2.5GHz 45dB RF-Detecting Controllers

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Detailed Description

The MAX4000/MAX4001/MAX4002 family of logarithmic

amplifiers (log amps) is comprised of four main amplifi-

er/limiter stages each with a small-signal gain of 10dB.

The output stage of each amplifier is applied to a full-

wave rectifier (detector). A detector stage also pre-

cedes the first gain stage. In total, five detectors each

separated by 10dB, comprise the log amp strip. Figure

1 shows the functional diagram of the log amps.

A portion of the PA output power is coupled to RFIN of

the log amp controller, and is applied to the log amp

strip. Each detector cell outputs a rectified current and

all cell currents are summed and form a logarithmic

output. The detected output is applied to a high-gain

OUT is applied to the gain-control pin of the PA to close

the control loop. The voltage applied to SET determines

the output power of the PA in the control loop. The volt-

age applied to SET relates to an input power level

determined by the log amp detector characteristics.

Extrapolating a straight-line fit of the graph of SET vs.

RFIN provides the logarithmic intercept. Logarithmic

slope, the amount SET changes for each dB change of

RF input, is generally independent of waveform or ter-

mination impedance. The MAX4000/MAX4001/

MAX4002 slope at low frequencies is about 25mV/dB.

Variance in temperature and supply voltage does not

alter the slope significantly as shown in the Typical

Operating Characteristics.

The MAX4000/MAX4001/MAX4002 are specifically des-

igned for use in PA control applications. In a control

loop, the output starts at approximately 2.9V (with sup-

ply voltage of 3V) for the minimum input signal and falls

to a value close to ground at the maximum input. With a

portion of the PA output power coupled to RFIN, apply

a voltage to SET and connect OUT to the gain-control

pin of the PA to control its output power. An external

capacitor from the CLPF pin to ground sets the band-

width of the PA control loop.

Transfer Function

Logarithmic slope and intercept determine the transfer

function of the MAX4000/MAX4001/MAX4002 family of

log amps. The change in SET voltage per dB change in

RF input defines the logarithmic slope. Therefore, a

10dB change in RF input results in a 250mV change at

SET. The Log-Conformance plots (see Typical Oper-

ating Characteristics) show the dynamic range of the

log amp family. Dynamic range is the range for which

the error remains within a band of ±1dB.

The intercept is defined as the point where the linear

response, when extrapolated, intersects the y-axis of

the Log-Conformance plot. Using these parameters,

the input power can be calculated at any SET voltage

level within the specified input range with the following

equation:

where SET is the set-point voltage, SLOPE is the loga-

rithmic slope (V/dB), RFIN is in either dBm or dBV and

IP is the logarithmic intercept point utilizing the same

units as RFIN.

Applications Information

Controller Mode

Figure 2 provides a circuit example of the MAX4000/

MAX4001/MAX4002 configured as a controller. The

MAX4000/MAX4001/MAX4002 require a 2.7V to 5.5V

supply voltage. Place a 0.1µF low-ESR, surface-mount

Electrically isolate the RF input from other pins (espe-

cially SET) to maximize performance at high frequencies

(especially at the high-power levels of the MAX4002).

The MAX4000 has an internal input-coupling capacitor

RFIN

SET

SLOPE

IP

=+

Figure 1. Functional Diagram

OUT

N.C.

GND

CLPF

SET

RFIN

MAX4000

SHDN

DAC

RF INPUT

XX

POWER AMPLIFIER

ANTENNA

50

Ω

0.1

µF

Figure 2. Controller Mode Application Circuit Block

10dB

DET

10dB

10dB

10dB

DET

DET

DET

DET

OFFSET

COMP

LOW-

NOISE

BANDGAP

OUTPUT

ENABLE

DELAY

GND

(PADDLE)

gm

+

-

X1

V-I

RFIN

SHDN

OUT

CLPF

SET

MAX4000