Amplifiers

Amplifiers
Amplifiers make signals larger. Signal boost, or gain, is measured in decibels (dB). Amplifiers can be
broadly classified into three categories: low noise, high power, and everything else. Low-noise amplifiers
(LNAs) are usually connected to an antenna to boost the received signal to a level that is recognizable by
the electronics to which the RF system is connected. LNAs are also rated with a noise figure, which is the
measure of how much extraneous information the amplifier introduces to the signal-to-noise ratio (SNR).
Smaller noise figures enable the receiver to hear smaller signals and thus provide a greater range.
High-power amplifiers (HPAs) are used to boost a signal to the maximum power possible before
transmission. Output power is measured in dBm, which are related to watts. Amplifiers are subject to the
laws of thermodynamics; they give off heat in addition to amplifying the signal. The transmitter in an
802.11 PC card is necessarily low power because it needs to run off a battery if it is installed in a laptop,
but it is possible to install an external amplifier at feed APs. This amplifier can be connected to the power
grid where power is more plentiful. This is where things can get tricky with respect to compliance with
regulations. 802.11 devices are limited to 1 watt of power output and 4 watts of effective radiated power
(ERP). ERP multiplies the transmitter's power output by the gain of the antenna minus the loss in the
transmission line. With a 1 watt amplifier, an antenna that provides 8 dB of gain, and 2 dB of transmission
line loss, the result is an ERP of 4 watts; the total system gain is 6 dB, which multiplies the transmitter's
power by a factor of 4.
[1]Matthew Gast, 802.11b Wireless Networks: The Definitive Guide (Sebastopol, California: O'Reilly &
Associates, 2002), 316–322.