Datasheet

LTC6431-15
11
643115f
The LTC6431-15 is a highly linear fixed-gain amplifier
which is designed for ease of use. Implementing an RF
gain stage is often a multistep project. Typically an RF
designer must choose a bias point and design a bias
network. Next the designer needs to address impedance
matching with input and output matching networks and,
finally, add stability networks to ensure stable operation
in and out of band. These tasks are handled internally
within the LTC6431-15.
The LTC6431-15 has an internal self-biasing network
which compensates for temperature variation and keeps
the device biased for optimal linearity. Therefore, input
and output DC blocking capacitors are required.
Both the input and output are internally impedance matched
to 50Ω from 20MHz to 1700MHz. Similarly, an RF choke is
required at the output to deliver DC current to the device.
The RF choke acts as a high impedance (isolation) to
the DC supply which is at RF ground. Thus, the internal
LTC6431-15 impedance matching is unaffected by the
biasing network. The open collector output topology can
deliver much more power than an amplifier whose collector
is biased through a resistor or active load.
Choosing the Right RF Choke
Not all choke inductors are created equal. It is always
important to select an inductor with low R
LOSS
, as this will
drop the available voltage to the device. Also look for an
inductor with high self-resonant frequency (SRF) as this
will limit the upper frequency where the choke is useful.
Above the SRF, the parasitic capacitance dominates and
the choke impedance will drop. For these reasons, wire
wound inductors are preferred, and multilayer ceramic
chip inductors should be avoided for an RF choke. Since
the LTC6431-15 is capable of such wideband operation,
a single choke value will probably not result in optimized
performance across its full frequency band. Table 1 lists
target frequency bands and suggested corresponding
inductor values.
Table 1. Target Frequency Bands and Suggested Inductor Values
FREQUENCY BAND
(MHz)
INDUCTOR VALUE
(nH)
MODEL
NUMBER MANUFACTURER
20 to 100 1500nH 0805LS Coilcraft
www.coilcraft.com
100 to 500 560nH 0603LS
500 to 1000 100nH 0603LS
1000 to 2000 51nH 0603LS
DC Blocking Capacitor
The role of a DC blocking capacitor is straightforward:
block the path of DC current and allow a low series imped-
ance path for the AC signal. Lower frequencies require a
higher value of DC blocking capacitance. Generally, 1000pF
to 10000pF will suffice for operation down to 20MHz.
The LTC6431-15 is relatively insensitive to the choice of
blocking capacitor.
RF Bypass Capacitor
RF bypass capacitors act to shunt AC signals to ground
with a low impedance path. It is best to place them as close
as possible to the DC power supply pins of the device.
Any extra distance translates into additional series in-
ductance which lowers the self-resonant frequency and
useful bandwidth of the bypass capacitor. The suggested
bypass capacitor network consists of two capacitors:
a low value 1000pF capacitor to handle high frequencies
in parallel with a larger 0.1µF capacitor to handle lower
frequencies. Use ceramic capacitors of an appropriate
physical size for each capacitance value (e.g., 0402 for
the 1000pF, 0805 for the 0.1µF) to minimize the equiva-
lent series resistance (ESR) of the capacitor.
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