Amplifier

An amplifier can be considered to be any device that uses a small amount of energy to control a larger amount, although the term today usually refers to an electronic amplifier. The relationship of the input to the output of an amplifier — usually expressed as a function of the input frequency — is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain.

General characteristics of amplifiers

Most amplifiers can be characterised by a number of parameters.

Gain

How much an amplifier increases the signal level is called the gain. This is usually measured in decibels (dB). Mathematically speaking, the gain is equal to the output level divided by the input level.

Output dynamic range

This is the range usually quoted in dB between the lowest useful; output and the largest useful output level. Since the lowest useful level is limited by output noise, this is quoted as the amplifier dynamic range.

Bandwidth and rise time

The bandwidth BW of an amplifier is usually defined as the difference between the lower and upper half power points. This is therefore also known as the −3 dB BW. Bandwidths for other response tolerances are sometimes quoted (−1 dB, −6 dB etc.

As an example, a good audio amplifier will have a −3 dB BW from around twenty hertz to about twenty kilohertz (the range of normal human hearing).

The rise time of an amplifier is the time taken for the out put to change from 10% to 90% of its final level when driven by a step input. For a Gaussian response system (or a simple RC roll off), the rise time is given by :

Tr = BW/0.35

where BW is in Hz and Tr is in seconds

Settling time and aberrations

Time taken for output to settle to within a certain percentage of the final value say 0.1%. Specified in high accuracy measurement systems.

Slew rate

This is the maximum rate of change of output variable, usually quotes in volts per second (or microsecond).

Sine wave distortion

The properties of amplifier circuits distort the signal. This distortion comes in several forms including harmonic distortion and intermodulation distortion.

Harmonic distortion is fairly easy to measure. The amplifier output is connected to a spectrum analyzer, (a device which graphs frequency against amplitude). Then a pure tone is applied to the amplifier input. Typically a sinusoidal signal of 1 kHz is used. The largest signal on your analyzer should be the input signal at 1 kHz. You will sometimes see humps at even intervals along the graph at even multiples of that base signal. These are the harmonics. The total harmonic distortion (THD) is the sum of these components relative to the signal.

Noise

How much noise is introduced by the amplification process? This is an undesirable thing that is the inevitable result of the electronics devices and components. It is measured in either decibels or the peak output voltage produced by the amp when no signal is applied.

Efficiency

How much of the input power is usefully applied to the amplifier's output? Class A amplifiers are very inefficient, in the range of 10–20% with a max efficiency of 25%. Modern Class AB amps are commonly between 35–55% efficient with a theoretical maximum of 78.5%. Commercially available class D amplifiers have reported efficiencies as high as 97%. The efficiency of the amplifier limits the amount of total power output that is usefully available. Note that more efficient amps run much cooler, and often do not need any fans even in multi-kilowatt designs.

Electronic amplifiers

There are numerous types of electronic amplifier depending upon the application.

The most common type of amplifier is the electronic amplifier, commonly used in radio and television transmitters and receivers, high-fidelity ("hi-fi") stereo equipment, microcomputers and other electronic digital equipment, and guitar and other instrument amplifiers. Its critical components are active devices, such as vacuum tubes or transistors.

Class

All amplifiers can be classified by the angle of flow of the input signal through the amplifying device; see electronic amplifier.

Where efficiency is not a consideration, most small signal linear amplifiers are designed as class A which means that one active device amplifies all portions (360deg) of the input signal. Class AB and class B are essentially the same, transmitting about 180deg of the input signal to the output with each device. These classes are usually used in efficient low frequency amplifiers (such as audio and hi-fi) owing to their relatively high efficiency. RF tuned amplifiers are usually Class C which means that they amplify less than 180° of the input signal. The signal is restored to almost sinusoidal shape by the tuned circuit.

Vacuum tube (valve) amplifiers

Today most sound systems use transistor amplifiers for economic reasons, but valve amplifiers remain popular for guitar amplification, for "high end" hi-fi systems and analog production and replay equipment in recording studios.

Valve amplifiers are widely, but not always correctly, associated with the valve sound. Some claim this sound has more to do with the circuit topology and circuit design of the amplifier, than to the use of valves rather than transistors as the active gain devices. In the earlier years of audio, vacuum tubes filled the active device role.

