voltage gain, but the output power is greater than the input power, the amplifier is a power
FREQUENCY RESPONSE OF AMPLIFIERS
5-16. In addition to being classified by function, amplifiers are classified by frequency
response. You may wonder why the frequency response is important. The frequency
response of an amplifier refers to the band of frequencies or frequency range that the
amplifier was designed to amplify.
5-17. The reason an amplifier, designed to amplify a signal of 100 Hz, does not work as
well at 1,000 MHz is because the components of the amplifier respond differently at
different frequencies. The amplifying device (such as the electron tube, transistor, magnetic
amplifier, and so forth) itself will have frequency limitations and respond in different ways
as the frequency changes. Capacitors and inductors in the circuit will change their
reactance as the frequency changes. Even the slightest amounts of capacitance and
and self-inductance) can become significant at high frequencies. Since the response of
components varies with the frequency, the components of an amplifier are selected to
amplify a certain range or band of frequencies.
amplifier, RF amplifier, and video amplifier. An audio amplifier is designed to amplify
frequencies between 15 Hz and 20 KHz. Amplifiers that are designed for this entire band
of frequencies or any band of frequencies contained in the audio range is considered to be
an audio amplifier. RF amplifiers are designed to amplify frequencies between 10 KHz and
100,000 MHz. A single amplifier will not amplify the entire RF range, but any amplifier
whose frequency band is included in the RF range is considered an RF amplifier. A video
amplifier is an amplifier designed to amplify a band of frequencies from 10 Hz to 6 MHz.
Since this is such a wide band of frequencies, these amplifiers are sometimes called wide-
band amplifiers. While a video amplifier will amplify a very wide band of frequencies, it
does not have the gain of narrower-band amplifiers. It also requires many more
components than a narrow-band amplifier to enable it to amplify a wide range of
5-19. A transistor amplifier is a current-control device. The current in the base of the
transistor (which is dependent on the emitter-base bias) controls the current in the collector.
A vacuum-tube amplifier is also a current-control device. The grid bias controls the plate
5-20. You might hear that a vacuum tube is a voltage-operated device (since the grid
does not need to draw current) while the transistor is a current-operated device. You might
agree with this statement, but the vacuum tube and the transistor are still current-control
devices. The whole secret to understanding amplifiers is to remember that fact. Once
current is controlled you can use it to give you a voltage gain or a power gain.
5-21. This chapter will use transistor amplifiers to present the concepts and principles of
amplifiers. These concepts apply to vacuum-tube amplifiers and, in most cases, magnetic
amplifiers as well as transistor amplifiers.
23 June 2005