The design and construction of an AC-coupled, ultralow noise amplifier (equivalent input noise voltage En=0.33 nV/ square root Hz, optimum source impedance 1 k Omega at 100 kHz) is presented. The amplifier employs nine Sony 2SK 152-4 JFETS in parallel to yield the low noise result. Measurements were taken to characterize the amplifier's bandwidth, the additive voltage noise versus frequency, and the current noise versus frequency. SPICE modelling parameters for the JFETs used will also be discussed. Finally, the authors will review the performance of the amplifier in a flux-locked SQUID application.

Many high precision experiments place severe requirements on the noise, linearity and slew rate of flux-locked dc SQUID systems (linearity requirement approaches 1 in 106 for Gravity Probe-B). A computationally efficient and accurate method of simulating a dc SQUID'S V -0 and I-V characteristics has proven valuable in evaluating and improving various SQUID readout methods. The simulation of the SQUID is based on fitting of previously acquired data from either a real or a modeled device using the Fourier transform of the V -0 curve. This method does not predict SQUID behavior, but rather is a way of replicating a known behavior efficiently with portability into various simulation programs such as SPICE. In this paper we discuss the methods used to simulate the SQUID and the flux-locking control electronics and present specific examples of this approach. Results include an estimate of the slew rate and linearity of a simple flux-locked loop using a characterized dc SQUID.

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