Application Information

1.0 The LMV243 as an RF Power Amplifier (PA) Control-

ler

The LMV243 is a member of the power loop controller family

of National Semiconductor, for a quad-band TDMA/GSM

solution. The typical application diagram demonstrates a

basic approach for implementing the quad-band solution

around the RF Power Amplifier. The LMV243 contains a 50

dB Logamp detector and interfaces directly with the direc-

tional coupler.

The LMV243 Base Band (control) interface consists of 2

signals:

TX_EN to bring the device out of shutdown status

within 5µs, and V

determining the desired Output Power level. The LMV243

gives maximum flexibility to meet GSM frequency and time

mask criteria for many different single supply Power Ampli-

fier types like HBT or, MesFET in GaAs, SiGe or Si technol-

ogy. This is accomplished by the Programmable Ramp char-

acteristic from the Base Band and the TX_EN signal along

with the external compensation capacitor.

Power consumption requirements are supported by the

TX_EN function which puts the entire chip into a Power

Saving Mode to enable maximum standby and talk time

while ensuring the output does not glitch excessively during

Power-up and Power-down.

2.0 A Typical GSM Power Amplifier Controlled Loop

This section should give a general overview and understand-

ing of how a typical Power Amplifier control loop works and

how to get rid of some of the most common problems

confronted in the design.

Figure 2 shows the generic com-

ponents of such a loop. Beginning at the output of the GSM

Power Amplifier (PA), this signal is fed, usually via a direc-

tional coupler, to a detector. The output current of the detec-

tor Idet drives the inverting input of an op amp, configured as

an integrator. A reference voltage drives the non-inverting

input of the op amp. Finally the output of the op amp inte-

grator drives the gain control input of the power amplifier.

Now to examine how this circuit works, we will assume

initially that the output of the PA is at some low level and that

the V

V

only come from the integrator capacitor C. Current flow in

this direction increases the output voltage of the integrator.

This voltage, which drives the PA, increases the gain (we

assume 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 detector output current equals the ramp current I

that point, the current through the capacitor will decrease to

zero and the integrator output will be held steady, thereby

settling the loop. If capacitor charge is lost over time, the

gain will decrease. However, this leakage will quickly be

corrected by additional integrator current from the newly

reduced detector current.

The key usefulness of this circuit lies in its immunity to

changes in the PA gain control function. From a static per-

spective at least, the relationship between gain and gain

control voltage is of no consequence to the overall transfer

function. Based upon the value of V

set the gain control voltage to whatever level is necessary to

produce the desired output level. Any temperature depen-

dency in the gain control function will be eliminated. Also,

non-linearity’s in the gain transfer function of the PA do not

appear in the overall transfer function (V

only requirement is that the gain control function of the PA be

monotonic. It is crucial, however, that the detector is tem-

perature stable.

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

duce a constant output level for varying input levels. The

only requirement is for it to be temperature stable for input

levels that correspond to the setpoint voltage V

detector used has a higher dynamic range, the circuit to

precisely set PA output levels over a wide dynamic range. To

do this, the integrator reference voltage, V

The voltage range on V

tor’s transfer function. For example, if the detector delivers

0.5V for an input of −7dBm, a reference voltage of 0.5V will

cause the loop to settle when the detector input is −7dBm

(the PA output will be greater than this amount by whatever

coupling factor exists between PA and detector). The dy-

namic range for the variable RF P

mined by the device in the circuit with the least dynamic

range (i.e. the gain control range PA or linear dynamic of

detector).

20029037

FIGURE 2. PA Control Loop

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