Using NPN or PNP transistors at micro amp bias and load levels. REPLY
Thanks for your response!
Measuring the gain at the µA bias level is a good suggestion since it is not published on the data sheet. Using a PNP 2N3907, with my Fluke it measured about 315. At about 0.60µA it had dropped to about 200, well in excess of what is needed to fully load the 10 mΩ collector resistor.
I am not that familiar with the use of MOSFET transistors. However, as I understand it, they are voltage sensitive as opposed to a transistor that is more driven by current. Originally I had the resistor in series with one of the probes feeding the gate of my inverter through a 10 mΩ resistor. The high Z of the inverter in conjunction with a slightly higher humidity level in the water vessel allowed enough leakage to driver the inverter gate into a false alarm condition. In addtion the voltage across the two stainless steel probes could drift high enough that if the water level dropped below the probes causing an alarm condition, when the water level rose and covered the probes, the voltage would not drop low enough to change the inverter state. So, I felt that I had to utilize an interface between the logic gate and the probes that would be less sensitive to voltage fluctuations. I hope that I have achieved that using transistors? After reading the responses I fell that using transistors and current biasing may be the best way to proceed? Or, is there a way to use a MOSFET that will be less sensitive yet more resistant to humidity and or contamination?
Thanks again! This is the first time I have used a Forum, what a great tool for learning!!