An inverting amplifier uses negative feedback to invert and amplify a voltage. The $R_f$ resistor allows some of the output signal to be returned to the input. Since the output is 180° out of phase, this amount is effectively subtracted from the input, thereby reducing the input into the operational amplifier. This increases the overall gain of the amplifier and is dubbed negative feedback.
While solving OPAMP circuits, follow two rules:
- No Current Flows into the Input Terminals
- The Differential Input Voltage is Zero as $V_+ = V_- $ (Virtual Earth)
(because 
is a virtual ground)
The gain of the amplifier is determined by the ratio of $R_f$ to $R_{in}$. That is:
