SLAAED5A June 2023 – September 2025 AFE11612-SEP , OPA4H199-SEP

5 Negative Biasing for GaN PAs

The AFE11612-SEP has an internal 2.5V reference that scales the DAC output range from 0V to 5V. 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 shifted to a negative voltage through the use of a differential op-amp circuit. The circuit example in Figure 5-1 uses the radiation hardened op-amp OPA4H199-SEP to offset and scale the DAC output to the negative range. 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 0V. The differential circuit outputs the most negative voltage to the GaN gate, thus ensuring the GaN PA turns off.

Figure 5-1 Differential Operational Amplifier Circuit

Resistor values are chosen based on the desired op-amp output, V_IN, and DAC range. The following equations provide a guideline for selecting the resistor values:

Equation 1.

V_{G S} = - (V_{I N} \times \frac{R 2}{R 1}) + D A C \times (\frac{R 4}{R 4 + R 3}) \times (\frac{R 1 + R 2}{R 1})

When DAC = 0V:

Equation 2.

V_{G S} (M I N) = - (V_{I N} \times \frac{R 2}{R 1})

V_GS(MIN) is selected to be –7.5V, and V_IN is selected to be 5V.

Equation 3.

- 7.5 V = - (5 \times \frac{R 2}{R 1})

Equation 4.

\frac{R 2}{R 1} = 1.5

Select R2 and R1 values in accordance with this ratio. In this example, R1 = 10kΩ and R2 = 15kΩ. To calculate R3 and R4, use the maximum desired DAC value and V_GS value. In this example, DAC = 5V, and V_GS(MAX) = 0V.

Equation 5.

V_{G S} (M A X) = - (V_{I N} \times \frac{R 2}{R 1}) + D A C \times (\frac{R 4}{R 4 + R 3}) (\frac{R 1 + R 2}{R 1})

Equation 6.

0 = - (5 \times \frac{15 k}{10 k}) + 5 \times (\frac{R 4}{R 4 + R 3}) \times (\frac{10 k + 15 k}{10 k})

Equation 7.

7.5 = 12.5 \times (\frac{R 4}{R 4 + R 3})

Reducing the equation gives the resistor ratio in Equation 8.

Equation 8.

\frac{R 4}{R 3} = 1.5

Since this is the same ratio as R2/R1, the same values will be used for R3 and R4 : R3 = 10kΩ and R4 = 15kΩ. Figure 5-2 shows the DAC vs V_GS output with these resistor values.

Figure 5-2 Differential Operational Amplifier Output