A constant-gm and high-slew-rate operational amplifier for an LCD driver

To drive the backplane of a liquid crystal display device and achieve different kinds of grey levels, a high-slew-rate operational amplifier with constant-gm input stage is presented. A Zener-diode structure is inserted between the tails of the complementary input pairs to keep the gm of the input stage constant. A novel slew rate enhancement circuit is implemented to achieve a very high slew rate. The chip has been implemented in a 0.5 μm CMOS process and the chip area of the operational amplifier circuit is 0.11 mm2. The testing results indicate that in the 5–8 V input range, the maximum gm fluctuation is only 4.2%. The result exhibits a high slew rate of 111 V/μs and 102 V/μs for the rising and falling edges under a 20 pF capacitance load, and the low frequency gain is up to 109 dB with a phase margin of 70 °C.