Self-balancing a.c. bridge using field-effect transistors as variable resistors. And its application to a process transmitter

The field-effect transistor (f.e.t.) can be utilised as a variable resistor whose value is controlled by the gate potential. This relationship is inherently nonlinear, but good linearity can be achieved if two are used together as a potential divider to form two ratio arms of a bridge. This feature opens the way to the design of bridge-type instruments which can convert the ratio of two resistances, inductances or capacitances into a proportional d.c. signal. The principle is embodied in a 4¿20 mA 2-wire process transmitter in which the process variable is arranged to unbalance such a bridge. Out of balance is detected by an a.c. amplifier, whose output is demodulated to control the transmission current and the gate potentials of the two f.e.t.s to re-establish bridge balance, thus providing a truly self-balancing system. Solid-state circuitry is used throughout, and operation over an ambient temperature range of ¿30°C to +80°C with very low temperature coefficient is obtained by suitable compensation of the f.e.t. potential divider.