PA2

COUPLER

ANTENNA

50:

RFIN

OUT2

LMV242

PIN2

VRAMP

DETECTOR

ERROR AMP1

IDET

IRAMP

COMP1

VAPC2

C

V/I

ERROR AMP2

BS

PA1

PIN1

VAPC1

COMP2

OUT1

SWITCH

-

-

+

+

+

LMV242

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SNWS014C – APRIL 2004 – REVISED MAY 2013

Figure 25. PA Control Loop

We will assume initially that the output of the PA is at some low level and that the VRAMP voltage is at 1V. The V/I

converter converts the VRAMP voltage to a sinking current IRAMP. This current can only come from the integrator

capacitor C. Current flow from this direction increases the output voltage of the integrator. The output voltage,

which drives the VAPC of 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 IRAMP. At that point, the current through the

capacitor will decrease to zero and the integrator output will be held constant, thereby settling 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 IDET = IRAMP thereby creating a direct relation

between the VRAMP set voltage and the PA output power, independent of the PA's VAPC-POUT characteristics.

Power Control Over Wide Dynamic Range

The circuit as described so far, has been designed to produce 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, VRAMP, is varied. To estimate the response of POUT vs.

VRAMP, PIN vs. VRAMP of the LMV242 should be known (POUT = PIN + attenuation as discussed in ATTENUATION

BETWEEN COUPLER AND LMV242 DETECTOR).

The relation between PIN and VRAMP can be constructed out of 2 curves:

•

ICOMP vs, VRAMP

•

VOUT vs. RF Input Power (detection curve)

IOUT can be calculated by dividing the VOUT of the detection curve by the feedback resistor used for measuring.

With the knowledge that ICOMP = IOUT in a closed loop the resulting function PIN vs. VRAMP is shown in Figure 26.

Extra attenuation should be inserted between PA output and LMV242’s PIN to match their dynamic ranges.

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