How do you measure the impedance of a broadband transformer for a RF
transistor amplifier? I'm referring to the input and output broadband
tranformers just before and after the transistor finals.

From: Starz and Stripes on Jul 2, 8:50 pm


How do you measure the impedance of a broadband transformer for a RF
transistor amplifier? I'm referring to the input and output broadband
tranformers just before and after the transistor finals.

With a broadband network analyzer and a set of resistive
terminations...over a wide bandwidth of frequencies. To be
very precise. But, you don't have to be that precise.

It's no different than checking an old audio transformer, such as
a vacuum tube output amplifier's transformer. Just count the
turns in each, square the ratio of the number of turns, and
you have the "impedance ratio." That's a rough approximation
which usually works well for most things.

All the answers depend on what you want to find out about this
solid-state RF amplifier. Please give more details on desires.

Most of the Motorola App-Note designs are detailed enough to
get the data direct, including designing your own particular
broadband transformer thingy. Helge Granberg did a nice job
very detailed in those App Notes.

How do you measure the impedance of a broadband transformer for a RF
transistor amplifier? I'm referring to the input and output broadband
tranformers just before and after the transistor finals.

You don't! The broadband transformer reflects the load impedance back to the
driving stage. In other words, if the turns-ratio is 1:1 then the input
impedance of the transformer is equal to the load impedance.

If your load is 50 Ohms and you want to match a 10-watt transmitter, with a
12v supply, then you need to calculate the impedance ratio. The transmitter
output stage impedance (in Ohms) is

(Vsupply squared) / Watts

With 12 v supply, the output impedance of a 10- watt amplifier is
(12 x 12) / 10 = 144 / 10 = 14.4 Ohms.

The transformer impedance ratio is therefo
14.4 : 50 = 1 : 3.5

The transformer turns ratio is therefore Sqrt ( impedance ratio)
= 1: 1.8 (close enough to 1:2) step-up.