Apparatus and method for receiving high-common mode voltage range signals with low-voltage devices
AssigneeNational Semiconductor Corporation
InventorsWillem J. Kindt, Gertjan V. Sprakelaar
Life of this patent
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Abstract
An input stage employs low-voltage MOSFETs as input devices for an operational amplifier circuit that operates at common mode voltages that may exceed the gate-oxide breakdown voltage of the input devices. Also, the input stage is arranged for relatively low noise. The input stage is arranged to detect the input common mode voltage and to feed back the detected input common mode voltage to a base of a bipolar folded cascode transistor that is coupled to the drain of the input devices. Accordingly, the input devices are bootstrapped such that they are protected from gate-oxide breakdown.
Claims27
A circuit for receiving high common mode voltage signals with low-voltage devices, the circuit comprising:
- a first transconductance stage that includes an output, a differential input, and a source, wherein the first transconductance stage is arranged to provide a first current at the output of the first transconductance stage in response to a differential input signal that is received at the differential input, and wherein the first transconductance stage is arranged to provide, at the source of the first transconductance stage, a first common mode signal that is associated with an input common mode voltage of the differential input signal;
- a load circuit that is arranged to provide a first voltage from the first current;
- and
- a first cascode transistor that includes a first cascode base, a first cascode emitter, and a first cascode collector, wherein the first cascode emitter is coupled to the output of the first transconductance stage, the first cascode base is arranged to receive a cascode base signal such that a voltage at the output of the first transconductance stage substantially tracks the input common mode voltage to prevent a breakdown condition from occurring if a closed loop operation is being employed;
- the cascode base signal is based, in part, on the first common mode signal;
- and wherein an output voltage is based, at least in part, on a voltage that is associated with the first cascode collector.
A circuit for receiving high common mode voltage signals with low-voltage devices, the circuit comprising:
- a first transconductance stage, including:
- a first transistor that includes a first gate, a first source, and a first drain;
- and
- a second transistor that includes a second gate, a second source, and a second drain, wherein the second source is coupled to the first source;
- and
- a first cascode transistor that includes a first cascode base, a first cascode emitter, and a first cascode collector, wherein the first cascode base is coupled to the first source, and wherein the first cascode emitter is coupled to the first drain.
A circuit for receiving high common mode voltage signals with low-voltage devices, the circuit comprising:
- a means for providing a first current in response to a differential input signal;
- a means for providing a common mode signal that is associated with an input common mode voltage of the differential input signal;
- a means for providing a first voltage from the first current;
- a means for providing the first voltage to an emitter of a cascode transistor;
- and
- a means for providing a signal that is based, at least in part, on the common mode signal, to the base of the cascode transistor.
Dependencies are inferred from the claim text.