Fully Differential Broadband LNA with Active Impedance Matching for SQUID Readout

In this paper, we present the development and characterization of a fully differential SiGe BiCMOS low noise amplifier (LNA) with active impedance matching for the readout of superconducting, quantum interference devices (SQUIDs). Impedance matching is particularly important when using the LNA over a broad frequency range and it is achieved using the Miller effect by adding a negative feedback loop. This approach avoids the degradation of the noise performance that is generated by simply using a parallel resistor at the LNA input. Furthermore, this impedance matching implementation preserves the signal to noise ratio (SNR) because both the signal and the noise are divided by 2 due to the negative feedback loop. This was verified by measuring a lower input voltage noise with the LNA input loaded with a 100 Ω resistor at 77 K compared to a short-circuit. In addition, we present simulations and measurements of the LNA frequency response, input impedance and input voltage noise. The obtained performances for the LNA show a flat gain of 173 V/V with a cut-off frequency of 26MHz and a referred input voltage white noise spectral density level of 0.34 nV/ √(Hz) with a corner frequency of 72 Hz in input matching condition. These values are in good agreement with the simulations. Finally, a discussion about the impact of the impedance matching on the SQUID biasing is also presented.