Input and output matching - UK Vintage Radio Repair and Restoration Discussion Forum

dmowziz · 19th Dec 2022, 5:50 pm

Came across this (highlighted) yesterday... Except to transfer max power, is matching unimportant?
I've always seen that a mixer should be terminated properly (although no experience with any problem created).

If max power is not transferred and perfect reactive elements are used for matching, where will the remaining power go to? (am I missing something?)

Right now, past 2 days been trying to match the large signal input impedance of a transistor to 50 ohms and at the same time, also predict the voltage at the base of the transistor.
I'm trying to do this to predict the output current at the 2nd harmonic (given the base voltage) and also provide impedance match at the input... Is this waste of energy?

Thanks

G6Tanuki · 19th Dec 2022, 5:57 pm

Assuming we're talking resonant systems, power not absorbed by the load is reflected... but contrary to popular belief regarding "SWR" it is _not_ dissipated in the source, it gets reflected again.

Though the resultant voltages/currents can be damaging to both the power-source and the matching components.

"Loaded Q" being the important thing here.

Smart designers dealing with potential harmonic currents use a LPF/HPF network to separate them out [just as a loudspeaker crossover-network separates the audio and feeds it appropriately to your woofers and tweeters] and dumps the unwanted harmonic energy safely into a load-resistor.

19th Dec 2022, 5:50 pm	#1
dmowziz Pentode Join Date: Nov 2021 Location: Toronto, Canada Posts: 131	Input and output matching Came across this (highlighted) yesterday... Except to transfer max power, is matching unimportant? I've always seen that a mixer should be terminated properly (although no experience with any problem created). If max power is not transferred and perfect reactive elements are used for matching, where will the remaining power go to? (am I missing something?) Right now, past 2 days been trying to match the large signal input impedance of a transistor to 50 ohms and at the same time, also predict the voltage at the base of the transistor. I'm trying to do this to predict the output current at the 2nd harmonic (given the base voltage) and also provide impedance match at the input... Is this waste of energy? Thanks Attached Thumbnails

19th Dec 2022, 5:57 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Input and output matching Assuming we're talking resonant systems, power not absorbed by the load is reflected... but contrary to popular belief regarding "SWR" it is _not_ dissipated in the source, it gets reflected again. Though the resultant voltages/currents can be damaging to both the power-source and the matching components. "Loaded Q" being the important thing here. Smart designers dealing with potential harmonic currents use a LPF/HPF network to separate them out [just as a loudspeaker crossover-network separates the audio and feeds it appropriately to your woofers and tweeters] and dumps the unwanted harmonic energy safely into a load-resistor. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.