EMRFD Message Archive 5261

Message Date From Subject
5261 2010-09-27 09:36:26 Tim Push-pull output magnetics styling
Having looked at WA2EBY's MOSFET amp, and NM0S's recently published 50 Watt homebrew challenge design (MOSFET class E done with envelope restoration... gotta learn that!), I notice the output magnetics both are very similar.

The DC power comes to the MOSFET drains comes in through a bifiliar (effectively center-tapped) toroid transformer... kinda like a balanced choke.

But the RF power output is not drawn through a winding on that choke. Instead the RF output power goes out through a binocular transformer that is not center tapped; it simply bridges drain to drain on the MOSFET.

This keeps DC current out of the output transformer. Is this a big advantage in the 50W to 100W range? I know for audio output transformers, it is harder to do a broadband single-ended transformer than a push-pull transformer because of unbalanced DC currents... but for RF both NM0S and WA2EBY sidestep this whole issue by having a bifiliar choke for power and a binocular core (no net DC current) for RF output. Are there other subtleties to this kind of design that I'm not catching at first glance?

In particular, why not simply tap the RF output winding onto the same transformer/choke already in there?

Tim N3QE
5267 2010-09-27 18:15:32 kb1gmx Re: Push-pull output magnetics styling