Bias circuit and differential amplifier having stabilized output swing

In a differential amplifier circuit comprising a pair of amplifier transistors having load transistors, said circuit receives as input a first input voltage and a second input voltage and outputs a differential output voltage indicative of which input voltage is greater, a bias circuit that stabilizes the differential output against variations in a first supply voltage and transistor transconductance, said bias circuit comprising:

• a bias load transistor approximately equal in size to sizes of the load transistors, a gate node of the bias load transistor coupled to gate nodes of the load transistors;
• a feedback loop to maintain a predetermined voltage at a first node of the bias load transistor and a current through a feedback path, said feedback loop maintaining said predetermined voltage at the first node of the bias load transistor independent of variations of said first supply voltage and transconductance of transistors in the circuit, and
• a current mirror coupled to the feedback loop to receive the current through the feedback path, said current mirror further coupled to the differential pair of transistors to provide a constant current value through said differential pair of transistors;
• wherein the differential output is stabilized against variations in said first supply voltage and transistor transconductance.

In a differential amplifier circuit comprising a pair of amplifier transistors having load transistors, said circuit receives as input a first input voltage and a second input voltage and outputs a differential output voltage indicative of which input voltage is greater, a method for biasing the circuit that stabilizes the differential output against variations in supply voltage and transistor transconductance, said method comprising the steps of:

• connecting a bias load transistor to the load transistors, said bias load transistor approximately equal in size to sizes of the load transistors, a gate node of the bias load transistor coupled to gate nodes of the load transistors;
• maintaining a predetermined voltage at a first node of the bias load transistor and a current through a feedback path, said feedback loop maintaining said predetermined voltage at the first node of the bias load transistor independent of variations of said supply voltage and transconductance of transistors in the circuit, and
• mirroring the current to the pair of amplifier transistors to provide a constant current value through said pair of amplifier transistors;
• wherein the differential output is stabilized against variations in said supply voltage and transistor transconductance.