![]() ![]() 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. Equivalent 2N4401 transistors are 2N3904 and 2N22A. 2N3904 is complementary to 2N3906 PNP bipolar junction transistor. It is widely used for low power amplification and switching. And from those, you can calculate the optimum source impedance. 2N3904 Transistor is a popular NPN bipolar junction transistor used in low power and current, medium voltage applications. BJT, NPN, 40V, 600MA, TO-92 Transistor BJT. This switches ON the transistor and the transistor will activate the relay. Download schematic symbols, PCB footprints, pinout & datasheets for the 2N4401 by Central Semiconductor Corp. ![]() 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. 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. Working Explanation The working of this circuit is quite simple. Ignore low frequency noise specifications because they include flicker noise which does not apply at higher frequencies. 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. 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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