john23,
I am uncertain how to answer this.
You referred to Figure 12 of a datasheet for the lower two transistors, which is a GaN RF power amp.? I downloaded the datasheet from alldatasheet.com that you referred to.? Figure 12 is a plot of LSG (large signal gain?) versus frequency and Vd.? What does that have to do with how you used it?? Is there a different figure that you meant to use?
I am somewhat uncomfortable replacing the two RF transistors with two 3 ohm resistors.? I think what you wish to simulate is the switching circuit that supplies switched power to the two RF transistors.? M1 (or M1 and M2) switches the DC power on and off to the RF transistors represented by M4 and M5, or by the 3 or 1.5 ohm resistors which are the effective DC load that the RF transistors present to M1.? I think it's likely that the effective DC load presented by the RF transistors can vary a lot, so 3 ohms would be a VERY rough approximation.? And of course you also have the filtering of the switched supply voltage.
So, if I assume that the source pin of M1 represents the switched "DC" power to the RF transistors, then I think I understand your connection from the "VSS" pin of the UCC5304 to the source of M1.? Now, everything on that half of the UCC5304 should be referenced to that pin and that net.? The power source to the UCC5304's "VDD" pin also should be referenced to its "VSS" pin.? If it is not, imagine what happens when your circuit drives M1 to switch ON.? Its source pin pulls up towards +45 V.? Now you have a problem, because the UCC5304's VSS pin is around +40 V, but its VDD pin is only at +10 V (relative to the same Ground), so it has backwards DC power and it doesn't work.
I don't know how well the UCC5304 functions when its two halves move up and down fast.? That's something you need to worry about, to make sure that it works.? Also, there is likely more to the RF part of the circuit, such as supply filtering and bypassing.
What is the question (the "riddle")?? Is there still a question?? In your earlier circuit, you did not have a connection or path from the UCC5304's "VSS" pin to ground.? But in the photo 11.png, it does.
Andy
