Input stage for constant gm amplifier circuit and method
First Claim
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1. An amplifier input stage comprising:
- first and second transistors having respective bases coupled to first and second amplifier stage inputs and having emitters coupled to a first common node;
third and fourth transistors having respective bases coupled to the first and second amplifier input stage inputs and having emitters coupled to a second common node;
a comparator circuit, separate from the first, second, third and fourth transistors, and configured to compare a first signal indicative of the input applied to the amplifier input stage inputs with a reference signal; and
tail current source circuitry configured to selectively source current to the first and second common nodes in response to the comparator circuit.
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Abstract
An amplifier input stage having a constant input gm and including first and second differential transistor pair so as to provide operation with inputs at or near the upper and lower power supply rails. A comparator circuit operates to control which of the transistor pair is active based upon the relative magnitudes of the amplifier input stage inputs and a reference voltage.
14 Citations
12 Claims
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1. An amplifier input stage comprising:
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first and second transistors having respective bases coupled to first and second amplifier stage inputs and having emitters coupled to a first common node;
third and fourth transistors having respective bases coupled to the first and second amplifier input stage inputs and having emitters coupled to a second common node;
a comparator circuit, separate from the first, second, third and fourth transistors, and configured to compare a first signal indicative of the input applied to the amplifier input stage inputs with a reference signal; and
tail current source circuitry configured to selectively source current to the first and second common nodes in response to the comparator circuit. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. An amplifier input stage comprising:
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a first differential transistor pair having respective bases coupled to first and second amplifier input stage inputs;
a second differential transistor pair having respective bases coupled to the first and second amplifier input stage inputs;
a comparator circuit, which includes first and second transistors which form a third differential transistor pair, with the first transistor having a base for receiving a reference signal and the second transistor having a base for receiving a signal indicative of the input applied to the amplifier input stage inputs, with the comparator circuit switching between first and second states in response to relative magnitudes of the reference signal and the signal indicative of the input; and
tail current source circuitry configured to provide current to the first differential transistor pair and not the second differential pair when the comparator circuit is in the first state and to provide current to the second differential pair and not the first differential pair when the comparator circuit is in the second state.
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12. A method of controlling an amplifier input stage which includes a first differential transistor pair having respective bases coupled to the input stage inputs and a second differential transistor pair having respective bases coupled to the input stage inputs, said method comprising:
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providing a third differential transistor pair;
coupling a reference signal to a base of a first transistor of the third differential transistor pair;
coupling an input signal indicative of the input stage inputs to a base of a second transistor of the third differential pair;
providing an output having first and second states based upon relative magnitudes of current flow through the first and second transistors;
coupling a tail current to the first differential pair, and not the second differential pair, when the output is in the second state; and
coupling a tail current to the second differential pair, and not the first differential pair, when the output is in the first state.
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Specification