5 Negative GaN Biasing

The AFE11612-SEP features twelve 12-bit DACs. The device has an internal 2.5-V reference that scales the DAC output range from 0 V to 5 V. GaN PAs require negative gate voltage to be properly biased, with pinch-off voltages being more negative than the on voltages. The DAC output can be converted to a negative voltage through the use of a differential op-amp circuit that is powered by a negative power supply (V_SS). The following circuit uses the op-amp OPA4H199-SEP to offset and scale the 5-V output range to -7.5 V to 0 V. A differential op-amp circuit is used to protect the PA in case of an alarm shutdown. In an alarm state, the DAC drives the voltage to 0 V. The differential circuit outputs -7.5V to the GaN gate, thus ensuring the GaN PA turns off. The use of an inverting amplifier configuration is not recommended as the off state drives the output to 0 V, which can damage the PA in an alarm state.

The AFE11612-SEP 2.5 V reference output provides the offset voltage for the differential circuit. The resistors in the circuit were selected with the purpose of providing a -7.5 V to 0 V output range, while not significantly loading V_REF. The OPA4H199-SEP has a 40-V supply range, and the circuit can be modified to support greater negative-voltage applications if needed.