3-22
You can compute the values for view (C) and prove that point A in that circuit is also at virtual
ground.
The whole point is that the inverting input to the operational amplifier shown in figure 3-13 is at
virtual ground since it is at 0 volts (for all practical purposes). Because the inverting input is at 0 volts,
there will be no current (for all practical purposes) flowing into the operational amplifier from the
connection point of R1 and R2.
Given these conditions, the characteristics of this circuit are determined almost entirely by the values
of R1 and R2. Figure 3-15 should help show how the values of R1 and R2 determine the circuit
characteristics.
Figure 3-15.—Current flow in the operational circuit.
NOTE: It should be stressed at this point that for purpose of explanation the operational amplifier is
a theoretically perfect amplifier. In actual practice we are dealing with less than perfect. In the practical
operational amplifier there will be a slight input current with a resultant power loss. This small signal can
be measured at the theoretical point of virtual ground. This does not indicate faulty operation.
The input signal causes current to flow through R1. (Only the positive half cycle of the input signal
is shown and will be discussed.) Since the voltage at the inverting input of the operational amplifier is at 0
volts, the input current (Iin) is computed by:
The output signal (which is opposite in phase to the input signal) causes a feedback current (Ifdbk) to
flow through R2. The left-hand side of R2 is at 0 volts (point A) and the right-hand side is at Eout.
Therefore, the feedback current is computed by:
(The minus sign indicates that Eout is 180 degrees out of phase with Ein and should not be confused
with output polarity.)
