Phase Noise in RF and Microwave Amplifiers
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.