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 Measuring Input and Output Impedance

If you are using a digital multimeter, you should check the frequency response of the meter by measuring the output voltage of the tone generator at various frequencies. Some multimeters are designed to measure only a small range of A.C. frequencies. If the voltages change significantly (more than 1 or 2 percent), you must take that into consideration in the various measurements you make. If you have a 'true RMS' meter, you are likely going to get accurate readings over the entire audio spectrum.

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=10.1 mv and V1=10mV then Iin=0.1mV/10k = 10uA.
The input impedance would then be 10mV / 10uA = 10kOhms.

Calculate imput impedance

R1 = Ohm kOhm MOhm
V1 = mV V
V2 = mV V
Input Impedance = Ohms

Output impedance may also be determined using a similar technique. A fixed load resistor is used and the output voltage is measured first with full load, then without the load.
In the diagram above, Zo is the output impedance of the network to be measured. The term network is a general term, as the circuit could be an amplifier, filter oscillator, etc. The network is shown as a the vin in source.
  • Firstly the load resistor, Rl is removed and output voltage measured (V).
  • Next the load resistor,Rl is placed in the circuit and the voltage measured again (Vo).
  • The voltage drop across the output impedance,Zo becomes V - Vo, the series current,
  • Io becomes Vo / Rl,
  • and the output impedance is (V-Vo) / Vo/Rl or rearranging Zo= Rl(V-Vo) / Vo.

Calculate output impedance

Rl = Ohm kOhm MOhm
V = mV V
Vo = mV V
Zo = Ohms