  Understanding the amplifier phase noise is a critical issue in numerous
fields of engineering and physics, like oscillators, frequency synthesis,
telecommunications, radars, spectroscopy, in the emerging domain of microwave
photonics, and in more exotic domains like radio astronomy, particle
accelerators, etc.
  This article analyzes the two main types of phase noise in amplifiers, white
and flicker. White phase noise results from adding white noise to the RF
spectrum around the carrier. For a given amount of RF noise added, noise is
proportional to the inverse of the carrier power. By contrast, the 1/f
coefficient is a constant parameter of the amplifier, in a wide range of
carrier power. This fact has amazing consequences on different amplifier
topologies. Connecting m equal amplifiers in parallel, flicker is 1/m times
that of one device. Cascading m equal amplifiers, flicker is m times that of
one amplifier. Recirculating the signal in an amplifier so that the gain
increases by a power of m (a factor of m in dB) due to positive feedback
(regeneration), which for integer m is similar to the case of m amplifiers, we
find that flicker is m^2 times that of the amplifier alone.
  Starting from the fact that near-dc flicker exists in all electronic devices,
although generally not accessible from outside, the simplest model for the 1/f
phase noise is that the near-dc 1/f noise phase-modulates the carrier through
some parametric effect in the semiconductor. This model predicts the behavior
of the (simple) amplifier and of the different amplifier topologies. Numerous
measurements on amplifiers from different technologies and frequencies (HF to
microwaves), also including some obsolete amplifiers, validate the theory.
