An electronic amplifier, amplifier, or (informally) amp is an electronic device that increases the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude. In this sense, an amplifier modulates the output of the power supply.
There are four basic types of electronic amplifier: the voltage amplifier, the current amplifier, the transconductance amplifier, and the transresistance amplifier. A further distinction is whether the output is a linear or nonlinear representation of the input. Amplifiers can also be categorized by their physical placement in the signal chain.
Amplifiers are described according to their input and output properties.They exhibit the property of gain, or multiplication factor that relates the magnitude of the output signal to the input signal. The gain may be specified as the ratio of output voltage to input voltage (voltage gain), output power to input power (power gain), or some combination of current, voltage, and power. In many cases, with input and output in the same unit, gain is unitless (though often expressed in decibels (dB)).
The four basic types of amplifiers are as follows:
- Voltage amplifier – This is the most common type of amplifier. An input voltage is amplified to a larger output voltage. The amplifier's input impedance is high and the output impedance is low.
- Current amplifier – This amplifier changes an input current to a larger output current. The amplifier's input impedance is low and the output impedance is high.
- Transconductance amplifier – This amplifier responds to a changing input voltage by delivering a related changing output current.
- Transresistance amplifier – This amplifier responds to a changing input current by delivering a related changing output voltage. Other names for the device are transimpedance amplifier and current-to-voltage converter.
In practice the power gain of an amplifier will depend on the source and load impedances used as well as the inherent voltage/current gain; while an RF amplifier may have its impedances optimized for power transfer, audio and instrumentation amplifiers are normally designed with their input and output impedances optimized for least loading and highest signal integrity. An amplifier that is said to have a gain of 20 dB might have a voltage gain of ten times and an available power gain of much more than 20 dB (power ratio of 100), yet actually be delivering a much lower power gain if, for example, the input is from a 600 ohm microphone and the output is connected to a 47 kilohm input socket for a power amplifier.
In most cases an amplifier will be linear; that is, the gain is constant for any normal level of input and output signal. If the gain is not linear, e.g., clipping of the signal, the output signal will be distorted. There are however cases where variable gain is useful. Exponential gain amplifiers are used in certain signal processing applications.
There are many differing types of electronic amplifiers used in areas such as: radio and television transmitters and receivers, high-fidelity ("hi-fi") stereo equipment, microcomputers and other digital equipment, and guitar and other instrument amplifiers. The essential components include active devices, such as vacuum tubes or transistors. A brief introduction to the many types of electronic amplifiers follows.
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