Differential amplifier having rail-to-rail input capability and square-root current control
First Claim
1. 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, a second end range extending to the second supply voltage, and an intermediate range extending between the end ranges, the circuit comprising:
- first differential means for amplifying a differential input signal by largely dividing a first tail current into a pair of first main currents whose difference is representative of the input signal when its common-mode voltage VCM is in the intermediate and first ranges;
second differential means for amplifying the input signal by largely dividing a second tail current into a pair of second main currents whose difference is representative of the input signal when VCM is in the intermediate and second ranges; and
square-root means comprising a pair of primary control FETs having respective gate-to-source voltage whose sum is held largely constant during normal circuit operation, for controlling the tail currents so that the sum of their square roots is largely constant during circuit operation as VCM traverses the entire intermediate range and the end ranges.
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Abstract
A differential amplifier contains a pair of differential portions (10 and 12) that together provide representative signal amplification across the full amplifier power-supply voltage range. Each differential portion normally contains a pair of like-polarity differentially coupled FETs (Q1 and Q2, Q3 and Q4) that divide a tail current (IN, IP) into a pair of main currents (I1 and I2, I3 and I4). The two FET pairs are complementary. A square-root circuit (24) controls the tail currents in such a way that the sum of their square roots is largely constant. Consequently, the amplifier transconductance is largely constant.
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Citations
32 Claims
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1. 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, a second end range extending to the second supply voltage, and an intermediate range extending between the end ranges, the circuit comprising:
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first differential means for amplifying a differential input signal by largely dividing a first tail current into a pair of first main currents whose difference is representative of the input signal when its common-mode voltage VCM is in the intermediate and first ranges; second differential means for amplifying the input signal by largely dividing a second tail current into a pair of second main currents whose difference is representative of the input signal when VCM is in the intermediate and second ranges; and square-root means comprising a pair of primary control FETs having respective gate-to-source voltage whose sum is held largely constant during normal circuit operation, for controlling the tail currents so that the sum of their square roots is largely constant during circuit operation as VCM traverses the entire intermediate range and the end ranges. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27)
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28. 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, a second end range extending to the second supply voltage, and an intermediate range extending between the end ranges, the circuit comprising:
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first differential means for amplifying a differential input signal by largely dividing a first tail current into a pair of first main currents whose difference is representative of the input signal when its common-mode voltage VCM is in the intermediate and first ranges; second differential means for amplifying the input signal by largely dividing a second tail current into a pair of second main currents whose difference is representative of the input signal when VCM is in the intermediate and second ranges; and square root means comprising four like-polarity control FETs, each having respective gate electrodes and sources, coupled together in a loop by way of their gate electrodes and sources, whereby the sum of their gate-to-source voltages around the loop equal zero, for controlling the tail currents so that the sum of their square roots is largely constant during circuit operation as VCM traverses the entire intermediate range and the end ranges. - View Dependent Claims (29, 30, 31, 32)
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Specification