3-23Since no current flows into or out of the inverting input of the operational amplifier, any currentreaching point A from R1 must flow out of point A through R2. Therefore, the input current (I_{in}) and thefeedback current (I_{fdbk}) must be equal. Now we can develop a mathematical relationship between the inputand 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 E_{out}: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 inputvoltage:Therefore, the voltage gain of the inverting configuration of the operational amplifier is expressed bythe equation:(As stated earlier, the minus sign indicates that the output signal is 180 degrees out of phase with theinput signal.)Noninverting ConfigurationFigure 3-16 shows a noninverting configuration using an operational amplifier. The input signal (E_{in})is applied directly to the noninverting (+) input of the operational amplifier. Feedback is provided by

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