The Improvements of signal-to-noise ratio (SNR) is very important in small signals measurements. And preamplifiers are used to amplify signals when they are affected by noise, generally in sensor-based measurements, where the purpose is to improve the SNR. Yet, on the other hand, preamplifiers are sometimes used to change the output impedance of sensors as well.
Choosing an appropriate preamplifier based on the purpose of application is the key to measure signals from sensors with better quality.
Why are there so many different types of preamplifiers?
Because there are various requirements for different measurements, it is necessary to choose a correct preamplifier based on the type of sensor used.
In the case of thermal sensors, we normally choose a voltage-input type preamplifier, because the sensors typically provide voltage outputs. In the case of photodiodes, a current-input type preamplifier would be suitable because the photodiodes for photodetection provides current outputs.
Furthermore, the requirements for preamplifiers’ output signals may vary depending on the purpose of application. At the same time, high-speed response is necessary for image processing, low noise is required for spectroscopic measurements of semiconductor properties and so on.
Which preamplifier should you choose?
There are so many different factors need to be considered when it comes to selecting the perfect product, but the main ones you absolutely need to know are right here:
Voltage input type or Current input type?
Selection depending on the sensor outputs, whether voltage output or current output.
AC coupling or DC coupling?
If only the AC signal is needed, DC signals need to be cut by an AC coupled preamplifier. In low frequency measurements, a DC coupled preamplifier is required because AC coupled preamplifiers cannot amplify low frequency signals.
Differential input or unbalanced input?
The differential output type sensor can reduce the influence of noise caused by the GND line, but the maximum effect can be obtained by combining it with the differential input type preamplifier.
Input impedance, larger or small?
Even with a voltage output type sensor, the impedance of the sensor may be large or small. The input impedance of the preamplifier depends on whether the input element is a FET or a bipolar transistor, a suitable type needs to be selected for the sensor.
Frequency bandwidth, too wide or too narrow?
If fast response is important, use a wide frequency band preamplifier. On the other hand, in the case of low frequency measurement, noise may be reduced by using a preamplifier in the corresponding band.