Measuring Amplifier Output Impedance


Measuring the output impedance of an amplifier does not require technical gear like a VNA, though that will make for a nice quick and accurate measurement. You can also do it using your multi meter and a scope. Here is how:

Connect the output of the amp to the swiper of a 10K pot or better yet, a decade resistor box.


And measure the resultant waveform on the scope.


Starting with the pot set to 10K ohm, adjust the pot until you have 50% of the amplitude of the signal.


Remove the pot from the circuit and measure its resistance, this value is equal to the output impedance of the amplifier.

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You can then use a transformer turns calculator to determine the turns ratio for transformer for your circuit. Not hard now is it.