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There will be a voltage drop from VDD to VDRAIN due to finite RDS_ON resistance of the internal switch. To

compensate for this voltage drop choose the VDD value as shown in equation (1).

VDD = VDRAIN + IDRAIN x RDS_ON

(1)

where VDRAIN is the supply voltage of the external amplifier and IDRAIN is the desired constant bias current through

the external amplifier.

Note that RDS_ON resistance of the internal FET switch can be adjusted through SW pad. RDS_ON is typically equal

to 5 Ohms when SW is pulled up to VDIG, and is typically equal to 10 Ohms when SW is pulled down to GND. If SW

is left floating, it is pulled up to VDIG through an internal weak pull-up. Recommended settings for the SW position

are given in Table-1. Not using the HMC981 in the recommended settings will increase the power dissipation of the

part and may increase the part-to-part variation.

Negative Voltage Generator (VNEGOUT)

The HMC981 has an internally regulated charge pump block to generate negative voltage (VNEGOUT) required for

depletion mode devices. The HMC981 generates -2.5V at the VNEGOUT output in default configuration. It requires

two diodes and two capacitors connected externally as shown in the sample application schematics. The HMC981 is

designed to reject the ripple on the VNEGOUT by isolating VNEGOUT from the VGATE. The negative voltage is only

required for depletion mode devices, and it can be disabled through the VGATEFB and VNEGFB pads. Where an

enhancement device is targeted or a negative supply is already available in the system, simply connect the available

negative supply to the VNEG pad. See Table-2 for detail on how to set this operation mode.

HMC981

v00.0611

ACTIVE BIAS CONTROLLER

Table 1. Recommended Current Range Configuration

Current Range (mA)

Condition

RDS_ON (Ohm)

20 to 80

SW=GND

10

80 to 200

SW=VDIG

5

Enable/Disable (EN)

The active bias control loop is enabled when EN is pulled up to VDIG, and it is disabled when it is pulled down to GND.

If EN is left floating HMC981 is enabled through an internal weak pull-up. Note that VNEG operation is independent

of EN condition. EN signal controls the operation of only VGATE, VG2 and VDRAIN outputs. When EN pulled down

to GND, the HMC981 discharges VDRAIN and VG2 down to GND and it pulls the VGATE down to VNEG. Please see

the “Active Bias Control Loop” section for detailed explanation.

Active Bias Control Loop

The HMC981 regulates the bias current (IDRAIN) of the amplifier under bias through VGATE output connected to

the gate of the external amplifier. In this closed loop operation the current passing through the amplifier under bias is

sampled and is used to adjust VGATE to achieve constant quiescent bias through the external amplifier.

The HMC981 continuously compensates for any supply, temperature, process variations and threshold drifts due to

aging. The part-to-part, temperature, and supply variation of the HMC981 is excellent. Thus, by using an accurate

sense resistor connected to the ISENSE pad, expensive calibration procedures in high volume production could be

avoided.