Page 60 - Op Amps Design, Applications, and Troubleshooting

inverting Amplifier 43

Input Impedance. The input impedance of the amplifier shown in Figure 23
is that resistance (or impedance) as seen by the source (i>j). You will recall that the
voltage between the (+) and (-) terminals of the op amp (%) will always be about
0 unless the amplifier is saturated. Since the (+) terminal is connected to ground
(via R B) in the inverting amplifier circuit, it is reasonable to assume that the (-) pin
will always be near ground potential even though it is not and cannot be connect-
ed directly to ground. But, since the (-) input is essentially at ground potential we
call this point in the circuit a virtual ground.
Considering that the (-) pin is a virtual ground, it becomes apparent that the
input impedance seen by the source is simply R/. That is, as far as current demand
is concerned, resistor R/ is effectively connected across the signal source. The equa-
tion for input impedance then is given by Equation (2.7).

For the case of the inverting amplifier shown in Figure 2.3, the input impedance is
computed as follows:

In general, as long as the (-) pin remains at a virtual ground potential, the input
impedance will be equal to the impedance between this pin and the source. If the
impedance is more complex (e.g., resistor capacitor combination), then you must
use complex numbers to represent the impedance. The basic method, however,
remains the same.

Input Current Requirement. Ohm's Law can be used to calculate the amount
of current that must be supplied by the source. Recall that essentially no current
flows into or out of the (-) terminal of the op amp. Therefore the only current sup-
plied by the source is that drawn by Rj. Since R/ is effectively in parallel with the
source due to the effect of the virtual ground, the input current can be computed
as follows: