For price, delivery, and to place orders, please contact Hittite Microwave Corporation:

2 Elizabeth Drive, Chelmsford, MA 01824 Phone: 978-250-3343 Fax: 978-250-3373

Order On-line at www.hittite.com

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Application Notes

Detailed Description

The HMC981 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 HMC981 employs an integrated control circuitry

to achieve safe power-up and power-down sequencing of the targeted amplifier. The HMC981 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 HMC981 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 HMC981 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 HMC981 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 HMC981 ensures the protection of the external amplifier during turn on/off by adjusting the sequence of VDRAIN,

VGATE and VG2 outputs. HMC981 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 HMC981 employs a SW pad 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 HMC981 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 HMC981 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 HMC981 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 HMC981 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.

HMC981

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ACTIVE BIAS CONTROLLER