Input and Output Impedances

Impedance is one of those concepts in radio that can be hard to fathom conceptually. If you read a description of what impedance is, it generally goes something like “The resistance in a circuit by the combined effects of resistance and reactance” With reactance being “The effect of capacitance and inductance causing the current to be out of phase with the EMF causing it

All in all, it sounds like a bunch of gobbledygook to me. What I think is important to know, as a total beginner like me, is that impedance exists and that mismatches in impedance between circuits leads to loss and inefficiency and at other times, probably some other nasties as well. So, if we know it exists, even if we cannot conceptualise the concept, how can we measure it so we can design transformers to match the output impedance of once circuit with the input of another.

All that is needed is a variable resistor or decade resistance box, signal source and an oscilloscope and understanding a basic principle that if you vary the input or output load resistance until you have a 50% drop in the output amplitude, the resulting resistance is equal to the input or output impedance.

Input Impedance


Starting with the variable resistor at 0 ohms and working up, output amplitude decreases, when amplitude is 50% reduced, resistance of the variable resistor = input impedance.


Output Impedance


With output impedance you want to start with variable resistor at maximum and decrease its value till the output amplitude is 50% of the no load value. At this point, resistance again equals output impedance.

Alternate Method

Do not have a scope or power meter, this is no problem either as you can acheive the same using nothing more than a fixed resistance and a multi meter. For this method you take 2 measurements, voltage at no load and under a fixed load. With those values you can then use the following equation to determine the output impedance.

Output Impedance = Rload(Vopen/Vload -1)

Or, Output Impedance is equal to the Load Resistance multiplied by the (open voltage divided by the load voltage minus 1). Both methods will give very similar results and if applied to things like amp chains, will allow the builder to match input and output impedances across stages and reduce losses in the system.

Images are from: HERE  Also this site is a very good resource for all things introduction to electronics theory and practice.



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