____________________________________________________ Video and Radio Frequency Amplifiers
Figure 6-1. Frequency-response Curve of Audio Amplifier
BANDWIDTH OF AN AMPLIFIER
6-8.
The bandwidth represents the amount or "width" of frequencies, or the "band of
frequencies," that the amplifier is most effective in amplifying. However, the bandwidth is
not the same as the band of frequencies that is amplified. The bandwidth of an amplifier is
the difference between the frequency limits of the amplifier. For example, the band of
frequencies for an amplifier may be from 10 KHz to 30 KHz. In this case, the bandwidth
would be 20 KHz. As another example, if an amplifier is designed to amplify frequencies
between 15 Hz and 20 KHz, the bandwidth will be equal to 20 KHz minus 15 Hz or 19,985
Hz (see Figure 6-1). The following shows how to compute bandwidth:
BW = f2 f1
BW = 20 KHz -15 Hz
BW = 20,000 Hz -15 Hz
BW = 19,985 Hz
6-9.
Also notice in Figure 6-1 that the frequency-response curve shows output voltage
known as HALF-POWER POINTS. The half-power points are the points at which the
output voltage (or current) is 70.7 percent of the maximum output voltage (or current). Any
frequency that produces less than 70.7 percent of the maximum output voltage (or current)
is outside the bandwidth and, in most cases, is not considered a usable output of the
amplifier. The reason these points are called "half-power points" is that the true output
power will be half (50 percent) of the maximum true output power when the output voltage
(or current) is 70.7 percent of the maximum output voltage (or current).
23 June 2005
TC 9-62
6-3
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