Re: Analyzing a PA


Timothy Fidler
 

The formula for a PP amp whether  CT type or phasing transformer type  is    Z  drain to drain  =  2 * ( Vcc- Vmin) squared /  Power out.   all in consistent units.   Vmin is sometimes taken to be  2 V but that is a guess.  It is whatever the in circuit parameters make it.  This is a completely different formula for single ended RF amps and older versions of the HB skipped around this issue which pretty poor and confused me way back in the day. Perhaps it is fixed now in that August document.

The Z thusly calculated is what the  reflected  load impedance on the secondary of the transformer needs to be matched to approximately.   ON top of that the  unloaded impedance of the PP transformer primary  needs to be high enough so it does not shunt all the supplied RF power.   Typically it  would  be  designed at  4 or 5 x the Z previously calculated at the lowest frequency of operation. By making that decision you may end up choking off the power at the HF end via mechanisms like the  parasitic inductance of the primary and secondary windings.  The elimination of parasitic inductance or almost elimination in the primary is one of the pluses of  Cu tube type high power RF transformers.

It's complicated. ... :-) )

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