Page 110 - Complete Wireless Design

Amplifier Design

Amplifier Design 109

External noise, produced by atmospheric upheavals like lightning, as well
as space noise caused by sunspots and solar flares and cosmic noise created by
interfering signals from the stars, is exacerbated by man-made electromag-
netic noise sources such as dimmer switches, neon lights, car ignitions, and
electric motors.

Amplifier design considerations. Since the data sheet is the dominant source of
information circuit designers have for selecting an active device for their own
specialized applications, it is especially important to understand data sheet
parameters as they apply to RF transistors.
Check out the device’s data sheet with your design to confirm that current,
voltage, and power limitations will not be exceeded in your wireless application;
whether in a nonsignal DC condition or under a maximum signal situation.
Obviously, the transistor’s f , P1dB, and G (maximum available gain) and,
T A(MAX)
for low-noise amplifier (LNA) applications, the NF, are all vital specifications.
The near maximum output power possible in an amplifier is the 1-dB com-
pression point (P1dB). This is the area where a linear amplifier begins to run
out of room for its maximum output voltage swing. Any amplifier will have
what is generally considered as a linear P until it reaches this P1dB point,
OUT
which occurs when a high enough input signal is injected into the amplifier’s
input. At P1dB, the gain of the amplifier will depart from the gain displayed
at lower input powers (Fig. 3.13), and for every decibel placed at the ampli-
fier’s input, no longer will there be a linear amplification of the signal. The
output gain slope flattens, and soon no significant increase in output power
is possible.

Figure 3.13 The third-order intercept and 1 dB compression points.

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