Linear Op-amp Operation-Inverting And Non-Inverting Configuration - Operational Amplifiers Tutorials Series

•An op-amp can use negative feedback to set the closed-loop gain as a function of the circuit external elements (resistors), independent of the op-amp gain, as long as the internal op-amp gain is very high

•Shown at left is an ideal op-amp in a non-inverting configuration with negative feedback provided by voltage divider R1, R2
•Determination of closed-loop gain: 

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–Since the input current is assumed zero, we can write v- = R1/(R1 + R2)vOUT
–But, since v+ =~ v- for the opamp operation in its linear region, we can write

v- = vIN = R1/(R1 + R2)vOUT

or, vOUT = ((R1 + R2)/R1)vIN 

•We can derive the same expression by writing

vOUT = A(v+- v-) = A{vIN – [R1/(R1 + R2)] vOUT}  and solving for vOUT with A>>1

Linear Op-amp Operation: Inverting Configuration

•An op-amp in the inverting configuration (with negative feedback) is shown at the left
–Feedback is from vOUT to v- through resistor R2
–vIN comes in to the v- terminal via resistor R1
–v+ is connected to ground
•Since v- = v+ = 0 and the input current is zero, we can write
–i1 = (vIN – 0)/R1 = i2 = (0 – vOUT)/R2 or,

vOUT = - (R2/R1) vIN
•The circuit can be thought of as a resistor divider with a virtual short (as shown below)
–If the input vIN rises, the output vOUT will fall just enough to hold v- at the potential of v+ (=0)
–If the input vIN drops, vOUT will rise just enough to force v- to be very near 0

Input Resistance for Inverting and Non-inverting Op-amps

•The inverting The non-inverting op-amp configuration of slide 2-4 has an apparent input resistance of infinity, since iIN = 0 and RIN = vIN/iIN = vIN/0 = infinity 

•op-amp configuration, however, has an apparent input resistance of R1
–since RIN = vIN/iIN = vIN/[(vIN – 0)/R1] = R1