MAX2472/MAX2473
500MHz to 2500MHz
VCO Buffer Amplifiers
10 ______________________________________________________________________________________
Applications Information
Input Considerations
The MAX2472/MAX2473 offer high-impedance inputs
ideal for low-distortion buffering of a VCO. For applica-
tions with discrete transistor-based oscillator designs,
simply AC-couple the oscillator directly to the input; the
buffer’s high input impedance results in minimal load-
ing on the oscillator. For use with 50ΩVCO modules,
determine the approximate input impedance (S11) of
the MAX2472/MAX2473 from the S parameter tables in
the
Typical Operating Characteristics
. Terminate the
buffer input with a shunt resistor “RTERM” such that
RTERM
||
RE[S11] = 50Ω, and use a series blocking
capacitor between the shunt resistor and the device
input. This provides a very stable 50Ωtermination and
increases reverse isolation. For those applications
needing both high gain and good input match, reac-
tively match the buffer inputs to 50Ωwith simple two-
element matching circuits. Figure 1 shows an equivalent
input circuit of the MAX2472/MAX2473 IN pin.
Output Considerations
The MAX2472/MAX2473 outputs are configured as
open-collector output stages, allowing maximum flexi-
bility. To achieve maximum gain and output drive, reac-
tively match these outputs with a shunt inductor/choke
to VCC followed by a series capacitor (DC block). For
applications not requiring high gain and output drive
(PLL prescaler), use a simple resistor to VCC followed
by a DC blocking capacitor. Figure 2 shows an equiva-
lent circuit of the MAX2472/MAX2473 OUT pin(s).
Figure 3 shows an equivalent circuit of the MAX2473
BIAS pin.
Output Matching Circuit
Table 1 lists recommended values for lumped-element
output matching circuits at certain frequencies. For fre-
quencies other than those specified, use the parame-
ters given in the
Typical Operating Characteristics
.
Layout and Power-Supply Bypassing
A properly designed PC board is essential to any
RF/microwave circuit. Be sure to use controlled imped-
ance lines on all high-frequency inputs and outputs.
Bypass the power supply with decoupling capacitors