Bridge-balancing system for measuring extremely low currents

Abstract

A bridge-balancing system for measuring minute current flows in devices having an extremely high impedance, the bridge having two sets of opposing arms to define input and output diagonals. A direct voltage is applied to the input diagonals and an amplifier having a high input impedance is connected to the output diagonals to yield an analog signal whose sense and magnitude depend on the degree to which the impedance of the device under test which forms one arm in one set differs from that of an adjustable high-impedance element forming the opposing arm of that set, a pair of fixed matching resistors defining the arms of the other set. The adjustable element is constituted by a field effect transistor operating in its variable resistance region wherein the extremely high impedance presented thereby depends on the applied gate potential. In the manual mode of the system, the impedance of the device under test is determined by varying the potential applied to the gate until a point is reached at which the signal is nulled and the bridge is in balance, at which point the impedance of the transistor matches that of the device under test and has a value which is a function of the applied gate potential. In the automatic mode, the analog signal is converted into a corresponding digital value which is evaluated in a microprocessor to determine the potential required to effect nulling, the microprocessor yielding a digital instruction that is converted into an analog control signal which is applied to a correction amplifier that imposes a nulling potential on the gate.