Supplement: Power Figure 3. Fixed drain current biasing (active bias control)


Figure 4. Power sequencing waveforms at turn on. Once VDD is applied, high EN signifies the start of the control loop. VGATE is turned ON first, followed by the VDRAIN


Figure 5. Power sequencing waveforms at turn off. When VDD is removed, EN will be low. VGATE will decrease to the minimum VNEG again and VDRAIN will decrease to zero. Then VGATE will eventually reach zero


it does not take into account the part-to-part variation in the VGATE to IDRAIN relationship of the RF amplifier. The part-to-part variation in drain current (assuming a fixed gate voltage) can be significant resulting in each circuit having a different drain current. Drain current variations typically impact compression (OP1dB) and third-order intermodulation distortion (OIP3) (gain will also be affected but to a lesser extent). One benefit of this approach is that the drain current will increase and decrease based on the RF input and RF output power present. So, if the RF input power is low, power consumption will be low and vice versa.


Active bias control


Active bias control is an alternative approach. Instead of fixing the gate voltage, this technique involves fixing the drain current. In Figure 3, an active bias controller regulates the drain current by measuring that current and varying the gate voltage to keep the current fixed even at different input RF conditions. The circuit consists of an LT8608 step-down regulator and the HMC920 active bias controller which can support drain voltages from 3V to 15V and total drain currents of up to 500mA.


The high voltage, high current linear regulator (LDOCC pin) inside the HMC920 can


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generate positive voltages from 3V to 15V and current up to 500mA. Its output is connected to the VDRAIN port via an internal MOSFET switch, which is used for power sequencing. To set the desired drain voltage for the power amplifier, the feedback resistors of the LDO regulator, R5 and R8, must be adjusted using Equation 1:


Where VDRAIN is the desired drain voltage value and IDRAIN is the desired drain current. The 0.5 constant is the RDS(ON) value of the internal MOSFET switch.


The internal charge pump generates the negative voltage for VGATE. By reading the voltage at RSENSE, the controller senses the drain current and varies the voltage at VGATE. To set the drain current, RSENSE (R4 and R19) must be varied using Equation 2:


When HMC920 is turned on by applying the supply voltage (VDD), a signal will be sent to the EN pin to start the control loop. VDRAIN is initially shorted to the ground to force it to zero. Meanwhile, the voltage at the VGATE is initially pulled to the minimum voltage of VNEG. After this, VDRAIN will increase to the drain voltage value that was set. A voltage drop will be generated on


Components in Electronics March 2025 31


RSENSE, which will result in the controller to vary the gate voltage. During turn-off, a logic low signal will be sent to the EN pin. VGATE will decrease to VNEG to cut off the amplifier and the voltage at VDRAIN will decrease to zero. The voltage at VGATE will eventually reach zero. This cycle follows the proper power sequencing for the safe operation of depletion mode amplifiers. It also has safety features such as over and under current alarm, short circuit protection, and power foldback. Other safety mechanisms of the bias controller are explained in detail in the HMC920 data sheet.


This bias controller was implemented as a


power management solution for the ADL8106 wideband low noise amplifier. The ADL8106 operates from 20GHz to 54GHz with a nominal drain voltage of 3V and a quiescent drain current of 120mA. Figures 4 and 5 show the associated turn on and turn off waveforms.


Noise and spur suppression The level of spurs and noise at the RF output of the RF amplifier will depend on the output noise and spurs of the HMC920 and on the power supply modulation ratio (PSMR) of the amplifier. Figure 6 shows the plots of the PSRR at the switching regulator (LT8608) inputs and Continues on page 32


Figure 6. The power supply rejection ratio of the LT8608 + HMC920 (VDD = 5V, VDRAIN = 3V, IDQ = 120mA, VGATE = –0.64V)


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