Fully differential broadband LNA with active impedance matching for SQUID readout
Date:
Low noise amplifiers (LNA) are key elements in most instrument’s detector readout chains. In this paper we present the development and characterization of a differential to differential SiGe BiCMOS LNA with active impedance matching for the readout of Superconducting Quantum Interference Devices (SQUIDs). The matching impedance is particularly important when using the LNA over a wide band 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 signal and the noise, are divided by a factor of 2 due to the impedance matching and 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 gain, input impedance and input voltage noise. The obtained performances for the LNA show a flat gain of 86 V/V with a cut-off frequency of 26 MHz and an input voltage white noise spectral density level lower than 0.5 nV/√ Hz with a corner frequency of 200 Hz. 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.