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Input Impedance

When calculating the output impedance we still assumed an infinite input impedance. In this section we will calculate the finite input impedance assuming a zero output impedance. We consider a model that assumes an internal resistor connecting the inverting and non-inverting input terminals of the op-amp as shown in figure 6.31

Consider an inverting amplifier and remove the input resistor so that the input impedance can be calculated directly at the amplifier's input terminals.

Figure 6.31:  Model for calculating the input impedance of the inverting amplifier.

The input impedance is defined by

and the current at the summing junction is

The current through the feedback resistor is

and the output voltage is related to by the open-loop gain

The resulting input impedance is thus

For large A

The closed-loop input impedance is thus small and almost independent of the large of the operational amplifier.

Now consider the non-inverting amplifier shown in figure 6.32.

Figure 6.32:  Model for calculating the input impedance of the non-inverting amplifier.

The student should calculate the input impedance by recognizing that is much less than , since is much greater than or . Your result should be

where is the closed-loop gain of the amplifier. Notice that in contrast to the low input impedance for the inverting amplifier, the non-inverting amplifier exhibits a closed-loop input impedance that is much larger than the open-loop value .

Doug Gingrich
Tue Jul 13 16:55:15 EDT 1999