A Bilateral Analog FET Optocoupler

Recently, optoelectronic switches for low-level applications were described by Bussolati (1) and Brown (2). The optoelectronic switch proposed in Bussolati´s paper consists of two phototransistors (PT) connected back-to-back with separate infrared emitting diodes (IRED´s). Although by varying the ratio of the currents in the IRED´s the offset voltage of the switch can then be zeroed, it is not a very practical device to use. An excellent performance, but technically complex and relatively costly hybrid to perform the same circuit function is detailed in Brown´s paper. It is the purpose of this paper to present a new device called an optically-coupled bilateral analog FET, which is an economically advantageous design with technical trade-offs compared to the previous devices. This new device features a monolithic detector, coupled to an IRED, and is newly available in sample quantities. It provides the function of an ideal bilateral analog FET with isolated current control of the output conductance. This provides the advantage of optical isolation to the analog electronic functions of a linear, electronically variable, resistor (from 20 ohms to hundreds of megohms) and a fast bilateral ac low-level signal switch. The IRED and the detector are enclosed in a 6-pin DIP, although other configurations are possible. The structure and characteristics of this device are described in this paper. Possible applications, as well as limitations, are thoroughly analyzed.