Chapter 14 - Noise Analysis

Abstract

Noise analysis is run in conjunction with an AC analysis, and calculates the output noise and equivalent input noise in a circuit. The output noise, at a specified output node, is the root mean square sum of the noise generated by all the resistors and semiconductors in the circuit. If the circuit is considered as noiseless, then the equivalent input noise is defined as the noise required at the input to generate the same output noise. This is the same as dividing the output noise by the gain of the circuit to obtain the equivalent input noise. Johnson noise or thermal noise is due to the random thermal agitation of electrons in a conductor, which increases with frequency and temperature. In PSpice, the thermal noise contribution from a resistor is represented by a current source in parallel with a noiseless resistor. Semiconductor noise is generally made up of thermal, shot, and flicker noise. Thermal noise is generated by the intrinsic parasitic resistances for the device. Shot noise, however, is a randomly fluctuating noise current generated when a current flows across a PN junction. After a noise analysis is run, the thermal, shot, and flicker noise contributions for resistors and semiconductors are made available as trace variables in Probe. Users have to run an AC analysis to run a noise analysis. In the simulation settings for an AC sweep, there is an option to enable the Noise Analysis.