AD8314

–13–

REV. 0

The setpoint voltage, in the range 0 V to 1.1 V, is applied to the

VSET pin of the AD8314. This will typically be supplied by a

Digital-to-Analog Converter (DAC). This voltage is compared

to the input level to the AD8314. Any imbalance is between VSET

and the RF input level is corrected by V_DN, which drives the

maximum value of approximately 1.9 V on a 2.7 V supply (this

will be higher for higher supply voltages) while delivering approxi-

dynamics of the power amplifier, something that is frequently

poorly characterized, so some trial and error may be necessary.

In this example, a 220 pF capacitor gives the loop sufficient

speed to follow the GSM and DCS1800 time slot ramping profiles,

while still having a stable, critically-damped response.

Figure 35 shows the relationship between the setpoint voltage,

function is linear in dB for a dynamic range of over 40 dB.

Figure 36 shows a similar circuit for a single band handset power

amplifier. The BGY241 (Phillips) is driven by a nominal power

level of 0 dBm. A 20 dB directional coupler, DC09-73 (Alpha) is

used to couple the signal in this case. Figure 37 shows the rela-

tionship between the control voltage and the output power at

0.9 GHz.

In both of these examples, noise on the V_DN pin can be reduced

control pin of the power amplifier. However, the value of the

resistor should be kept low to minimize the voltage drop across

it due to the dc current flowing into the gain control input.

VSET – Volts

–30

0

0.2

0.4

0.6

0.8

1.0

1.2

–20

–10

0

10

20

30

40

Figure 35. POUT vs. VSET at 0.9 GHz for Dual Mode

Handset Power Amplifier Application

ENBL

RFIN

AD8314

VSET

FLTR

VPOS

COMM

V UP

2.7V

0V–1.1V

RF INPUT

0dBm

MAX

ATTN

15dB

V DN

220pF

+35dBm

MAX

47 F

TO

ANTENNA

BGY241

+15dBm

2.2 F

680pF

0dBm

DC09-73

6

3

4

5

12

3.5V

0.1 F

52.3

Figure 36. A Single Mode Power Amplifier Control Circuit

VSET – Volts

–30

0

0.2

0.4

0.6

0.8

1.0

–20

–10

0

10

20

30

40

–40

–50

Figure 37. POUT vs. VSET at 0.9 GHz for Single Mode

Handset