Development of a multi-channel, stable, low-cost, low-noise, high precision, wide range current source for superconducting integrated circuits

Since the discovery of single flux quantum (SFQ) logic circuits, SFQ logic became one of the emerging technologies, being used in numerous applications. As the core element of an SFQ circuit is Josephson junction, they are current biased circuits. SFQ logic circuits provide faster operations with lower power consumption, using Josephson junctions as the switching devices. Complicated circuits need to be biased from many points in order to decrease the Joule heating effect related thermal load (I2R) as the current source is placed at the room temperature whereas the SFQ circuits are at the cryogenic temperatures. In addition, the requirements of the range of current bias may be in the order of μA as in the case of 1-bit comparator cell to a range of few Amperes as in the case of a microprocessor circuit. Semiconductor circuits operating at room temperatures do not have such requirements as they are voltage biased and they do not have a thermal load problem. Due to the small market associated to superconducting integrated circuits, it is not easy to find a compact, low noise, multi-channel and computer controller compact current sources sold off the shelf. So, in this study, we develop a low noise, high precision, wide range current source for the biasing of superconducting integrated circuits. Feedback algorithm is used to increase the stability and decrease the noise amount of the current source. Channels of the current source are controlled with an FPGA. Desired current can be set via the user interface which is running on the PC. The PC sends the required commands to FPGA, which controls the high speed feedback algorithm. For a 1 mA setting, the measured noise current is 13.9 nA/√Hz, in a bandwidth of 1.5 mHz to 10 Hz.