Q.Write a note on ideal and practical characteristics of an op-amp.

The ideal op-amp is an imaginary benchmark whose parameters are pushed to perfect limits so that a circuit's behaviour depends only on its external components. Its defining properties are: infinite open-loop voltage gain, infinite input impedance (it draws no input current), zero output impedance, infinite bandwidth, infinite common-mode rejection ratio, and zero input offset voltage. These lead to the two working assumptions used to analyse op-amp circuits: no current flows into either input, and (with negative feedback) the two inputs sit at the same voltage.

A practical op-amp such as the IC 741 comes close but cannot reach these limits. Its open-loop gain is very large but finite (of the order of $2\times10^{5}$); its input impedance is high but finite (about $2\ \text{M}\Omega$); its output impedance is small but non-zero (about $75\ \Omega$); its bandwidth and CMRR (about 90 dB) are finite.

Two extra real-world imperfections matter. Input offset voltage is the small DC voltage (a few millivolts) that must be applied between the inputs to force the output exactly to zero, since the input-stage transistors are never perfectly matched. Slew rate is the maximum rate at which the output voltage can change, $\left(\dfrac{dV_o}{dt}\right)_{max}$, about $0.5\ \text{V}/\mu\text{s}$ for the 741; it limits large-signal high-frequency operation. Comparing these terms to their ideal values shows how a real device departs from theory.