The task of the input stage is to process the differential signals present at the inputs of the operational amplifier while keeping the amount of interference that is added by the input stage as small as possible. Interference includes offset, noise, and distortion. Offset is caused by a mismatch of devices, while noise is generated by active components and resistors. Interference is also generated by cross-talk from common-mode input signals due to a finite common-mode rejection. If the amplifier is used in applications where the common-mode voltage varies, as a non-inverting amplifier for example, the finite common-mode rejection causes distortion. Due to the low-supply voltages used in modern electronics, the input-voltage range of a conventional input stage is very small. Especially if the applications of the operational amplifier include a buffer connected configuration, the common-mode input-voltage range should be as large as possible so that an input stage with rail-to-rail input-voltage range is needed. Basic input stages, input stages with rail-to-rail input range, and the biasing required by input stages are discussed in this chapter.
KeywordsBias Current CMOS Technology Charge Pump Current Mirror Input Stage
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