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Biasing with variable supply voltage
I’m designing a simple radiation detector using a PIN photodiode to pick up gamma radiation. When a gamma particle strikes the photodiode, it’ll produce a tiny pulse that the rest of the detector needs to amplify. Something like Maxim App Note 2236 (http://www.maximintegrated.com/app-notes/index.mvp/id/2236) but using discrete components.
The kicker is that it uses supercaps for power storage, which means the supply voltage will vary from about 4.5V down to about 2V, and I need to keep the quiescent current for the whole thing as small as possible, ideally around 1 uA.
I’ve been trying to figure out how to bias the transistors. To minimize quiescent current but remain sensitive they should be just at the threshold of turning on, but as they draw power from the supercaps the supply voltage will gradually decrease, so a traditional voltage divider would probably result in reduced sensitivity as the bias point drops with supply voltage. The zener diodes I looked at all had leakage currents on the order of 7 uA which is over budget for the design.
Is there some sort of self-biasing arrangement that would use negative feedback to keep the transistor at or just below conduction? Any other ideas? Either MOSFETs or BJTs would be acceptable in the design.
I’m mostly a digital and embedded software guy and am gradually trying to re-learn analog design, so suggestions and pointers would be much appreciated. Thanks in advance!