Inverting and non inverting amplifiers

Inverting amplifier

Definition

Inverting amplifier is one in which the output is exactly 1800 out of phase with respect to  input(i.e. if you apply a positive voltage, output will be negative). Output is an inverted(in terms of phase) amplified version of input.

Circuit operation

The inverting amplifier using opamp is shown in the figure below

inverting amplifier

inverting amplifier

Assuming the opamp is ideal and applying the concept of virtual short at the input terminals of opamp, the voltage at the inverting terminal is equal to non inverting terminal. The simplified circuit is shown in the figure below

Applying KCL at inverting node we get

                                              (0-Vi)/Ri+(0-Vo)/Rf = 0

By rearranging the terms we will get

                                           Voltage gain Av = Vo/ Vi = – Rf/Ri .

Gain

Gain of inverting amplifier Av= – Rf/Ri.

Non Inverting amplifier

Definition

Non Inverting amplifier is one in which the output is in phase with respect to  input(i.e. if you apply a positive voltage, output will be positive ). Output is an Non inverted(in terms of phase) amplified version of input.

Circuit operation

The inverting amplifier using opamp is shown in the figure below

non inverting amplifier

non inverting amplifier

Assuming the opamp is ideal and applying the concept of virtual short, the voltage at the inverting terminal is equal to non inverting terminal. Applying KCL at inverting node we get

                                     (Vi-Vo)/R2+(Vo-0)/R1 = 0

By rearranging the terms we will get

                                 Voltage gain Av = Vo/ Vi = (1+ Rf/Ri)

Gain

Gain of non inverting amplifier Av= (1+ Rf/Ri).

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