Superconducting Switching Amplifiers for High Speed Digital Data Links

In a superconductor digital-RF transmitter, the power amplifier chain can be implemented in hybrid temperature, hybrid technology environment, where the superconductor switching amplifier forms the first stage of the amplification chain. We have designed several flavors of SQUID based switching amplifiers, each targeted for different power output and speed. One of the amplifiers is a differential amplifier featuring dynamic equalization. To minimize the intersymbol interference, the amplifier employs a pre-emphasis technique, thereby compensating for the bandwidth limitations of the channel. The differential output voltage of this amplifier is 16 mV at low speed (200 Mbps), which rolls off to 2 mV at 16 Gbps data rate, for an “1100” pattern (no consecutive transitions on the input data sequence). For high speed operation of the SQUID amplifiers, simultaneous switching of SQUIDs is desired to reduce the output rise and fall times. Hence, in another amplifier design, called the Differential H-Tree Amplifier, the SQUIDs are arranged in an “H-tree” structure, to equalize the propagation delay of the control signal to each SQUID. The amplifier's differential output voltage is 8 mV at low speed (200 Mbps), and rolls off to 2 mV at 10 Gbps data rate. A third type of amplifier is a differential SFQ-to-DC amplifier; it consists of a pair of synchronously driven SFQ-to-DC converters that produce complementary positive and negative voltage waveforms respectively. The differential output voltage of this amplifier is 0.8 mV, and it can be operated at very high speeds. Moreover, the complexity of this amplifier enables it to be yielded in higher critical current density process. Both, the speed and the output voltage scale as the square root of the process critical current density.