AD7294 datasheet(45/48 Pages) AD

45 / 48 page

AD7294

Data Sheet

Rev. I | Page 44 of 47

GAIN CONTROL OF POWER AMPLIFIER

In gain control mode, a setpoint voltage, proportional in dB to

the desired output power, is applied to a power detector such as

the AD8362. A sample of the output power from the power

amplifier (PA), through a directional coupler and attenuator (or

by other means), is fed to the input of the AD8362. The VOUT

is connected to the gain control terminal of the PA, see Figure 60.

Based on the defined relationship between VOUT and the RF

input signal, the AD8362 adjusts the voltage on VOUT (VOUT

is now an error amplifier output) until the level at the RF input

corresponds to the applied VSET. The AD7294 completes a

feedback loop that tracks the output of the AD8362 and adjusts

the VSET input of the AD8362 accordingly.

VOUT of the AD8362 is applied to the gain control terminal of

the power amplifier. For this output power control loop to be

stable, a ground referenced capacitor must be connected to the

CLPF pin. This capacitor integrates the error signal (which is

actually a current) that is present when the loop is not balanced.

In a system where a variable gain amplifier (VGA) or variable

voltage attenuator (VVA) feeds the power amp, only one AD8362

is required. In such a case, the gain on one of the devices (VVA,

PA) is fixed and VOUT feeds the control input of the other.

1nF

0.1nF

1:4

AD8362

AD7294

CLPF

ENVELOPE OF

TRANSMITTED

SIGNAL

DIRECTIONAL

COUPLER

ATTENUATOR

POWER

AMPLIFIER

RF IN

VIN

VOUT

VSET

INLO

INHI

Figure 60. Setpoint Controller Operation

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