Preamplifiers

An amplifier can be considered to be any device that uses a small amount of energy to control a source of a larger amount of energy, although the term today usually refers to an electronic amplifier. more...

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%). This is usually specified in oscilloscope vertical amplifiers and high accuracy measurement systems.

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