Transistor amplifiers

Most common active devices in transistor amplifiers are bipolar junction transistors (BJTs); metal oxide semiconductor field-effect transistors (MOSFETs) are also used. The essential role of this active element is to magnify an input signal to yield a significantly larger output signal. The amount of magnification (the "forward gain") is determined by the external circuit design as well as the active device.

Operational amplifiers (op-amps)

An operational amplifier is a solid state integrated circuit amplifier which employs external feedback for control of its transfer function or gain

Musical instrument amplifiers

See main page: instrument amplifier.

Audio amplifier

An audio amplifier is usually used to amplify signals such as music or speech.

Carbon microphone

One of the first devices to amplify signals was the carbon microphone. By channeling a large electric current through the compressed carbon granules in the microphone, a small sound signal could produce a much larger electric signal. The carbon microphone was extremely important in early telecommunications until other types of amplifiers were available.

Other amplifier types

Magnetic amplifier

A magnetic amplifier is a transformer-like device that makes use of the saturation of magnetic materials to produce amplification. It is a non-electronic electrical amplifier with no moving parts. The bandwidth of magnetic amplifiers extends to the tens of kilohertz.

An Amplidyne or Rototrol is a rotating machine like an electrical generator that provides amplification of electrical signals by conversion of mechanical energy to electrical energy.

Optical amplifiers

Optical amplifiers amplify light, through the process of stimulated emission.

Miscellaneous types

There are also mechanical amplifiers, such as the automotive servo used in braking.
Relays can be included under the above definition of amplifiers, although their transfer function is not linear (that is, they are either open or closed).
Another type of amplifier is the fluidic amplifier, based on the fluidic triode.

See also: electronic amplifier, low noise amplifier, preamplifier, satellite in-line amplifier.

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See also: electronic amplifier, low noise amplifier, preamplifier, satellite in-line amplifier. The retro appeal of old Bakelite products, especially kitchenware and toys, has made them quite collectible in recent years: A quick search of, for example, eBay turns up hundreds of listings for all things Bakelite, ranging from radios to poker chips to telephones. Optical amplifiers amplify light, through the process of stimulated emission. An exception to the overall decline is the use in small precision-shaped components where their specific properties are required, such as molded disc brake cylinders, saucepan handles, electrical plugs and switches, and electrical iron parts. An Amplidyne or Rototrol is a rotating machine like an electrical generator that provides amplification of electrical signals by conversion of mechanical energy to electrical energy. Phenolics are little used in general consumer products today due to the cost and complexity of production and their brittle nature. The bandwidth of magnetic amplifiers extends to the tens of kilohertz. It was demolished in 1998.

It is a non-electronic electrical amplifier with no moving parts. Around 1928, A new factory opened in Tyseley, Birmingham in September 1931. A magnetic amplifier is a transformer-like device that makes use of the saturation of magnetic materials to produce amplification. Bakelite Limited was formed in 1927 from the amalgamation of three suppliers of phenol formaldehyde materials: the Damard Lacquer Company Limited of Birmingham; Mouldensite Limited of Darley Dale and Redmanol Limited of London. The carbon microphone was extremely important in early telecommunications until other types of amplifiers were available. in 1938. By channeling a large electric current through the compressed carbon granules in the microphone, a small sound signal could produce a much larger electric signal. The company was acquired by Union Carbide and Carbon Corp.

One of the first devices to amplify signals was the carbon microphone. and Redmanol Co. An audio amplifier is usually used to amplify signals such as music or speech. was formed in 1922 from General Bakelite Co., Condensite Corp. See main page: instrument amplifier. Bakelite Corp. An operational amplifier is a solid state integrated circuit amplifier which employs external feedback for control of its transfer function or gain. Several patterns were made in 1942, but steel was used instead in 1943 and recycled shell casings in 1944 and 1945.

The amount of magnification (the "forward gain") is determined by the external circuit design as well as the active device. Due to its hardness and durability, it was considered as a material for making pennies in the United States during World War II, due to copper being needed for shell casings. The essential role of this active element is to magnify an input signal to yield a significantly larger output signal. It was used for its nonconductive and heat-resistant properties in radio and telephone casings and electrical insulators. Most common active devices in transistor amplifiers are bipolar junction transistors (BJTs); metal oxide semiconductor field-effect transistors (MOSFETs) are also used. Formed by the reaction under heat and pressure of phenol and formaldehyde, generally with a wood flour filler, it was the first plastic made from synthetic polymers. In the earlier years of audio, vacuum tubes filled the active device role. Leo Baekeland.

