Phase noise in RF and microwave amplifiers

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. We analyze the two main types of phase noise in amplifiers, white and flicker. Using the polynomial model, the phase-noise power spectral density is S_φ(f) = b₀ + b_-1/f. Phase noise results from adding white noise to the RF spectrum around the carrier. For a given amount of RF noise added, 60 is proportional to the inverse of the carrier power P₀. By contrast, b_-1 is a constant parameter of the amplifier, in a wide range of carrier power. Accordingly, the flicker phase noise b_-1/f is independent of P₀. This fact has amazing consequences on different amplifier topologies. Connecting m equal amplifiers in parallel, b_-1 is 1/m times that of one device. Cascading m equal amplifiers, b_-1 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 b_-1 is m² times that of the amplifier alone. 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. This model also applies to other devices, including passive components.