S2079: Drivers for GaAs FET Switches and Digital Attenuators

Many of M/A-COM's GaAs FET switches and digital attenuators cannot operate directly with simple TTL or CMOS logic, but instead require external circuits to provide appropriate control voltages. This application note, an update of M539, Drivers for GaAs FET MMIC Switches and Digital Attenuators, provides information on M/A-COM's SW-109 and SWD-119 drivers and other commercially available digital logic IC's for control of switches and digital attenuators. GaAs MMIC control devices such as switches and digital attenuators typically employ Field Effect Transistors (FET's). The most common FET is the n-channel depletion mode device, which has low source-to-drain resistance in the absence of a gate bias, and allows a current IDSS to flow. With the application of a negative gate bias voltage, the electric field below the gate causes the conduction channel to narrow, increasing the source-to-drain resistance. The gate voltage that creates a high enough resistance to reduce the source-to-drain current to (typically) 1 - 2 percent of IDSS is known as the pinch-off voltage.