Rail-to-rail input stage circuit with constant transconductance

A method in a rail-to-rail input stage circuit comprising;

• receiving an input signal at a first differential pair circuit comprising a first pair of matched transistors having control terminals coupled to a pair of input terminals and a first current source transistor;
• shifting up the voltage level of the input signal using a level-shifted differential pair circuit coupled with the first differential pair circuit, the level-shifted differential pair circuit having control terminals coupled to the pair of input terminals;
• generating a substantially constant transconductance in the rail-to-rail input stage circuit using a constant transconductance generation circuit, the constant transconductance generation circuit:
• generating a current through a first transistor corresponding to a current in the first differential pair circuit, the first transistor having a control terminal coupled to a control terminal of the first current source transistor in the first differential pair circuit;
• receiving the current through the first transistor in a second transistor, the second transistor configured in series with the first transistor and having a control terminal coupled to a voltage based on one or more voltages on the pair of input terminals;
• and
• mirroring a second current through a current mirror transistor to the level-shifted differential pair circuit, the current mirror transistor having a first terminal coupled to a bias current and the current in the first and second transistors, wherein the second current is based on a difference between the bias current and the current in the first and second transistors.

A rail-to-rail input stage circuit comprising;

• means for receiving an input signal at a first differential pair circuit comprising a first pair of matched transistors having control terminals coupled to a pair of input terminals and a first current source transistor;
• means for shifting up the voltage level of the input signal using a level-shifted differential pair circuit coupled with the first differential pair circuit, the level-shifted differential pair circuit having control terminals coupled to the pair of input terminals;
• means for generating a substantially constant transconductance in the rail-to-rail input stage circuit using a constant transconductance generation circuit, the constant transconductance generation circuit comprising:
• means for generating a first current corresponding to a current in the first differential pair circuit;
• means for coupling the first current configured in series with the means for generating the first current, said means for coupling the first current being activated by a voltage based on one or more voltages on the pair of input terminals;
• and
• means for mirroring a second current to the level-shifted differential pair circuit, wherein the second current is based on a difference between a bias current and the first current from the means for coupling the first current.