3-23
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.
Mathematically:
If you multiply both sides of the equation by R1:
If you divide both sides of the equation by Eout:
By inverting both sides of the equation:
You should recall that the voltage gain of a stage is defined as the output voltage divided by the input
voltage:
Therefore, the voltage gain of the inverting configuration of the operational amplifier is expressed by
the equation:
(As stated earlier, the minus sign indicates that the output signal is 180 degrees out of phase with the
input signal.)
Noninverting Configuration
Figure 3-16 shows a noninverting configuration using an operational amplifier. The input signal (Ein)
is applied directly to the noninverting (+) input of the operational amplifier. Feedback is provided by
