Post #3738001

2026-07-10 09:02 UTC

Hello #hamradio #electronics folks, maybe you can help me - I have a serious problem in my understanding of RF power transformers. Often you see designs that have two ferrite sleeves, and there is a copper or brass tube on each side forming the primary windings connected to the transistors, along the lines of the attached picture. Now, in push pull designs I've often seen proposals which use the left brass tube as the primary winding for Transistor 1, and the right brass tube as the primary winding for Transistor 2. The secondary windings go through both holes. Now, my problem with this construction is the following: if I have one transistor on for each half wave and one transistor off (i.e. something lile Class B or AB), one of the brass tubes ideally is always an open circuit. This means that the secondary windings going through that side basically form a high impedance ferrite choke with no induction from a primary. The result being that the output impedance is gigantic and no powerful current can form. So in my mind, this design does not work, or if it does, only by coincidence when mosfet drain output impedances are low even in the off state because of output capacitance. What am I missing?

Replies (2)

@upofadown@mstdn.ca 2026-07-10 17:05

@Quantensalat@scicomm.xyz Would this fall under generic transformer magic? A transformer driven on the primary without a load on the secondary is just an inductor. That inductance keeps the transformer from taking power from the source. Adding a load to the secondary cause the changing magnetic flux in the coil to induce current in the secondary. That makes the primary no longer look like a pure inductance. It now looks somewhat resistive. So is the answer: because of the load?

Open ##3738000
@Quantensalat@scicomm.xyz 2026-07-11 05:20

Can we for concreteness discuss this DIY arrangement from https://homebrewcorner.blogspot.com/p/pa-150-hf-linear-200w-amplifier.html just as an example for the kind of design I am talking about. This apparently uses such a push pull configuration with two separate core halves. The OM says they get appropriate power levels out of this and there is no reason to doubt the measurements shown. But how? Is this due to high quiescent current settings making the "off" transistor still do some work on the other half wave, or do the parallel caps work some magic here? #hamradio #electronics

Open ##4188216