Some claim this sound has more to do with the circuit topology and circuit design of the amplifier, than to the use of valves rather than transistors as the active gain devices. Bakelite is a brand named material based on the thermosetting phenol formaldehyde resin polyoxybenzylmethylenglycolanhydride, developed in 1907-1909 by Dr. Valve amplifiers are widely, but not always correctly, associated with the valve sound. Patent 942809 -- Condensation product and method of making same. Today most sound systems use transistor amplifiers for economic reasons, but valve amplifiers remain popular for guitar amplification, for "high end" hi-fi systems and analog production and replay equipment in recording studios. U.S. The signal is restored to almost sinusoidal shape by the tuned circuit.

RF tuned amplifiers are usually Class C which means that they amplify less than 180° of the input signal. These classes are usually used in efficient low frequency amplifiers (such as audio and hi-fi) owing to their relatively high efficiency. Class AB and class B are essentially the same, transmitting about 180deg of the input signal to the output with each device. Where efficiency is not a consideration, most small signal linear amplifiers are designed as class A which means that one active device amplifies all portions (360deg) of the input signal.

All amplifiers can be classified by the angle of flow of the input signal through the amplifying device; see electronic amplifier. Its critical components are active devices, such as vacuum tubes or transistors. The most common type of amplifier is the electronic amplifier, commonly used in radio and television transmitters and receivers, high-fidelity ("hi-fi") stereo equipment, microcomputers and other electronic digital equipment, and guitar and other instrument amplifiers. There are numerous types of electronic amplifier depending upon the application.

Note that more efficient amps run much cooler, and often do not need any fans even in multi-kilowatt designs. The efficiency of the amplifier limits the amount of total power output that is usefully available. Commercially available class D amplifiers have reported efficiencies as high as 97%. Modern Class AB amps are commonly between 35–55% efficient with a theoretical maximum of 78.5%.

How much of the input power is usefully applied to the amplifier's output? Class A amplifiers are very inefficient, in the range of 10–20% with a max efficiency of 25%. It is measured in either decibels or the peak output voltage produced by the amp when no signal is applied. How much noise is introduced by the amplification process? This is an undesirable thing that is the inevitable result of the electronics devices and components. The total harmonic distortion (THD) is the sum of these components relative to the signal.

These are the harmonics. You will sometimes see humps at even intervals along the graph at even multiples of that base signal. The largest signal on your analyzer should be the input signal at 1 kHz. Typically a sinusoidal signal of 1 kHz is used.

Then a pure tone is applied to the amplifier input. The amplifier output is connected to a spectrum analyzer, (a device which graphs frequency against amplitude). Harmonic distortion is fairly easy to measure. This distortion comes in several forms including harmonic distortion and intermodulation distortion.

The properties of amplifier circuits distort the signal. This is the maximum rate of change of output variable, usually quotes in volts per second (or microsecond). Specified in high accuracy measurement systems. Time taken for output to settle to within a certain percentage of the final value say 0.1%.

where BW is in Hz and Tr is in seconds. Tr = BW/0.35. For a Gaussian response system (or a simple RC roll off), the rise time is given by :. The rise time of an amplifier is the time taken for the out put to change from 10% to 90% of its final level when driven by a step input.

As an example, a good audio amplifier will have a −3 dB BW from around twenty hertz to about twenty kilohertz (the range of normal human hearing). Bandwidths for other response tolerances are sometimes quoted (−1 dB, −6 dB etc. This is therefore also known as the −3 dB BW. The bandwidth BW of an amplifier is usually defined as the difference between the lower and upper half power points.

Since the lowest useful level is limited by output noise, this is quoted as the amplifier dynamic range. This is the range usually quoted in dB between the lowest useful; output and the largest useful output level. Mathematically speaking, the gain is equal to the output level divided by the input level. This is usually measured in decibels (dB).

How much an amplifier increases the signal level is called the gain. Most amplifiers can be characterised by a number of parameters. . The relationship of the input to the output of an amplifier — usually expressed as a function of the input frequency — is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain.

An amplifier can be considered to be any device that uses a small amount of energy to control a larger amount, although the term today usually refers to an electronic amplifier. Another type of amplifier is the fluidic amplifier, based on the fluidic triode. Relays can be included under the above definition of amplifiers, although their transfer function is not linear (that is, they are either open or closed). There are also mechanical amplifiers, such as the automotive servo used in braking.