classification is by their frequency response; in other words, what frequencies they are
designed to amplify.
5-10. If you describe an amplifier by these two classifications, you will have a good
working description of the amplifier. You may not know what the exact circuitry is, but
you will know what the amplifier does and the frequencies that it is designed to handle.
VOLTAGE AMPLIFIERS AND POWER AMPLIFIERS
5-11. All amplifiers are current-control devices. The input signal to an amplifier controls
the current output of the amplifier. The connections of the amplifying device (such as the
electron tube, transistor, magnetic amplifier, and so forth) and the circuitry of the amplifier
determine the classification.
5-12. A voltage amplifier is an amplifier in which the output signal voltage is larger than
the input signal voltage. In other words, a voltage amplifier amplifies the voltage of the
5-13. A power amplifier is an amplifier in which the output signal power is greater than
the input signal power. In other words, a power amplifier amplifies the power of the input
signal. Most power amplifiers are used as the final amplifier (stage of amplification) and
control (or drive) the output device. The output device could be a speaker, an indicating
device, an antenna, or the heads on a tape recorder. Whatever the device, the power to
make it work (or drive it) comes from the final stage of amplification.
5-14. Figure 5-2 shows a simple block diagram of a voltage amplifier with its input and
output signals and a power amplifier with its input and output signals. Notice that in view
(A) the output signal voltage is larger than the input signal voltage. Since the current
values for the input and output signals are not shown, you cannot tell if there is a power
gain in addition to the voltage gain. In view (B), the output signal voltage is less than the
input signal voltage. As a voltage amplifier, this circuit has a gain of less than 1. However,
the output power is greater than the input power. Therefore, this circuit is a power
Figure 5-2. Block Diagram of Voltage and Power Amplifiers
5-15. The classification of an amplifier, as a voltage or power amplifier, is made by
comparing the characteristics of the input and output signals. If the output signal is larger
in voltage amplitude than the input signal, the amplifier is a voltage amplifier. If there is no
23 June 2005