We will assume initially that the output of the PA is at some

low level and that the V

converter converts the V

I

pacitor C. Current flow from this direction increases the

output voltage of the integrator. The output voltage, which

drives the V

that the PA’s gain control input has a positive sense, that is,

increasing voltage increases gain). The gain will increase,

thereby increasing the amplifier’s output level until the de-

tector output current equals the ramp current I

point, the current through the capacitor will decrease to zero

and the integrator output will be held constant, thereby set-

tling the loop. If capacitor charge is lost over time, output

voltage will decrease. However, this leakage will quickly be

corrected by additional current from the detector. The loop

stabilizes to I

between the V

independent of the PA’s V

Power Control Over Wide Dynamic Range

The circuit as described so far, has been designed to pro-

duce a temperature independent output power level. If the

detector has a high dynamic range, the circuit can precisely

set PA output levels over a wide power range. To set a PA

output power level, the reference voltage, V

To estimate the response of P

of the LMV242 should be known (P

as discussed is section 3).

The relation between P

of 2 curves:

•

I

•

V

I

curve by the feedback resistor used for measuring. With the

knowledge that I

function P

tion should be inserted between PA output and LMV242’s

P

Using a closed loop to control the PA has benefits over the

use of a directly controlled PA. Non-linearity’s and tempera-

ture variations present in the PA transfer function do not

appear in the overall transfer function, P

response of a typical closed loop is given in Figure 3. The

shape of this curve is determined by the response of the

controller’s detector. Therefore the detector needs to be

accurate, temperature stable and preferably linear in dB to

achieve a accurately controlled output power. The only re-

quirement for the control loop is that the gain control function

of the PA has to be monotonic. With a linear in dB detector,

the relation between V

linear in dB as well, which makes calibration of the system

easy.

The response time of the loop can be controlled by varying

the RC time constant of the integrator. Setting this at a low

level will result in fast output settling but can result in ringing

in the output envelope. Setting the RC time constant to a

high value will give the loop good stability but will increase

settling time.

ATTENUATION BETWEEN COUPLER AND LMV242

DETECTOR

Figure 4 shows a practical RF power control loop realized by

using the National’s LMV242 with integrated RF detector.

The RF signal from the PA passes through a directional

coupler on its way to the antenna. Directional couplers are

characterized by their coupling factor, which is in the 10 dB

to 30 dB range, typical 20 dB. Because the coupled output

must in its own right deliver some power (in this case to the

detector), the coupling process takes some power from the

main output. This manifests itself as insertion loss, the inser-

tion loss being higher for lower coupling factors.

It is very important to choose the right attenuation between

PA output and detector input to achieve power control over

the full output power range of the PA. A typical value for the

output power of the PA is +35.5 dBm for GSM and +30 dBm

for PCS/DCS. In order to accommodate these levels into the

LMV242 detection range the minimum required total attenu-

ation is about 35 dBm (please refer to typical performance

characteristics in the datasheet and Figure 2). A typical

coupler factor is 20 dB. An extra attenuation of about 15 dB

should be inserted.

20079521

FIGURE 2. P

20079522

FIGURE 3. Closed Loop Response

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