What Are Amplifiers?

Amplification of anything implies taking some known physical quality (like sound), and increasing or enlarging it. You can amplify your voice with a simple megaphone. But while that may be an elegant solution for some needs, it is not entirely useful for most applications.

In electric circuits or devices, an amplifier (amp) is a generic term that refers to an electronic device that produces an output signal that is an increased version of an input signal (electric current). It essentially turns a small electric current into a larger one. This function can be used to do many things, such as boost volume in audio equipment, strengthen a radio or TV signal, or make a cell phone easier to hear on a crowded train. An electronic amplifier can either be a separate piece of equipment or a circuit contained in a device.

If you are amplifying a constant electric voltage you would use a transformer, which can turn large voltages into smaller ones, and also boost small voltages into larger ones. In the case of a fluctuating signal, such as a person’s voice or a TV signal, you would use a transistor-based amplifier. A transistor is a device that can control a strong current with a weak voltage. Simply put, a current flowing through a transistor can open a larger flow of current, or in effect amplify a signal.

The ability of an amplifier circuit to increase the power of a signal from the input to the output is measured as gain, and is expressed in decibels (dB). For example, if the input signal is one volt and the output is 50 volts, then the gain is 50. An amplifier is a circuit that has a power gain greater than one. The larger the gain, the bigger the current boost in relation to the input signal. Large increases in gain, however, can negatively affect both the bandwidth and the frequency response of the amplifier. Bandwidth is the range of frequencies that an amplifier can most effectively amplify, and frequency response is the band of frequencies that an amplifier is designed to amplify. These terms are often used interchangeably.

Depending on the application, amplifiers can be solid state devices or larger systems composed of discrete components. They are usually classified depending on the size of the input signal, the physical configuration of the amplifier, and how it processes the input signal it receives. There are many types of circuits that are classified as amplifiers, from operational and small signal amplifiers to large signal and power amplifiers.

Audio power amplifiers are also classified according to their circuit configurations and mode of operation. They are designated by letters indicating class of operation such as class “A”, class “B”, class “C”, class “AB”, etc. These classifications range from devices with a near-linear output but low efficiency, to ones with a non-linear output and a high efficiency.

Amplifying any signal electronically, however, can also lead to distortion, or “clipping”. This happens when an amp is driven to over-amplify a signal beyond its power rating. Designers usually correct this by using multiple amps that feed signals to each other. These are known as multistage amplifiers. Some audio equipment also uses a pre-amplifier and a main amplifier on the same signal to avoid clipping problems.

There are many amplifier types, each of which is matched to particular applications. Some of these various types include:

• Solid State - uses transistor circuits to amplify the signal (as opposed to vacuum tubes)
• Power - increases the magnitude of a signal in order to drive heavier loads, like loudspeakers
• Operational Amplifier (Op amp) - integrated circuit that uses transistors and resistors to multiply an input signal to a much larger output
• Distributed - uses circuit designs that seek to produce a larger gain and greater bandwidth than conventional amplifier circuits
• Switched Mode - circuit in which the input signal is converted into pulses and switched by the transistors back and forth to increase the efficiency of the amplifier (also known as Class-D)
• Negative Resonance - feeds back a small portion of the signal to the amp to extend frequency response and reduce distortion
• Video - specifically designed to process video signals and have varying bandwidths
• Microwave/RF - high power amplifiers that increase the input signal without noticeable distortion
• Audio - amplify low power audio signals (microphone, radio receiver) in order to drive loudspeakers
• Linear - an amplifier where the output is proportional to the input signal (if the input is increased the output proportionally increases) while providing increased power to the load (with output power measured in watts or kilowatts)
• Nonlinear - an amplifier where an increase in the input signal does not produce a proportional increase in the output signal
• Wideband - produces precise amplification over a wide range of frequencies
• Narrowband - amplifies only a narrow range of frequencies

Some of the terms used in discussing the properties of various kinds of amplifiers include the following;

• Gain - ratio between the output and input signals
• Bandwidth - the range of frequencies that an amplifier is most effective in amplifying
• Frequency response - the band of frequencies that an amplifier is designed to amplify

• Noise - an undesired signal in the output of the amplifier
• Efficiency - the ratio between the power of the output and total power consumption of the amplifier
• Linearity - the equal proportionality between the input amplitude and the output amplitude
• Output dynamic range - ratio of amplifier range between the smallest and largest useful output levels, usually given in dB
• Slew rate - the maximum rate of change in the output voltage caused by a step change in the input
• Rise time - the time taken by a signal being amplified to change from a specified low value to a specified high value
• Stability - an amplifiers immunity to cause false oscillations in itself

Amplifiers function very well in the specific applications for which they are designed. Some of these many applications include radio and television transmitters and receivers, RF and microwave  equipment, wireless communications equipment, stereo gear, microcomputers and other digital equipment, music and audio equipment, and instrumentation.