Fully differential cryogenic transistor amplifier

• We have built a fully differential cryogenic dc-coupled amplifier.
• The amplifier has high bandwidth, low noise temperature and high common-mode rejection.
• Our amplifier is suitable for readout of SQUIDs and superconducting detectors.

We have constructed a dc-coupled differential amplifier capable of operating in the 4.2 K–300 K temperature range. The amplifier can be operated at high-bias setting, where it dissipates 5 mW, has noise temperature TN ≈ 0.7 K at RS ≈ 5 kΩ and >40 MHz bandwidth at 4.2 K bath temperature. The bias setting can be adjusted: at our lowest tested setting the amplifier dissipates <100 μW, has noise temperature TN ≈ 2 K at RS ≈ 25 kΩ and >2 MHz bandwidth. The 1/f noise corner frequency is a few times 10 kHz. We foresee the amplifier to have an application in the readout of Superconducting Quantum Interference Devices (SQUIDs), Superconducting Tunnel Junction Detectors (STJs) and Transition Edge Sensors (TESes). We have verified the practical use of the amplifier by reading out a 4.2 K 480-SQUID array with 40 MHz bandwidth and <8 × 10−8 Φ0/Hz1/2 flux noise.