Low voltage, beta immune variable gain amplifier with high supply voltage rejection
First Claim
1. An amplifier for operation between a first supply and a second supply having a lower potential than the first supply, comprising:
- a) a current multiplier circuit includinga first, differentially coupled input pair of transistors each having a base for receiving an input current of the multiplier cell, a collector coupled to the first supply, an emitter and a common base-collector junction;
a second, differentially coupled output pair of transistors each having a base coupled to said base of a respective input transistor, a collector for output ting an output current of the multiplier circuit, and an emitter; and
b) a bias circuit which biases the emitters of said input transistors with a first bias current and the emitters of said output transistors with a second bias current to control the ratio of the differential output current to the differential input current at a selected value, wherein the ratio of (i) the second bias current and (ii) the first bias current minus a current substantially equal to the base currents of the output transistors is substantially equal to the selected value of said ratio of the differential output currents to the differential input currents.
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Accused Products
Abstract
A variable gain amplifier includes a Gilbert cell multiplier cell having a pair of bipolar input transistors and a pair of bipolar output transistors. A bias circuit provides a first bias current to the input transistors and a second bias current to the output transistors, the ratio of the second to the first bias current determining the gain of the multiplier cell. The bias circuit couples the first and second bias currents to the emitters of the input and output transistors and references these currents to ground, which provides excellent supply rejection. Additionally, high beta immunity is achieved by increasing the ratio of the second bias current to the first bias current by subtracting a current from the first bias current substantially equal to the base currents of the output transistors.
29 Citations
12 Claims
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1. An amplifier for operation between a first supply and a second supply having a lower potential than the first supply, comprising:
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a) a current multiplier circuit including a first, differentially coupled input pair of transistors each having a base for receiving an input current of the multiplier cell, a collector coupled to the first supply, an emitter and a common base-collector junction; a second, differentially coupled output pair of transistors each having a base coupled to said base of a respective input transistor, a collector for output ting an output current of the multiplier circuit, and an emitter; and b) a bias circuit which biases the emitters of said input transistors with a first bias current and the emitters of said output transistors with a second bias current to control the ratio of the differential output current to the differential input current at a selected value, wherein the ratio of (i) the second bias current and (ii) the first bias current minus a current substantially equal to the base currents of the output transistors is substantially equal to the selected value of said ratio of the differential output currents to the differential input currents. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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8. A variable gain amplifier for operation between a first supply and a second supply having a lower potential than the first supply, said variable gain amplifier comprising:
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a) a multiplier circuit comprising differential input terminals which receive a differential input current; differential output terminals which output a differential output current; a pair of bipolar input transistors each having a base coupled to a respective one of said differential input terminals, a collector coupled to a first supply, an emitter, and a common base-collector junction; a pair of bipolar output transistors each having a base commonly coupled, respectively, to said base of one of said input transistors and one of said differential input terminals, a collector coupled to a respective one of said differential output terminals, and an emitter coupled to said second supply; and b) a biasing circuit comprising first and second current sources; a first, input current mirror having an input coupled to the first current source, and an output coupled to said emitter of each of said input pair of transistors, said output outputting a first, said input current mirror having a first pair of current mirror transistors having bases coupled to each other, a second, output current mirror having an input coupled to the second current source and an output coupled to each emitter of said output pair of transistors to provide a second, bias current to each of said output pair of transistors, said output current mirror having a second pair of current mirror transistors having bases coupled to each other, and a third current mirror, said third current mirror having an input coupled to said bases of said second pair of current mirror transistors, and an output coupled to said output of said first current mirror, said third current mirrors subtracting from said output of said first current mirror a third current substantially equal to the base currents of said second pair of current mirror transistors, said first current less said third current equaling a first bias current applied to said emitters of said input pair of transistors. - View Dependent Claims (9, 10)
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11. A method of biasing a multiplier cell having
a pair of bipolar input transistors each having a base coupled to a respective one of said differential input terminals, a collector coupled to a first supply, an emitter, and a common base-collector; - and
a pair of bipolar output transistors each having a base commonly coupled, respectively, to one of said input transistors and one of said differential input terminals, a collector coupled to a respective one of said differential output terminals, and an emitter, said method comprising the steps of; controlling the gain of the multiplier cell equal to a ratio between the output currents and the input currents by biasing the emitters of said input transistors with a first bias current and the emitters of said output transistors with a second bias current, wherein the ratio of (ii) the second bias current and (ii) the first bias current minus a current substantially equal to the base currents of the output transistors is substantially equal to said ratio of the output currents to the input currents. - View Dependent Claims (12)
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Specification