RF transformer design

[froten]

Hello all,

when designing a class A&AB RF amplifiers (for example for the range from 3Mhz to 30Mhz) using a transformer to match the laod to the transistor output impedence, the ratio of the turns can be easly calculated, but:

Q1) how the actual number of the windings is calculated?
Q2) in case the primary of the transformer is tuned , is the number of turns remains the same for the highest Q value of the tuned circuit?
Q3) what is the prefered transformer to use? is it the balun, transmition line transformer, classic transformer...?

thanx for any help

 

[FvM]

Seems that you are intending a wideband linear amplifier. It most likely shouldn't have a tuned transformer, but the output transistor(s) may have a frequency dependant impedance. Not an easy design!

Polyfet has a lot of interesting reference designs of this type, mostly using transmission line transformers. In any case, you should consider, that a RF transformer has a considerable leak inductance, it can be regarded rather as pair of coupled inductances.

 

[vfone]

https://www.frenning.dk/OZ1PIF_HOMEPAGE/AN749.pdf