On the calculation of noise in multistage amplifiers

It is shown that it can be a fallacy that the noise figure of a multistage amplifier is primarily determined by the noise figure of the first stage if the available power gain of the first stage is sufficiently high. The expression for the equivalent input noise voltage of a multistage amplifier is derived, where each stage is represented by the V/sub n/-I/sub n/ amplifier noise model. It is shown that the noise contributed by stages following the first stage is lowest when the first stage has a high transconductance, an input resistance that is high compared to the source resistance, and a high open-circuit voltage gain. In addition, the second stage must have a low input current noise. Circuit topologies which meet these conditions are described.<
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