ELT / TCT Reference Sheet

Measuring Input Impedance


Knowing the input impedance, ZIN, of a device is often useful to the technician. The technique outlined below will allow the technician to easily and accurately measure the input impedance of a unit.


Procedure For Measuring Input Impedance:


  1. Connect the input source, usually a function generator, to the input of the unit under test (UUT) through a series resistor, RS, as shown below. The value of the resistor should be approximately equal to the theoretical input impedance of the UUT.

  2. Set the output of the signal source to an acceptable range. In the case of an amplifier, be sure the input is not so large that it will result in distortion of the amplifier output signal.

  3. Carefully measure the signal at the output of the signal source, VS , and the signal at the input of the UUT, VIN.

  4. The input impedance may now be found using: