Fully Differential Rail-to-rail Input Opamp With Novel Constant-transconductance Control In 120nm CMOS Technology

A fully differential operational amplifier featured with rail-to-rail input and output performance, realized by a novel method of constant small- and large-signal behavior control as well as a new symmetrical differential class-AB output is proposed. The small-signal deviation is suppressed within only 1.6%, which is the best result achieved among all the constant behavior rail-to-rail input-stage designs. With suitable compensation, the opamp is able to drive heavy ohmic load down to 100ohm or capacitive load higher than 50pF. Also the total harmonic distortion is reduced owing to the symmetrical push-pull structure for the output stage. With a 1.5V power supply and a capacitive load of 10pF, the low-frequency gain and unity-gain bandwidth of the opamp are 77dB and 4.7MHz, respectively. The chip was fabricated in 120nm digital CMOS technology

INTRODUCTION

In low-power designs in 120nm CMOS, where low supply voltage is essential, rail-to-rail inputs are desirable. As a core and fundamental block of the analog section, many operational amplifiers also require the rail-to-rail input stage. This leads to a favourable signal-to-noise ratio in the analog section of a system-on-chip. Moreover, a noisy environment, in which other factors than electronic noise dominate the signal quality, may also call for rail-to-rail construction. Additionally, constant small-and large-signal behavior over the entire common-mode input range becomes indispensable in design of rail-to-rail input, because the inconstant transconductance and slew rate of an operational amplifier cannot keep a fixed unity gain frequency and will further launch issues in behavioral modelling for the analog cells in the system level[1], [6]. However, due to the transistor threshold voltage of about 0.4V and the supply voltage of 1.5V at maximum, the common-mode input voltage range of conventional op-amp topologies in 120nm CMOS is limited to about 0.2V to 0.4V, which is unacceptable for many applications.