Bipolar rail-to-rail input stage with transconductance correction

A bipolar rail-to-rail input amplifier with transconductance correction, comprising:

• an input stage, comprising:
• a first differential pair comprising first and second PNP transistors having their bases connected to receive a differential input signal and arranged to conduct respective collector currents which vary with said differential input signal, said differential input signal having a common mode voltage Vcm,
• a second differential pair comprising first and second NPN transistors having their bases connected to receive said differential input signal and arranged to conduct respective collector currents which vary with said differential input signal,
• a first emitter degeneration resistor connected between the emitter of said first PNP transistor and a first common mode node,
• a second emitter degeneration resistor connected between the emitter of said second PNP transistor and said first common mode node,
• a third emitter degeneration resistor connected between the emitter of said first NPN transistor and a second common mode node,
• fourth emitter degeneration resistor connected between the emitter of said second NPN transistor and said second common mode node,
• a tail current source circuit which receives a reference voltage Vref and provides a tail current IT to said first common mode node when Vcm<Vref, provides said tail current IT to said second common mode node when Vm>Vref, and provides tail current IT/2 to each of said first and second common mode nodes when Vcm=Vref, and
• a current shunt circuit, comprising:
• third and fourth PNP transistors, the emitters of which are connected to the emitters of said first and second PNP transistors, respectively, and the bases and collectors of which are connected together at a summing node,
• third and fourth NPN transistors, the emitters of which are connected to the emitters of said first and second NPN transistors, respectively, and the bases and collectors of which are connected together at said summing node,
• said current shunt circuit arranged to sense the difference in voltage between said common mode nodes and to shunt current around said first and second differential pairs to reduce the tail current said pairs would otherwise conduct and thereby lower their respective transconductances when Vcm≈Vref, the magnitude of said current reduction varying with said sensed voltage, and an output stage connected to receive the collector currents of said first and second differential pairs and to produce an output voltage in response.