Patent ID: 7265621

Claim:
An operational amplifier comprising: a pair of differential input transistors, each transistor having a source terminal, a gate terminal and a drain terminal, the pair of differential input transistors being connected together at the source terminals, the gate terminals of the pair of differential input transistors receiving a pair of differential input signals; a pair of cascode transistors, each cascode transistor having a source terminal, a gate terminal and a drain terminal, each cascode transistor being connected to the respective input transistor so as to form a cascode, the drain terminals of the pair of cascode transistors providing a pair of differential output signals; and a keep-alive circuit connected to provide first and second bias currents to the source terminals of the pair of cascode transistors and a third bias current to the source terminals of the pair of differential input transistors, the keep-alive circuit comprising: a tail current transistor having a source terminal connected to a ground potential, a drain terminal connected to the source terminals of the pair of differential input transistors, and a gate terminal receiving a first bias voltage the tail current transistor conducting the third bias current; and a pair of keep-alive transistors, a first keep-alive transistor having a source terminal connected to the ground potential, a gate terminal and a drain terminal connected together and to the source terminal of a first cascode transistor of the pair of cascode transistors to provide the first bias current, and a second keep-alive transistor having a source terminal connected to the ground potential, a gate terminal and a drain terminal connected together and to the source terminal of a second cascode transistor of the pair of cascode transistors to provide the second bias current, wherein each of the first and second bias currents is a variable current being a function of a voltage at the source terminal of the respective cascode transistor and is a variable fraction of the third bias current conducted by the tail current transistor.