The gate-turn-on threshold voltage for GaAs MESFETs (gallium-arsenide metal-semiconductor field-effect transistors) varies considerably from part to part, even within a given lot. That behavior makes biasing difficult, especially if you want to design the device into a high-volume product. To overcome this drawback, you can introduce a current sensor that monitors the bias current and provides feedback to the gate input (Figure 1). IC1 combines a current sensor and an error amplifier. Intended as a power-control IC for power amplifiers, it senses the drain-source current, IDS, at the source; compares and integrates the difference between voltage drops across RSENSE and RG1; and feeds back an output voltage to the MESFET gate. The feedback adjusts IDS until the two voltage drops are equal, thereby achieving uniform source current, regardless of the MESFET's gate-threshold characteristics. The expression for drain-source current is:
|Figure 1.||A smart-bias IC ensures uniform bias for GaAs FETS in high-volume products.|
Current through RG1 depends on a voltage, VPC, with respect to the negative supply, VEE, applied to the power-control input at Pin 4. You can implement VPC with a voltage divider, a reference, or a variable-voltage source. Because the gate voltage is negative with respect to the source, you must modify IC1’s supply voltage to ensure a negative gate drive for the MESFET: Connect the VCC pin to ground and the ground pin to VEE. You can easily modify this uniform-bias circuit for biasing bipolar transistors and MOSFETs, as well.