This is the first part in the series of differential amplifier. this part gives the details of the fundamental concepts of differential amplifier. it covers difference between single ended and differential operation. it gives the advantages of differential operation. The biggest advantage of differential operation is its noise immunity. Another advantage , direct coupling (without the use of capacitors) can be used to couple different stages of Differential amplifier. This makes it most suitable candidate for IC fabrication. Differential amplifier accepts two input signals and amplifies the difference between these input signals. If a common mode signal (same signal to both the inputs) is applied then the output is zero ideally. Practically, the transistor pair used in differential amplifier is not symmetrical ,so the amplifier gives some output corresponding to common mode signal. common mode signals are noise signals picked up by both inputs of differential amplifier. As differential amplifier rejects the common mode signal, it is used as the first stage of amplification in OP-AMPs.Common mode rejection ration is the figure of merit of a differential amplifier. It is the ratio of differential gain to common mode gain. More the value of CMRR, better is the common mode signal rejection of this amplifier.