ADL8106ACEZ-R7 datasheet(23/26 Pages) AD

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Data Sheet

ADL8106

APPLICATIONS INFORMATION

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Rev. B | 23 of 26

Figure 88 shows the basic connections for operating the ADL8106,

and the configuration used to characterize and qualify the device.

The RFIN and RFOUT pins are internally ac-coupled. However, if

the dc bias level of the input signal is not equal to 0 V, externally

ac-couple the RFIN pin. Likewise, if the RFOUT pin is driving an

input with a bias level other than 0 V, externally ac-couple this pin.

Because VDD1, VDD2 and VGG1 are internally decoupled, no

external decoupling components are required on these pins.

VGG1 sets the drain current of the amplifier (VGG1 ~ −0.65 V for

an IDQ of 120 mA). VDD1 and VDD2 provide the drain current (3 V

nominal) with equal currents on each pin.

To avoid damaging the device, set VGG1 before VDD1 and VDD2

are applied. Set VGG1 to −2 V before turning on VDD1 and

VDD2. VGG1 can then be adjusted upwards until the desired IDQ

is achieved. Then, apply the RF input signal. If the desired gate

voltage is known, VGG1 can be set to that voltage value directly

without taking it to the pinch off voltage (−2 V).

To turn off the device, turn off the RF input signal, turn off VDD1

and VDD2 and then turn off VGG1.

See the ADL8106-EVALZ user guide for information on using the

evaluation board.

Figure 88. Basic Connections

BIASING THE ADL8106 WITH THE HMC920

The HMC920 is designed to provide active bias control for depletion

mode amplifiers, such as the ADL8106. The HMC920 measures

and regulates drain current to compensate for temperature changes

and part to part variations in the drain current to gate voltage

relationship. Additionally, the HMC920 properly sequences gate

and drain voltages to ensure safe on and off operation and offers

circuit self protection in the event of a short circuit. The active

bias controller contains an internal charge pump that generates the

negative voltage needed to drive the VGG1 pin on the ADL8106.

Alternatively, an external negative voltage can be provided.

For more information regarding the use of the HMC920, refer to the

HMC920 data sheet and the AN-1363 Application Note.

Application Circuit Setup

Figure 89 shows the application circuit for bias control of the

ADL8106 using the HMC920. The HMC920 drain current is meas-

ured and the VGATE output voltage adjusts until the set point drain

current is achieved. The various external component values around

the HMC920 are calculated as follows.

The target drain current must first be determined and set. This

current must be set based on the maximum drain current that is

expected to be required during operation, including when the device

is generating the maximum expected output power. This current

is set by the resistor connected between the ISENSE pin on the

HMC920 (Pin 25) and ground using the following equation:

IDRAIN (A) = 166/RSENSE + 0.0135

To ensure adequate headroom, the supply voltage for the HMC920

must be set higher than the target drain voltage to the ADL8106 (3

V). Accordingly, the supply voltage to HMC920 is set to 5 V.

The voltage on the LDOCC pin (Pin 29) on the HMC920 drives the

VDRAIN pins which in turn drive the VDDx pins of the ADL8106.

Because the LDOCC output is connected to the VDRAIN output

through an internal metal-oxide semiconductor field effect transistor

(MOSFET) switch with an on resistance of 0.5 Ω, the LDOCC

voltage must be set slightly higher than the target drain voltage to

the ADL8106. To determine the required LDOCC voltage, use the

following equation:

VLDOCC = VDRAIN + IDRAIN × 0.5

Therefore, VLDOCC = 3 V + (0.14 × 0.5) = 3.07 V.

To set VLDOCC to 3.07 V, use the following equation with R5 set to

10 kΩ:

R8 = (R5/2) × (VLDOCC − 2)

Therefore, R8 = (10000/2) × (3.07 – 2) = 5.350 kΩ (choose 5.36 kΩ

standard value).

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