High-resolution dc-voltage-biased ac conductance bridge for tunnel junction measurements

Abstract

A simple, low-cost conductance bridge circuit, which provides high-resolution measurements of dI/dV and d 2I/dV2 for a voltage-biased load, is presented. A resolution in dI/dV for a 90-Omega tunnel junction of several parts in 10(5) has been measured using a 35-microV-rms modulation level and a 1-s postdetection time constant (Bn=0.25 Hz). The output impedance of the voltage source and the error in the conductance measurement are discussed. The output impedance of the circuit with an AD119 operational amplifier is 0.02 Omega. A detailed discussion of Johnson-Nyquist, tunnel junction, and amplifier noise is presented.