The impedance transformation network is used to transform the 50-ohm load to be Zopt of the transistor for maximum output power. How could it be used to match the 50-ohm load (i.e., making Zx = 50 ohm) at the same time ???

I think from your point of view, it is true for LNA where we conjugately match the input and output. However, for the PA, we typically don't do conjugate matching at the output, since this will result in non-optimum Pout.

Thanks again,

DYL

Re: Very curious about optimum impedance from load-pull in P

Yes, matching for power is different than matching for gain.