Detailed Description

The MAX9930–MAX9933 family of logarithmic ampli-

fiers (log amps) comprises four main amplifier/limiter

stages each with a small-signal gain of 10dB. The out-

put stage of each amplifier is applied to a full-wave rec-

tifier (detector). A detector stage also precedes 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 logarithmic amplifier controller/detector, and is

applied to the logarithmic amplifier strip. Each detector

cell outputs a rectified current and all cell currents are

summed and form a logarithmic output. The detected

buffered and then applied to OUT. For the

MAX9930/MAX9931/MAX9932, OUT is applied to the

gain-control input 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 voltage applied

to SET relates to an input power level determined by

the log amp detector characteristics. For the MAX9933,

OUT is applied to an ADC typically found in a base-

band IC which, in turn, controls the PA biasing with the

output (Figure 2).

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 termi-

nation impedance. The MAX9930/MAX9931/MAX9932

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 MAX9930/MAX9931/MAX9932 are specifically

designed for use in PA control applications. In a control

loop, the output starts at approximately 2.9V (with supply

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

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

tion of the PA output power coupled to RFIN, apply a volt-

age to SET (for the MAX9930/MAX9931/MAX9932) and

connect OUT to the gain-control pin of the PA to control

its output power. An external capacitor from CLPF to

ground sets the bandwidth of the PA control loop.

Transfer Function

Logarithmic slope and intercept determine the transfer

function of the MAX9930–MAX9933 family of log amps.

The change in SET voltage (OUT voltage for the

MAX9933) per dB change in RF input defines the loga-

rithmic slope. Therefore, a 10dB change in RF input

results in a 250mV change at SET (OUT for the

MAX9933). The Log Conformance vs. Input Power plots

(see

Typical Operating Characteristics) show the dynam-

ic 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 vs. Input Power plot. Using these

parameters, the input power can be calculated at any SET

voltage level (OUT voltage level for the MAX9933) within

the specified input range with the following equations:

RFIN = (SET / SLOPE) + IP

(MAX9930/MAX9931/MAX9932)

RFIN = (OUT / SLOPE) + IP

(MAX9933)

where SET is the set-point voltage, OUT is the output

voltage for the MAX9933, SLOPE is the logarithmic slope

(V/dB), RFIN is in either dBm or dBV and IP is the loga-

rithmic intercept point utilizing the same units as RFIN.

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OUT

N.C.

GND

50Ω

50Ω

RFIN

SHDN

CLPF

GND

DAC

ADC

0.01µF

XX

PA

BASEBAND

IC

TRANSMITTER

MAX9933

Figure 2. MAX9933 Typical Application Circuit

2MHz to 1.6GHz 45dB RF-Detecting

Controllers and RF Detector

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