CMOS class AB folded cascode operational amplifier for high-speed applications

A low noise, high slew rate CMOS operational amplifier receiving an input signal equal to the difference between first and second input signals, the low noise, high slew rate CMOS operational amplifier comprising:

• (a) a translinear transconductance stage including first and second input transistors and third and fourth input transistors, a source of the first input transistor being coupled to a source of the fourth input transistor, a source of the second input transistor being coupled to a source of the third input transistor, gates of the first and third input transistors being coupled to change in response to the first input signal, gates of the second and fourth input transistors being coupled to change in response to the second input signal, the translinear transconductance stage being operative to produce a square-law first current equal to the sum of drain currents of the first and second input transistors;
• and
• (b) a folded cascode operational amplifier including a differential input stage coupled to receive the input signal, a folded cascode stage coupled to the differential input stage, and a class AB output stage coupled to the folded cascode stage,
• the differential input stage receiving a tail current equal to or proportional to the square-law first current to provide input transconductance substantially more constant than is achievable using a constant value of the tail current for the low noise, high slew rate CMOS operational amplifier for a substantial range of magnitudes of the input signal.