LTC6431-20
10
643120f
For more information www.linear.com/LTC6431-20
OPERATION
APPLICATIONS INFORMATION
The LTC6431-20 is a highly linear, fixed-gain amplifier that
is configured to operate single ended. Its core signal path
consists of a single amplifier stage, minimizing stability
issues. The input is a Darlington pair for high input imped
-
ance and high
current gain. Additional circuit enhancements
increase the output impedance and minimize the effects
of internal Miller capacitance.
The LTC6431-20 starts with a classic RF gain block to
-
pology but adds enhancements to dramatically improve
linearity.
Shunt and series feedback are added to lower the
input/output impedance and match them simultaneously
to the 50Ω source and load. Meanwhile, an internal bias
controller optimizes the internal operating point for peak
linearity over environmental changes. This circuit archi
-
tecture provides low noise, excellent RF power handling
capability and wide bandwidth — characteristics that are
desirable for IF signal chain applications.
The LTC6431-20 is a highly linear fixed gain amplifier which
is designed for ease of use. Implementing an RF gain stage
is often a multi-step project. Typically an RF designer must
choose a bias point and design a bias network. Next we
need 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 LTC6431-20.
The LTC6431-20 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 1400MHz. Similarly, an RF choke
is required at the output to deliver DC current to the de
-
vice. The RF choke acts as a high impedance (isolation)
to
the DC supply which is at RF ground. Thus, the internal
LTC6431-20 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 LTC6431-20 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 to100 1500nH 0603LS Coilcraft
www.coilcraft.com
100 to 500
560nH 0603LS
500 to1000 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 impedance
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
LTC6431-20 is relatively insensitive to the choice of block
-
ing 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 de
-
vice. Any
extra distance translates into additional series
inductance
which lowers the self-resonant frequency and
