The term "video" comes from video amplifiers that are used to amplify signals that
represent video information. Video is the "picture" portion of a television signal. The
"sound" portion is audio. Video signals are not only used for television, they are also used
for radar systems. Therefore, radar systems use video amplifiers. Video amplifiers are also
used in video recorders and some communication and control devices. In addition to using
video amplifiers, televisions use RF amplifiers. Many other devices also use RF amplifiers
(such as radios, navigational devices, and communications systems). Almost any device
that uses broadcast or transmitted information will use an RF amplifier.
Remember, RF amplifiers are used to amplify signals between 10 KHz and
100,000 MHz (not this entire band of frequencies, but any band of frequencies within these
limits). Therefore, any device that uses frequencies between 10 KHz and
100,000 MHz will most likely use an RF amplifier.
You will need to learn a little more about the frequency response of an amplifier
and frequency-response curves before you learn about video and RF amplifiers. In
chapter 5 you were shown the frequency-response curve of an audio amplifier. There is a
frequency-response curve associated with every amplifier. Frequency-response curves are
used because they provide a "picture" of the performance of an amplifier at various
frequencies. You will probably never have to draw a frequency-response curve. However,
in order to use one, you should know how a frequency-response curve is created. The
amplifier for which the frequency-response curve is created is tested at various frequencies.
At each frequency, the input signal is set to some predetermined level of voltage (or
current). This same voltage (or current) level for all of the input signals is used to provide a
standard input and to allow evaluation of the output of the circuit at each of the frequencies
tested. For each of these frequencies, the output is measured and marked on a graph. The
graph is marked "frequency" along the horizontal axis and "voltage" or "current" along the
vertical axis. When points have been plotted for all of the frequencies tested, the points are
connected to form the frequency-response curve. The shape of the curve represents the
frequency response of the amplifier.
Some amplifiers should be "flat" across a band of frequencies. In other words, for
every frequency within the band, the amplifier should have equal gain (equal response).
For frequencies outside the band, the amplifier gain will be much lower. For other
amplifiers, the desired frequency response is different. For example, perhaps the amplifier
should have high gain at two frequencies and low gain for all other frequencies. The
frequency-response curve for this type of amplifier would show two "peaks." In other
amplifiers the frequency-response curve will have one peak indicating high gain at one
frequency and lower gain at all others.
Figure 6-1 shows a frequency-response curve. This is the frequency-response
curve for an audio amplifier as described in chapter 5. It is "flat" from 15 Hz to 20 KHz.
Also notice that the lower frequency limit is labeled f1 and the upper frequency limit is
labeled f2. Notice also the portion inside the frequency-response curve marked
23 June 2005