Low flicker-noise dc amplifier for 50 Ω sources

This article analyzes the design of a low-noise dc amplifier primarily intended as the post-detection front-end for phase noise measurements. Low residual flicker when the amplifier follows a source with 50 Ω impedance is the most desired performance. This feature can only be appreciated if white noise is sufficiently low and if an appropriate design ensures dc stability. An optimal solution is proposed, in which the low-noise and dc-stability are achieved at a reasonable complexity and without need of temperature stabilization. Gain is accurate in amplitude and phase up to more than 100 kHz. This is relevant to dual-channel Fourier transform measurements, that is, the measurement of transfer functions and of cross spectra. The proposed amplifier turns out to be a general-purpose laboratory tool, useful in a variety of measurements other than phase noise.