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For price, delivery, and to place orders: Analog Devices, Inc., One Analog Way, Wilmington, MA 01887

Phone: 781-937-1428 • Order online at www.analog.com

Application Support: Phone: 1-800-ANALOG-D

Application Notes

Detailed Description

The HMC981LP3E is a fully-integrated Active Bias Controller (ABC) that automatically adjusts the gate voltage of

amplifiers operating in the Class-A regime. With an internal feedback the automatic gate voltage control achieves

constant quiescent bias through the amplifier under bias, independent of temperature and amplifier threshold

variations. The quiescent current is adjusted with a resistor connected externally. The HMC981LP3E employs an

integrated control circuitry to achieve safe power-up and power-down sequencing of the targeted amplifier. The

HMC981LP3E can provide auto-bias solution virtually any amplifier in the market (both enhancement and depletion

type) with a quiescent current of up to 200 mA and a supply voltage of up to 12V.

The HMC981LP3E has an integrated negative voltage generator to synthesize negative voltages required to drive

depletion mode amplifiers. If an external negative supply is already available or an enhancement mode device is

targeted, the negative voltage generator can be disabled.

The HMC981LP3E achieves excellent bias stability over supply and temperature variations with low supply voltage

down to 4V. The gate control can both sink and source current (±0.8 mA) to achieve constant bias current over input

power variations applied to the amplifier. The HMC981LP3E also generates a second gate voltage (VG2). VG2 can be

adjusted through a resistor divider connected to VDD for amplifiers which require second gate voltage.

The HMC981LP3E ensures the protection of the external amplifier during turn on/off by adjusting the sequence of

VDRAIN, VGATE and VG2 outputs. HMC981LP3E controls the bias current of the amplifier under bias with the gate

control driver. The current passing through the amplifier is continuously sampled and is used to control the VGATE

voltage connected to the gate of the amplifier.

The HMC981LP3E employs a SW pin to control RDS_ON resistance of the internal switch between VDD and VDRAIN.

Refer to the section under the “Supply and Drain Voltage” section for details.

The HMC981LP3E has a built-in short circuit protection feature to protect both itself and the amplifier under bias

against short circuit conditions at the VDRAIN output. Refer to the section under the “Short Circuit Protection” section

for details.

Digital Power Supply (VDIG)

The HMC981LP3E requires an external low voltage bias rail (3.3V to 5.0V). VDIG powers the internal logic circuitry.

VDIG draws an average of 3 mA from a 3.3V. VDIG can accept voltages up to 5.0V.

Supply and Drain Voltage (VDD and VDRAIN)

The VDD supply to the HMC981LP3E is directly connected to the VDRAIN output through an internal MOSFET switch.

This internal MOSFET is controlled through power-up sequencing which ensures that no voltage is applied to drain

of the external amplifier until the gate voltage is pulled down to VNEG(ensuring external amplifier is pinched-off). The

VDRAIN output of the HMC981LP3E should be connected to the drain (collector) of the amplifier under bias for the

active bias control feedback and power-up/down sequencing to work.

HMC981LP3E

v02.1222

ACTIVE BIAS CONTROLLER