Quantum mechanics limits the noise performance of linear amplifiers, and these limits, in turn, restrict the effects of active feedback on the noise level of any device which uses feedback from such an amplifier. I present a simple quantum-mechanical characterization of feedback, and use this method to determine the limiting noise levels of active filters and oscillators. The results clarify the role of the amplifier in the measurement process and illustrate the limits of a Langevin treatment of quantum noise.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy (miscellaneous)