Compact low-voltage power-efficient operational amplifiers are describ
ed that are very suitable as very-large-scale-integration library cell
s because of the small die area of 0.08 mm(2) and the minimum supply v
oltage of 1.8 V. A key part of the circuit is the rail-to-rail class-A
B output stage with folded mesh feedback control that combines power e
fficiency with operation down to 1.8 V and allows sufficient gain in a
compact two-stage topology. A version with rail-to-rail input stage f
eatures a rail-to-rail input range for supply voltages down to 2.5 V.
The dc gain of the op-amps is more than 80 dB while driving 10 k Omega
, and the unity-gain frequency is 4 MHz with phase margin of 67 degree
s while driving 5 pF. The equivalent input noise voltage is 38 nV/root
Hz at a frequency of 100 kHz. The amplifiers have been implemented in
a standard digital 1.6-mu m complementary metal-oxide-semiconductor p
rocess.