Re: LDMOS mounting inductance confusion.


Chris Bartram G4DGU
 

Hello Ian,

Am I missing something, or what's wrong with bolting the PCB flat down onto the spreader and filling the gap between the gate leads and the topside of the board with honest, reliable metal?
Certainly, with a balanced amplifier, the whole point is to present the correct differential impedance to the devices. That will be present at the device terminals even if the microstripline groundplane is 'floating'.

Connecting the groundplane firmly to ground will make little difference to the differential impedance, but will - if the 'designer' has even thought about the problem - cause a significant change in the common-mode impedance presented to the devices. That could, in turn, lead to 'parallel-mode' instabilities.

What's missing in your analysis is the change of characteristic impedance presented in both common and differential mode impedances by changing the effective thickness of the tracks. Conductor thickness is a significant factor when designing microstriplines.

The lossy nature of many components at RF, and particularly microwaves, can hide many potential problems!

If I seem a little jaundiced regarding the design of amateur amplifiers, I am! However, I'm not prepared to discuss details in public ...

73

Chris G4DGU

73

Chris G4DGU

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