In todays IC industry a solid understanding of semiconductor devices is essential for both analog as well as digital designs. In digital designs MOS transistors are considered as simple switches but in analog designs detailed understanding of the device is required because many of the second order effects directly impact the performance of circuit. Also as in the new generation of IC technologies the device sizes are shrinking these non ideal effects becomes more significant. In many of the circuit designs circuit designer has to decide which effects can be neglected in a given circuit, to decide this insight into device operation proves invaluable. Analog integrated circuits plays an important role in analog signal processing. After the invention of BJT the analog circuits based on Bipolar technology are popular and covered most of the market of microelectronics industry. In sixties it was not clear what importance CMOS technology would have on analog circuits. Now, it has become very clear that CMOS technology has become the technology of choice for analog circuit design in mixed signal environment. The CMOS technology is popular in mixed-signal implementation because it provides density and power saving on digital side and a good mix of components for analog design. Amplification is an essential sub circuit of signal processing systems. The analog signals are amplified because the signal strength is too small to drive the load by overcoming the noise of subsequent stages. The differential amplifier topology is the most important subcircuit which has become the dominant choice of todays high performance analog and mixed signal circuits. In this chapter we will analyse and design the CMOS differential amplifier pairs. The single stage and differential stage amplifier uses the current source. The current mirrors are used for both the purposes i.e. as a biasing element and signal processing component, these are discussed after detailed discussion on differential amplifiers. Operational amplifiers are an integral part of many analog and mixed signal systems.