Texas Instruments TRF1500 - Manual (Page 18)

SWRA004A
TRF1500 Integrated Dual-Band RF Receiver User’s Guide 19
Low-Band LNA
In a typical down-conversion receiver, the LNA is usually placed
directly after the antenna and a band-select filter. The purpose of
the LNA is to amplify the desired signal being received while
adding as little undesired noise and distortion as possible. The
TRF1500 LNA is a common emitter amplifier, designed to operate
on a single 3.75 volt supply. The LNA has two selectable gain
states, normal state or strong signal state, which are controlled
with the digital CMOS control lines. The strong signal state, which
disables the LNA, is provided for operation in a high signal
environment such as near the base station. Operating near the
base station in the normal state could cause an increase in the
intermodulation product levels and thus cause undesired noise
and distortion in the receiver. Stand-alone LNA performance can
be ascertained by reconfiguring the evaluation board as noted on
the datasheet.
Low-Band LNA Turn on Time
The turn on time can be adjusted by changing the values of C10,
R6 and R7, as shown in Figure 3 and Figure 4. The resistors form
a voltage-divider network across the supply, Vcc. The function of
this network is to provide a bias condition near the ideal operating
region at the base of the common emitter amplifier. By providing
this bias condition, the charge time of the series capacitor, C10,
can be adjusted. Changing the value of resistors should not affect
gain, IIP3 or noise figure (NF) performance.
Figure 3. Voltage Divider at Low-Band LNA Input