Circuit Note

CN-0050

Circuit Designs Using Analog Devices Products

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Devices Connected/Referenced in this Circuit Note

ADL5330

Variable Gain Amplifier (VGA)

AD8318

70 dB Logarithmic Detector/Controller

Stable, Closed-Loop Automatic Power Control for RF Applications

Rev. B

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CIRCUIT FUNCTION AND BENEFITS

The circuit described in this document provides closed-loop, auto-

matic power control using a VGA (ADL5330) and a log detector

(AD8318). Due to the high temperature stability of the AD8318,

this circuit provides stability over temperature because the AD8318

RF detector ensures the same level of temperature stability at the

output of the ADL5330 VGA. The addition of the log amp detector

converts the ADL5330 from an open-loop variable gain amplifier

to a closed-loop output power control circuit. Because the AD8318,

setpoint transfer function also follows a linear-in-dB characteristic.

CIRCUIT DESCRIPTION

Although the ADL5330 variable gain amplifier provides accurate

gain control, precise regulation of output power can be achieved

with an automatic gain control (AGC) loop. Figure 1 shows the

ADL5330 operating in an AGC loop. The addition of the AD8318

log amp allows the AGC to have improved temperature stability

over a wide output power control range.

To operate the ADL5330 VGA in an AGC loop, a sample of the

output RF must be fed back to the detector (typically using a

directional coupler and additional attenuation). A setpoint voltage

is applied by a DAC to the VSET input of the detector while VOUT

is connected to the GAIN pin of the ADL5330. Based on the

detector’s defined linear-in-dB relationship between VOUT and the

RF input signal, the detector adjusts the voltage on the GAIN pin

(the detector’s VOUT pin is an error amplifier output) until the

level at the RF input corresponds to the applied setpoint voltage.

GAIN settles to a value that results in the correct balance between

the input signal level at the detector and the setpoint voltage.

INLO

ETC1-1-13

(M/A-COM)

ETC1-1-13

(M/A-COM)

INHI

GAIN

OPLO

OPHI

DIRECTIONAL

COUPLER

ATTENUATOR

VPSx

COMx

ADL5330

+5V

+5V

+5V

CMxP

VOUT

VPSx

VSET

INHI

INLO

CLPF

AD8318

LOG AMP

DAC*

RF INPUT

SIGNAL

RF OUTPUT

SIGNAL

412

Ω

1k

Ω

SETPOINT

VOLTAGE

220pF

1nF

1nF

120nH

120nH

100pF

100pF

100pF

100pF

*SEE COMMON VARIATIONS SECTION

Figure 1. ADL5330 Operating in an Automatic Gain Control Loop in Combination with the AD8318 (Simplified Schematic: Decoupling and

All Connections Not Shown)