Stable, Closed-Loop, Automatic Power Control for RF Applications

The circuit shown in this application note provides closed-loop, automatic power control using a variable gain amplifier (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 radio frequency (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, like the ADL5330, has a linear in dB transfer function, the output power (POUT) vs. the setpoint transfer function also follows a linear in dB characteristic.

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