Method and apparatus to improve frequency stability of an integrated circuit oscillator
First Claim
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1. A circuit, comprising:
- a capacitor coupled to be alternatingly charged and discharged by first and second current sources;
a first voltage follower circuit including a first bipolar transistor having a base coupled to the capacitor, the first bipolar transistor biased such that a voltage at an emitter of the first bipolar transistor follows a voltage on the capacitor; and
a current mirror having first and second current paths, the first current path coupled to the base of the first bipolar transistor, the first current path providing substantially all of a base current received by the base of the first bipolar transistor such that the base current received by the base of the first bipolar transistor includes substantially zero current received from the capacitor.
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Abstract
A method and apparatus providing a simple low cost integrated circuit oscillator with improved frequency stability over a range of selected frequencies by reducing the impact of process and temperature variations on a base current of bipolar transistor of the integrated circuit oscillator. A voltage follower circuit included in an integrated circuit oscillator provided to is injected with a base current provided by a current mirror instead of a capacitor such that variations in the base current of the bipolar transistor over the range of operating temperature do not substantially alter the charge in the capacitor to change the frequency of the oscillator.
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Citations
21 Claims
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1. A circuit, comprising:
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a capacitor coupled to be alternatingly charged and discharged by first and second current sources; a first voltage follower circuit including a first bipolar transistor having a base coupled to the capacitor, the first bipolar transistor biased such that a voltage at an emitter of the first bipolar transistor follows a voltage on the capacitor; and a current mirror having first and second current paths, the first current path coupled to the base of the first bipolar transistor, the first current path providing substantially all of a base current received by the base of the first bipolar transistor such that the base current received by the base of the first bipolar transistor includes substantially zero current received from the capacitor. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. An integrated circuit, comprising:
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a capacitor coupled to a first current source and to a switch, the switch coupled to a second current source, a magnitude and a direction of the first current source and the second current source causing a voltage on the capacitor to change in one direction when the switch is open and to change in an opposite direction when the switch is closed; a voltage follower circuit including an input coupled to the capacitor; a comparator circuit having an input coupled to an output of the voltage follower circuit, an output of the comparator circuit coupled to the switch such that the switch is opened when the voltage on the capacitor reaches a first threshold and the switch is closed when the voltage on the capacitor reaches a second threshold; a first bipolar transistor included in the voltage follower circuit, the first bipolar transistor having an emitter coupled to a third current source, a base of the first bipolar transistor coupled to the capacitor; a second bipolar transistor substantially the same as the first bipolar transistor, an emitter of the second bipolar transistor coupled to a fourth current source, the fourth current source substantially the same as the third current source; and a current mirror circuit coupled to a base of the second bipolar transistor such that an output current of the current mirror circuit is substantially equal to a base current of the second bipolar transistor, the output current of the current mirror circuit coupled to the base of the first bipolar transistor. - View Dependent Claims (13, 14, 15, 16)
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17. An integrated circuit oscillator comprising
a capacitor coupled to a first current source and a second current source, magnitudes and directions of the first current source and the second current source, respectively, causing a voltage on the capacitor to change in one direction until the voltage on the capacitor reaches a first threshold and to change in the opposite direction when the voltage on the capacitor reaches a second threshold; -
a first bipolar transistor having a base coupled to the capacitor; a second bipolar transistor having a current density substantially equal to a current density of the first bipolar transistor; a current mirror circuit coupled to a base of the second bipolar transistor, the current mirror circuit having an output substantially proportional to a base current of the second bipolar transistor, wherein the output of the current mirror circuit is substantially equal to a base current of the first bipolar transistor, wherein the output of the current mirror circuit is coupled to the base of the first bipolar transistor. - View Dependent Claims (18, 19, 20, 21)
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Specification