Noise in RF Amplifiers
7.1 SOURCES OF NOISE
The dynamic range of a communication transmitter or receiver circuit is usually limited at the high-power point by nonlinearities and at the low-power point by noise. Noise is the random fluctuation of electrical power that interferes with the desired signal. There can be interference with the desired signal by other unwanted deterministic signals, but at this point only the interference caused by random fluctuations will be considered. There are a variety of physical mechanisms that account for noise, but probably the most common source is thermal (also referred to as Johnson noise or Nyquist noise). This can be illustrated by simply examining the voltage across an open-circuited resistor (Fig. 7.1). The resulting voltage is not zero! The average voltage is zero but not the instantaneous voltage. At any temperature above absolute 0 K the Brownian motion of the electrons will produce random instantaneous currents. These currents will produce random instantaneous voltages, and this leads to noise power.
Noise arising in electron tubes, semiconductor diodes, bipolar transistors, or field-effect transistors come from a variety of mechanisms. For example, for tubes, these include random times of emission of electrons from a cathode (called shot noise), random electron ...