MOSFET single-pair differential amplifier having an adaptive biasing scheme for rail-to-rail input capability

A circuit, comprising:

• a first rail for receiving a first supply voltage;
• a second rail for receiving a second supply voltage;
• a differential amplifier circuit coupled to the first rail and the second rail and having a differential input for receiving a common-mode input voltage, the differential amplifier circuit including a differential pair of transistors;
• wherein the differential amplifier circuit comprises:
• a pair of like-polarity differentially coupled field effect transistors, the first and second transistor of the pair differentially respond to the input signal by dividing the first tail current between the currents;
• a pair of like-polarity common-gate coupled field effect transistors, the third and fourth transistors of the pair provide an active load to the differential means, the pair of like-polarity common-gate coupled field effect transistors are of opposite polarity to the pair of like-polarity differentially coupled field effect transistors;
• and
• a fifth field effect transistor that provides tail current for the pair of like-polarity differentially coupled field effect transistors;
• and
• a bias circuit coupled to said differential amplifier circuit for applying a bias voltage to the differential pair of transistors, the bias circuit controlling the threshold voltage of the differential pair of transistors in the response to the common-mode input voltage to turn the differential pair of transistors on when the common-mode input voltage is in a range extending from the first supply voltage to the second supply voltage.

A circuit, comprising:

• a first rail for receiving a first supply voltage;
• a second rail for receiving a second supply voltage;
• a differential amplifier circuit coupled to the first rail and the second rail and having a differential input for receiving a common-mode input voltage, the differential amplifier circuit including a differential pair of transistors;
• a bias circuit coupled to said differential amplifier circuit for applying a bias voltage to the differential pair of transistors, the bias circuit controlling the threshold voltage of the differential pair of transistors in the response to the common-mode input voltage to turn the differential pair of transistors on when the common-mode input voltage is in a range extending from the first supply voltage to the second supply voltage,
• wherein the biasing circuit comprises:
• a replica circuit coupled to the differential amplifier circuit for replicating half of the differential pair of transistors and for applying a voltage across the backgates of the differential pair of transistors to supply a threshold voltage such that when the common-mode voltage VCM is in the second-end range the differential pair of input transistors is operable;
• and
• a switching circuit coupled to the differential amplifier circuit and the replica circuit to turn the replica circuit on when the input common-mode voltage VCM is in the second-end range beyond the threshold voltage of the differential pair of transistors and to turn the replica circuitry off when the common-mode voltage VCM is in the first-end range.

An electronic circuit operable between first and second supply voltages which constitute a power supply range including a first-end range extending to the first supply voltage and a second-end range extending to the second supply voltage, the circuit comprising:

• a differential means having a differential pair of transistors having a first threshold voltage for amplifying a differential input signal when its common-mode voltage VCM is in the first-end range;
• a replica circuitry means coupled to the differential means for replicating half of the differential pair of transistors and for applying a voltage across the backgates of the differential pair of transistors to supply a second threshold voltage such that when the common-mode voltage VCM is in the second-end range the differential pair of input transistors is operable;
• and
• a switching means coupled to the differential means and the replica circuitry means to turn the replica circuitry means on when the input common-mode voltage VCM is in the second-end range beyond the first threshold voltage of the differential pair of input transistors and to turn the replica circuitry off when the common-mode voltage VCM is in the first-end range.