FREQUENCY RESPONSE OF AMPLIFIERS
In addition to being classified by function, amplifiers are classified by frequency response. The
frequency response of an amplifier refers to the band of frequencies or frequency range that the amplifier
was designed to amplify.
You may wonder why the frequency response is important. Why doesn't an amplifier designed to
amplify a signal of 1000 Hz work just as well at 1000 MHz? The answer is that the components of the
amplifier respond differently at different frequencies. The amplifying device (electron tube, transistor,
magnetic amplifier, etc.) 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 slight amounts of capacitance and inductance between the circuit wiring and other
components (interelectrode capacitance 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.
NOTE: For explanations of interelectrode capacitance and self-inductance see NEETS Modules 2
Introduction to Alternating Current and Transformers; 6Introduction to Electronic Emission, Tubes,
and Power Supplies; and 7Introduction to Solid-State Devices and Power Supplies.
The three broad categories of frequency response for amplifiers are AUDIO AMPLIFIER, RF
AMPLIFIER, and VIDEO AMPLIFIER.
An audio amplifier is designed to amplify frequencies between 15 Hz and 20 kHz. Any amplifier that
is designed for this entire band of frequencies or any band of frequencies contained in the audio range is
considered to be an audio amplifier.
In the term rf amplifier, the "rf" stands for radio frequency. These 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.
Because 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 a great many more components than a narrow-band
amplifier to enable it to amplify a wide range of frequencies.
Q-4. In what two ways are amplifiers classified?
Q-5. What type of amplifier would be used to drive the speaker system of a record player?
Q-6. What type of amplifier would be used to amplify the signal from a radio antenna?
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 current. These facts are expanded upon in
NEETS Module 6, Introduction to Electronic Emission, Tubes and Power Supplies, and Module 7,
Introduction to Solid-State Devices and Power Supplies.