inductioner
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Hi Aaron,
I think Gmax is usually used for transformer characterization, or as one of passive device merits of figure. When using XFMR for design, T or Pi model can be more useful to gain more circuit insight.
I designed a few CMOS PAs with integrated or off-chip XFMR. I was pretty much following the old-time PA design methodology : Use load-pull technique to do all the followings : 1. Power match to maximize PA output power 2. PAE match to target at power-aided efficiency 3. For linear systems (especially OFDM ones) linear match is also one of the constraints to check AM-AM and AM-PM.
In so doing, I worked on load-pull analysis (after a few some basic checks on P1dB, AM-AM and AM-PM, stability, etc.) for PA core. excluding XFMR, which I consider as part of PA output matching netowork in the configuration I am figuring. The partition is arbitrary. I had included on-chip XFMR as part of PA core, the result was the same, as it should be. But the parasitics of PA core can play a major role in matching optimization especially when dealing with > 5GHz operating frequency.
So, back to you question : I normally don't worry about transformer characteristic precisely from initial design phase; only the first order model parameters such as turn ratio, magnetically coupling coefficient k, rough L/C parasitic values and so on you will deal with. In the mean time, transformer can be designed to meet your electric and magnetic needs with the best achievable insertion loss (determined mainly by substrate rho, resistive loss, low magnetically coupling coefficient, etc.)
In final stage of design, surely we need to put everything together to see if the result is good.
Hope the above is useful to you.
cheers, inductioner
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