1-4
etc.) and the circuitry of the amplifier determine the classification. Amplifiers are classified as voltage or
power amplifiers.
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 input signal.
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
which is a power amplifier.
Figure 1-3 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.
Figure 1-3A.—Block diagram of voltage and power amplifiers.
Figure 1-3B.—Block diagram of voltage and power amplifiers.
In view (B) of the figure the output signal voltage is less than the input signal voltage. As a voltage
amplifier, this circuit has a gain of less than 1. The output power, however, is greater than the input
power. Therefore, this circuit is a power amplifier.
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 voltage gain, but the output power is greater
than the input power, the amplifier is a power amplifier.
