A Study on CMOS Differential Amplifier

Differential amplifier is one of the versatile circuits in analog circuit design. In CMOS technology the differential amplifier can be realized using a NMOS or PMOS differential pair. There are several tradeoffs that determine which differential pair to be used. One criterion that is considered in making the choice is the common mode input range. This specifies that over what range of commonmode voltages the differential amplifier continues to sense and amplify the difference signal with the same gain. Moreover in order to design a versatile Op-amp amplifier its input stage must consist of rail to rail input mode range capability. The simple differential pair, either made of NMOS or PMOS, cannot meet the requirements of rail to rail common mode input. A possible solution to the problem is to use both NMOS and PMOS differential pairs simultaneously [1]. The resulting compound amplifier is called as Complementary differential amplifier. This complementary differential amplifier is used as the core element of base band amplifiers [4]. With slight modifications, this complementary differential amplifier can be used in various analog and digital CMOS circuits.