7-66. 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 using the

following formula:

- E OUT

I fdbk =

R2

NOTE: The minus sign indicates that EOUT is 180 degrees out of phase with

EIN and should not be confused with output polarity.

7-67. Since no current flows into or out of the inverting input of the operational

amplifier, any current reaching point A from R1 must flow out of point A through R2.

Therefore, the input current (IIN) and the feedback current (Ifdbk) must be equal. Now we

can develop a mathematical relationship between the input and output signals and R1 and

R2 by computing the following:

IIN = Ifdbk

By substitution:

E IN - E OUT

=

R2

R1

If you multiply both sides of the equation by R1:

-(EOUT)(R1)

EIN =

R2

If you divide both sides of the equation by EOUT:

E IN

R1

=-

R2

E OUT

By inverting both sides of the equation:

EOUT

R2

=-

E IN

R1

You should recall that the voltage gain of a stage is defined as the output voltage divided

by the input voltage:

E OUT

E IN