2.5GHz, 22dBm/20dBm Power Amplifiers with

Analog Closed-Loop Power Control

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9

RF Input/SHDN

RFIN/SHDN is a dual-function input for a 2.4GHz to

2.5GHz RF signal and a DC-coupled shutdown function.

The input port is internally matched to 50

Ω, making it

simple to interface the PAs to a 50

Ω source without

external matching components. The PAs are designed to

amplify input signal levels of 0 to 4dBm and, although

the PAs function for input signals outside this range, out-

put power and efficiency degrade. Note: Ensure that the

RF signal is present at the input when the PA is enabled.

If the RF signal is not present at startup, the PA functions

like any closed-loop control system and automatically

goes into a high-gain state, amplifying and transmitting

noise. Avoid this mode of operation.

The second function of the RFIN/SHDN is shutdown con-

trol. A DC voltage at the input port digitally controls the

on/off state with standard CMOS levels. The PA is in low-

current shutdown when the DC voltage is a valid logic

low and is active for a valid logic high. Connect the

SHDN signal to the RFIN/SHDN through a 1k

Ω resistor.

Connect the RF signal to the RFIN/SHDN with a 10pF

capacitor in series to block any DC from corrupting the

SHDN signal.

Output Matching

The output structure of these nonlinear PAs is an open-

collector transistor that requires external impedance

matching and pullup inductance for biasing. The recom-

mended output matching network is shown in the Typical

Application Circuits (Figure 1). The impedance presented

to the RFOUT pin is shown in Figure 2 and Table 2. This

impedance is specified relative to a reference plane at

the amplifier output into the matching network and load.

The matching network is for impedance transformation

that transforms 6

Ω to 50Ω with the specified maximum

output power. The network also forms a lowpass filter that

provides attenuation for the 2nd and 3rd harmonics. A

shunt capacitor (C7) is needed to perform the transforma-

tion, and the inductive 50

Ω transmission line (T2) is need-

ed to match that capacitance. A larger capacitor can be

used to increase the maximum output power, but the

transmission line also must be increased to maintain a

match with C7. A DC-blocking capacitor (C6) of 5pF to

10pF is necessary between the PA output and the trans-

mission line.

main purposes: it resonates out the capacitive PA output,

provides biasing for the output stage, and becomes a

high-frequency choke to reduce RF energy from coupling

Ω trans-

mission line (T1); however, chip inductors can be used

instead. The typical application circuit terminates the

transmission line with a capacitor (C6).

Analog Power Control (PC)

The PAs use a closed-loop power-control system for

consistent output power across input power, supply volt-

age, and temperature. Output power is internally moni-

tored and compared to the desired setting on PC. The

control amplifier then adjusts the first-stage variable-gain

amplifier until the output power matches the desired set-

ting. The result is that the output power is controlled by

the voltage applied to PC.

The power-control voltage range at PC for the

MAX2244/MAX2246 is 0 to 2V. Output power remains at

mately 0.4V, output power increases exponentially until

20dBm (MAX2246). See Figures 3a and 3c for the rela-

MAX2244 and MAX2246, respectively.

Likewise, the MAX2245 output power is controlled by

control voltage range of the MAX2245 is 0 to 2.2V, with

for the MAX2245.

MATCHING IMPEDANCE FOR RFOUT PIN

MAX2244: 6.26

Ω + j13.56Ω AT 2.45GHz

MAX2245: 6.35

Ω + j13.25Ω AT 2.45GHz

MAX2246: 5.50

Ω + j13.50Ω AT 2.45GHz

SMITH CHART

Figure 2. Impedance of Matching Network at RFOUT Pin

MAX2244

MAX2245

MAX2246

FREQUENCY

GHz

REAL

(

Ω)

IMAG

(

Ω)

REAL

(

Ω)

IMAG

(

Ω)

REAL

(

Ω)

IMAG

(

Ω)

2.40

6.47

13.2

6.61

12.94

5.73

13.01

2.45

6.26

13.5

6.35

13.25

5.50

13.50

2.50

6.06

13.9

6.11

13.59

5.27

14.02

Table 2. Matching Network Impedance