![]() This switches ON the transistor and the transistor will activate the relay. When the probes will detect the moisture in the substance than the resistor between these two probes will decrease and it will let the voltage pass through it. There is also the base spreading resistance which is a significant source of noise at low impedance, but this would need to be measured in most cases few transistors specify it. Working Explanation The working of this circuit is quite simple. This transistor like all can be used either as a switch or as an amplifier. And from those, you can calculate the optimum source impedance. Texas Instrumentss 2N4401 is npn general purpose amplifier gp bjt transistor in the bipolar transistors, gp bjt category. So you can calculate the input noise voltage from the collector current, and you can calculate the input current noise from the base current, which is the collector current divided by the hfe. Ignore low frequency noise specifications because they include flicker noise which does not apply at higher frequencies. ![]() Exact specifications should be obtained from the product data sheet. ![]() The input current noise depends on the base current so transistors with higher hfe are ultimately lower noise, and they are sometimes advertised that way. Bipolar (BJT) Single Bipolar Transistors NTE Electronics, Inc 2N4401 Share Image shown is a representation only. I have only done this at lower frequencies but the ratio of the input voltage noise density and input current noise density defines the optimum source impedance for lowest noise, and adjusting the collector current alters this ratio so it can be adjusted for lowest noise in a given design.
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