From the AC impedance triangle, the input or output impedance of a two terminal network can be determined by measuring the small signal AC currents and voltages. For the input, the voltage is measured across the input terminals and the current measured by inserting the meter in series with the signal generator.
Use a fixed frequency say 1kHz and set the generator level to around 20 mV RMS. For example, if you read 20mV RMS and 10uA for current, then the impedance is 2k. With high impedance circuits, the current will become very small and difficult to measure, so an alternative method is called for.
An easy way to measure small input currents,is to use a fixed resistor, as in the diagram above. Measure the AC voltage at points V1 and V2, then the input current, Iin becomes: (V2 – V1) / R1. The input impedance of the circuit under test is then found from V1 / Iin.
If you use a 10k resistor for R1 and measure V2=10.1mv and V1=10mV then Iin=0.1mV/10k =10uA. The input impedance would then be 10mV / 10uA or 1K.
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