Download Measuring Input Impedance If we want to measure the input

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Transcript
Measuring Input Impedance
If we want to measure the input impedance of a circuit, we expect that it will be pulling only a small
current. Regular current meters are not terribly good for accurately measuring small currents.
R1
+
+
+
V2
-
V1
Zin
-
An easy way to measure small input currents is to use a fixed resistor, as in the diagram above. Measure
the voltage at V1 and V2, so the input current is then Iin = (V2 – V1) / R1. The input impedance of the
circuit under test is then found from Zin = V1/Iin. The impedance may be frequency dependent, do the
voltage source can be DC or AC, depending on which impedance you want to measure.
Measuring Output Impedance
Output impedance may also be determined using a similar technique. A fixed load resistor is used and
the output voltage is measured first without the load (not pictured), then with the load (as pictured).
Vout, Zout
+
R1
V0
-
In the diagram above, Zout is the output impedance of the circuit to be measured. If you measure the
open circuit voltage V (with no load), then add the load resistor, R1 and the measured voltage is now V0
as pictured. The voltage drop across Zout is V – V0, the output current is Iout = V0/R1, and so the output
impedance is Zout = (V-V0) / (V0/R1).