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# Inverting Amplifier

An Op-Amp circuit producing the output which is out of phase with respect to its input by 180° is called Inverting Amplifier. This means that if the input pulse is positive, then the output pulse will be negative and vice versa. Figure 1 shows an inverting amplifier built using an Op-Amp and two resistors. Here the input signal is applied to the inverting terminal of the Op-Amp via the resistor Ri while its non-inverting terminal is grounded. Further the feedback necessary to stabilize the circuit, and hence to control the output, is provided through the feedback resistor Rf.

Mathematically the voltage gain offered by the circuit is given as Where, However, it is to be noted that ideal OpAmps have infinite input impedance due to which the currents flowing into its input terminals are zero i.e. I1 = I2 = 0. Thus, Ii = -If. Further, as the non-inverting terminal is grounded (V2 = 0), one gets V1 = 0 due to the fact that the voltages at the inverting and the non-inverting terminals of an ideal OpAmp are equal. Hence, Substitution V1 and If in equation of Vo yields This indicates that the voltage gain of the inverting amplifier is decided by the ratio of the feedback resistor to the input resistor; with the minus sign indicating the phase-reversal. Further, it is to be noted that the input impedance of the inverting amplifier is nothing but Ri. Inverting amplifiers exhibit excellent linear characteristics which make them ideal to be used as DC amplifiers. Moreover they are often used to convert input current to output voltage in the form of Transresistance or Transimpedance Amplifiers. Further these can also be used in audio mixers when used in the form of Summing Amplifiers.

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