Basics of Amplifiers
Basics, noise figure, output power capability and
gain
Amps have four fundamental qualities. noise figure, output
capability, gain and bandwidth. Noise Figure (NF) is a measure of the random
noise (entropy) the amplifier contributes to the signal. This is something you
really don't want, but every amp. adds noise to the signal. The less noise the
better. Common units of measure for this quality are "Noise Figure" (NF) in dB,
"Noise Temperature" in degrees kelvin, or equilavent noise input power. Less is
better. Output power is a measure of how much oomph the amp can put out before
it poops out. No amplifier is perfectly faithful to the input signal, each amp
contributing distortion like that you have heard on an overloaded PA. As output
power increases beyond its capability, the amplifier generates much more of its
own signal, that is to say distortion. An amp capable of more output power and
less distortion is better. Usual measures of output capability for antenna
amplifiers are output rating in dBmv, or the more accurate and technical ip3
figure or 1 dB compression output level. 
Gain is how much stronger the signal is on the output than the input. More gain
is better, so long as the signals never distort (they do). Gain is measured in
deciBells, or dB. 10 dB=10 times, 20 dB=100 times,30 dB=1,000 times and so
forth. Design engineers know that improving one quality will degrade another.
All life is compromise. High power costs NF. More gain adds noise from more
stages and increases distortion. To conclude, what you want is lowest noise
figure (nf), highest output power (dBmv), highest gain (dB). At the antenna,
emphasis is on NF, for distribution to multiple outlets emphasis is on power
output.
Why not to use an amplifier
An amplifier adds
noise, and distorts the signals.
Why to use an
amplifier
An amplifier will boost signal strength to over come noise
in the first amplifier built into your receiver. This can help if your coax lead
in is very long with resulting signal loss, or if you must split the signal to
various taps around the building.
Indoor Outdoor?
Use outdoor
amplifiers with very low noise figure to overcome loss in the coax lead in.
Boost the signal at the antenna where the weak signal will better overcome amp
noise. Use outdoor amplifiers where all signals are weak within the band over
which the amplifier works. Use indoor amplifiers with high output power to boost
signal before splitting. For example, if you split 10 ways, and coax loss is
about 5 dB, then you need 15 dB just to make up for splitting an coax loss.
However, gain must not push the amp into distortion, so it has to have adequate
output power. Some amplifiers have "tilt" controls allowing boost of high
frequencies to compensate for higher coax cable loss at higher frequencies.
Other Advice
For outdoor antenna mounted amps, look for
noise figure (NF) ratings below 3dB. If you can't find a NF rating, don't expect
much. If you have nearby stronger signals, as most of us do (two way radio, FM
stations, other TV signals) be sure to get an amp. with filters that limit
amplifier activity to a range of frequencies free of the strong signals. Best,
but most expensive is a single channel amp. Lo VHF (Band I), FM (Band II) high
VHF (Band III) and UHF (Band 4 and 5) filtering is not too expensive. For indoor
amps, we assume you have plenty of signal hitting the input of the amplifier, so
noise figure should not be important. When you hit the distortion limit of an
amplifier you will observe signals that were not there to begin with. These new
signals will be mixtures of two or more real signals, On an FM radio you will
hear two stations at once. On analog TV expect to see garbled images floating by
in a sea of wiggles. If your amplifier has a gain control, expect to see these
things come up rapidly as you get into distortion.
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