 - An amplifier is an electronic device that
increases the voltage, current, or power of a signal. Amplifiers are used in wireless communications and
broadcasting, and in audio equipment of all kinds. They can be categorized as either weak-signal amplifiers or power amplifiers.
Weak-signal amplifiers are used primarily in wireless
receivers. They are also employed in acoustic pickups, audio tape players, and compact disc players. A weak-signal amplifier is designed to deal with exceedingly small input signals, in some cases measuring only a few nanovolts (units of 10-9 volt). Such amplifiers must generate minimal internal noise while increasing the signal voltage by a large factor. The most effective device for this application is the field-effect transistor. The specification that denotes the effectiveness of a weak-signal amplifier is sensitivity,
defined as the number of microvolts (units of 10-6 volt) of
signal input that produce a certain ratio of signal output to noise output (usually 10 to 1).
Power amplifiers are used in wireless transmitters,
broadcast transmitters, and hi-fi audio equipment. The most frequently-used device
for power amplification is the bipolar transistor.
However, vacuum tubes, once considered obsolete, are becoming
increasingly popular, especially among musicians. Many professional musicians believe that the vacuum tube (known as a "valve" in England) provides superior fidelity.
Two important considerations in power amplification are
power output and efficiency. Power output is measured in watts or kilowatts. Efficiency is the ratio of signal power output to
total power input (wattage demanded of the power supply or battery). This value is
always less than 1. It is typically expressed as a percentage. In audio
applications, power amplifiers are 30 to 50 percent efficient. In wireless communications and broadcasting transmitters, efficiency ranges from about 50 to 70
percent. In hi-fi audio power amplifiers, distortion is also an important
factor. This is a
measure of the extent to which the output waveform is a faithful
replication of the input waveform. The lower the distortion, in general, the better the
fidelity of the output sound.
| LAST UPDATED: |
26 Nov 2000
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