Measuring Amplifier AC Impedance



Knowing the impedance of an amplifier or any electronic circuit can be very useful, for lower frequency applications it’s desirable to have the source impedance be much lower than the load impedance to minimize voltage drop, in RF applications and where maximum power transfer is needed it’s better to match impedance to avoid reflections, the latter is a bit more complicated so in this article we will only be considering lower frequencies, typically below 10MHz.

Making the measurement

The only things you need to measure the impedance is a variable resistor (potentiometer), a signal generator and a multi-meter to measure resistance and AC voltage, an oscilloscope is also desirable as multi-meters generally do not measure signals of higher frequency that well.

Ensure your potentiometer is rated to handle the signal power

For measuring input impedance naturally you want the variable resistance to be at the input to your amplifier, while with output impedance you want it to be at the output, the goal is to adjust the resistance until the signal voltage drops by half the input value, the signal frequency should be the nominal expected frequency for the amplifier in question, I.E for audio you may want to start at 10kHz.

If you need to use an AC coupling capacitor make sure it’s large to avoid signal attenuation.

So for example to measure input impedance you set your frequency generator to 10kHz sine, with an output amplitude that is typical for your amplifier, let’s say 50mV RMS, you then measure the voltage on the amplifier side of the variable resistor, adjusting until you read an AC voltage of 25mV, then it’s just a simple case of measuring the resistance of the variable resistor which will give your impedance.

This works by forming a voltage divider between the signal source and the load, a voltage drop of half must mean the impedance is matched so both the variable resistor and the load will be of equal value.

This works great to give you the real component of the impedance, however it tells you nothing about the reactance, which in some situations, particularly RF is quite important to know, regardless this method is cheap and reasonably quick.