[Amps] 50V FET output transformer

[Larry Benko]

Dan,

I have built approximately 50 RF transformers in the past several 
years.  There are 2 types of loss in these circuits: dissipative loss 
(think heat) and mismatch loss (think uncontrolled leakage inductance, 
wrong transmission line Zo in the transformer etc.). Dissipative loss in 
the transformer consists of copper loss in the wires and heat in the 
ferrite cores.  Generally mismatch loss is the problem.  In your case 
the Zo of the 2 transformers needs to be 6.26 ohms and 25 ohms, leads 
need to be short, and the ferrite choking impedance needs to be large 
enough to balance the amp output currents.  Are you sure that if you 
loaded your amp with 3.125 + j0 ohms (50/16) you would be happy with the 
efficiency?  I generally use a 50 ohm (or 25 or 12.5 ohm) dummy loads 
connected to a large L network (very low loss) and use a VNA to get the 
desired impedance at the connection point to the amp.  Then you can see 
if your amp would work better with a load that is slightly reactive (it 
probably will).

Approaching this problem in a logical manner will show you where the 
problem is.

73,
Larry, W0QE


On 7/22/2014 11:49 AM, Dan Simmonds wrote:
> Has anyone built an output transformer for a 50V LDMOS FET (Freescale 
> device) that could lend some help with this?
>
> By my estimation a 16:1 constructed of (2) 4:1 Guanella’s should give 
> a satisfactory match to 50 ohms. The XFMR I've built currently only 
> gives about 33% efficiency.
>
> Thanks
>
> Dan KK3AN
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