The 4 times rule for matching transformer impedance

amdx · 2015-01-18T02:46:23+00:00

Hi all, I have always heard and used the 4 times rule for the impedance of a transformer winding. Example; To match 50 ohms to 400 ohms, the primary inductance should be 200 ohms at the lowest frequency of interest. 4 x 50 = 200 ohms XL. Tonight I'm searching for a reference for the 4 times. I found this; The math is more than I can handle, but his conclusion is; "In this article we have analyzed the impedance transformation properties of broadband RF transformers taking into account the finite inductive reactance of their primary and secondary windings. We have shown that the ?four times the impedance? design guideline for the inductive reactance of the transformer windings yields an impedance transformation that departs from its ?ideal? behavior by no more than 3%, and introduces a phase shift of no more than 14 degrees." Now some may quibble with the 3% and 14 degrees as being picked out of the air (me) but something needs to be picked and those numbers seems reasonable. Also, they reference Broadband RF transformers, Can I assume that scales to audio transformers? Mikek

Jan 18, 2015 21 Replies

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Tauno Voipio 11 years ago

Yes - the same applies when you scale the impedance down on the audio side. There is no free lunch ...

-TV

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bloggs.fredbloggs.fred 11 years ago

No originality at all- such a boring nobody of a non-entity- sad.

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