PIN NAME FUNCTION
1 BIAS
Resistor Bias Control. Connect a resistor, R
BIAS
, from BIAS to ground. R
BIAS
sets IP3 and supply
current (see Applications Information). The current through this pin is approximately 50mV divided
by R
BIAS
.
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.
4V
CC
Supply Voltage. Bypass with a 47pF capacitor directly to ground at the supply pin. Additional
bypassing may be necessary for long V
CC
lines.
6 RFOUT Amplifier Output. Internally matched to 50Ω. 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 33kΩ 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
decoupling capacitors located close to the device V
CC
pin. For long V
CC
lines, it may be necessary to add
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.
