900MHz SiGe, High IP3,

Low-Noise Amplifiers

6

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Pin Description

PIN

NAME

FUNCTION

1

BIAS

current (see Applications Information). The current through this pin is approximately 50mV divided

2, 5

GND

Ground. For optimum performance, provide a low-inductance connection to the ground plane.

3

RFIN

Amplifier Input. AC-couple to this pin with a DC-blocking capacitor. External matching network is

required for optimum performance.

Supply Voltage. Bypass with a 47pF capacitor directly to ground at the supply pin. Additional

6

RFOUT

Amplifier Output. Internally matched to 50

Ω. DC bias on this pin selects gain mode (MAX2642) or

shutdown mode (MAX2643) (see Applications Information).

Applications Information

Input Matching

Input matching is required for optimum performance.

The MAX2642/MAX2643 require a simple LC matching

network, as shown in the Typical Operating Circuit. To

further reduce cost and external component count,

replace the external inductor with a microstrip transmis-

sion line. The Typical Operating Circuit shows the rec-

ommended input-matching networks for the MAX2642/

MAX2643 at 900MHz. These values are optimized for

best simultaneous gain, noise figure, and return-loss

performance. To aid in the design of the matching net-

work for other frequencies, Tables 1–6 list typical

device S-parameters for various biases, and Tables 7,

8, and 9 list typical device noise parameters.

Attenuation Step (MAX2642)

The MAX2642’s DC bias voltage at RFOUT serves as

an attenuation step input. When the DC voltage at

RFOUT through a 33k

Ω resistor is less than +0.6V, the

device is in high-gain mode; if the DC voltage is greater

than +2.0V, the device is in low-gain mode. A standard

logic output can be applied as shown in the Typical

Operation Circuit. If no bias is applied, the device is in

high-gain mode.

Shutdown

For the MAX2643, the recommended shutdown method

is to set the DC voltage at the RFOUT pin in a manner

similar to the MAX2642’s attenuation step. That is, when

the DC voltage at RFOUT is below +0.6V, the device is

shut down; if the DC voltage is greater than +2.0V, the

device is enabled.

For the MAX2642, shutdown is achieved by leaving

BIAS unconnected. Figure 1 shows the suggested

shutdown methods. Avoid capacitance at the BIAS pin

by connecting the bias resistor from BIAS to the switch.

Table 10 summarizes the operational modes.

Layout Issues

A properly designed PC board is essential to any

RF/microwave circuit. Use controlled impedance lines

on all high-frequency inputs and outputs. Bypass with

additional decoupling capacitors. These additional

capacitors can be located farther away from the device

package. Proper grounding of the GND pins is essen-

tial. If the PC board uses a topside RF ground, connect

it directly to all GND pins. For a board where the

ground plane is not on the component layer, the best

technique is to connect the GND pins to the board with

a plated through-hole located close to the package.