Signal amplifiers are electronic devices that receive and amplify signals for processing or digitization. They produce only a small amount of internal noise while increasing signal strength by a significant factor. Signal amplifiers often include field-effect and bipolar junction transistors, electronic components that amplify signals without producing significant amounts of thermal noise. They may also include one or more operational amplifiers. An amplifier is frequently followed by a low pass active filter which reduced high frequency signal components, unwanted electrical interface noise, or electronic noise from the signal.
Isolation amplifiers allow measurement of small signals in the presence of a high common mode voltage. They are used to protect data acquisition components from common mode voltages, which are potential differences between instrument ground and signal ground.
Another important function of an amplifier is to increase the signal-to-noise ratio (SNR). Low-level voltage signals can be greatly affected by noise when the transducers are located far from the data acquisition board, amplifying the low-level signals before they are transmitted through the noisy environment increases the level of the required signal before they are affected by noise, thereby increasing the SNR of the signal for the same level of noise.
When selecting an amplifier it is important to consider the four categories of instrumentation amplifiers. Single-ended amplifiers have an unbalanced input, and are non-isolated. They are very inexpensive and are best used when the common mode voltages are zero or very small. Differential amplifiers have a balanced input and are non-isolated. This means they are suitable in applications where the sum of common mode and normal mode voltages remains within the measurement range of the amplifier. An amplifier known as single-ended, floating common is an isolated, quasi-balanced input that is suitable for off-ground measurements up to the breakdown voltage of the isolation barrier, and exhibits very good common more rejection. The last category is differential, floating common amplifiers. They use an isolated and balanced input which is suitable for off-ground measurements to the breakdown voltage of the isolation barrier. This type of amplifier also exhibits very good common more rejection.
In amplifier design, the input impedance of the amplifier is typically of prime concern. Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied. Input impedance is the equivalent impedance "seen" by a power source connected to a network. Signal amplifiers are used on thermocouples or piezo electric sensors. In general they will be used on sensors and electrodes or small signal AC and DC outputs from other equipment.
The output impedance of an amplifier is a measure of the impedance looking back into the amplifier.
Analog input channels inputs include AC and DC voltages, AC and DC currents, charges from piezoelectric devices, and frequencies or other specialized waveforms. Some signal amplifiers receive inputs from accelerometers, thermocouples, thermistors, resistance temperature detectors (RDTs), strain gauges, or bridges. Other devices receive signals from linear rotary differential transformers (LVDTs) and rotary variable differential transformers (RVDTs).
Gain is the factor by which input signals are multiplied, and is frequently greater than unity; however, gain can be fractional when a reduction (attenuation) of signal amplitude is desired. Gain may be specificed as the ratio of output voltage to input voltage, output power to input power, or some combination of current, voltage, and power.
Bandwidth the difference between the high and low limits of the frequency response, is a measurement that is typically defined by a variation from a nominal value by a stated value such as 3 dB.
There are a variety of form factors for signal amplifiers. Some devices mount on integrated circuits (ICs), standard DIN rails (as seen in image), or printed circuit boards (PCBs) that attach to enclosures or plug into computer backplanes. Others bolt into walls, cabinets, enclosures, or panels. Rack-mounted units fit inside a standard 19" telecommunications rack. Modular styles include stackable units that dock in bays, slots, or boxes. Benchtop or freestanding signal amplifiers often feature full casings or cabinets and integral interfaces.
Signal amplifiers are found in many industries and used for a wide variety of applications. There are some universal standards available such as QPL - 38449 for intercommunication amplifiers. Other standards, such as A-A-59003, are for specific audio frequency applications.
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