The Differences Between a Power Amplifier and a Voltage Amplifier
By John Papiewski
Amplifiers are electronic circuits that increase the strength of signals. Hundreds of amplifier designs exist, ranging from simple single-transistor circuits to elaborate digital systems. A power amplifier and a voltage amplifier represent two designs that strengthen signals in different ways. Understanding the difference between voltage and power and their applications helps you select the correct amplifier type for a project.
Voltage and Power
A circuit's voltage is the energy behind electric charges flowing through wires and components. The volt is the standard unit of voltage, defined as the potential energy difference across a resistance of one ohm, which produces a current flow of one amp. The greater the energy, the higher the voltage. When you multiply the voltage by the current you get the circuit's power, which is expressed in terms of watts: one watt is one volt times one amp. Power is the rate at which a circuit delivers energy -- circuits that handle more voltage and current produce more watts of power.
You can think of an amplifier as a valve controlled by an input signal. As the curve of the signal's wave increases, a valve connected to an independent voltage supply opens, passing more of the voltage to the output. When the wave decreases, the valve closes and the output sees less voltage. The output, though proportionally greater, is a mirror image of the input. You use an amplifier to increase the voltage or power of a weak input signal. If the signal's power is weak, you use a power amplifier. If the voltage is small, you multiply it with a voltage amplifier.
Power, Efficiency and Heat
Because no circuit is 100 percent efficient, an amplifier wastes some of its power as heat. The efficiency of various amplifier designs varies from about 20 percent to 90 percent or better; an amplifier of 80 percent efficiency which consumes 100 watts wastes 20 watts as heat. Although voltage amplifiers also have this issue, it is a greater problem with power amplifiers. A power amplifier uses robust components, cooling fans and other strategies to prevent overheating.
A transistor combined with a few resistors and capacitors acts as a simple amplifier. Some circuits use vacuum tubes for power or voltage amplification. Operational amplifiers, or op-amps, are integrated circuits that make excellent voltage amplifiers; however, they are too small to be good power amplifiers. Power amplifiers use large-sized transistors and other components to handle the extra heat without burning up. Power transistors come in hefty metal cases that conduct heat away from the device. Voltage amplifiers use smaller, plastic transistors which run at room temperature.
All amplifiers provide a certain amount of gain, or amplification factor. For example, a voltage amplifier with a gain of 100 produces a 1-volt output for a 10-millivolt input. Gain is also measured in decibels, or dBs; the decibel rating is the common log of the gain factor multiplied by 10. The common log of 100 is 2; multiplying it by 10 gives you 20 dB. Although power amplifiers also have gain, designers typically rate them by their maximum output wattage. Here, the "root mean square," or RMS rating, is the most conservative value, followed by average and peak. The RMS rating gives you a clear idea of the amplifier's output over several minutes; the peak rating is a measure of momentary ability.
Most power amplifiers drive loudspeakers and headphones, as it takes power to drive speakers to comfortable listening levels. Radio transmitters also use power amplifiers to deliver a strong signal to the broadcast antenna. Voltage amplifiers work best to increase very weak signals; they do not have enough power to drive speakers. Your cell phone contains two amplifiers: a voltage amplifier to pick up cellular radio signals and a power amplifier to drive the phone's speaker. A stereo receiver also has two amplifiers for the same reason.
Chicago native John Papiewski has a physics degree and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."