![]() And I must have hit some type of limit because the transistor burnt out. ![]() I was playing around with that until the pot started smoking (lol not me).īut it seemed like the sweet spot was above 100k started limiting to the range I needed, but I had to keep giving it more 100k resistors for every 10 more ma of current to limit. Like it didn't seem to start limiting current until I put 100k ohm resistor in. I think I noticed a sweet-spot in the circuit. And in fact, most real-world circuits (not just op-amp circuits) rely on the same principals so in production you don't have to tweak pots or select resistors. They have very-very high gain and with negative feedback the gain can be controlled with the resistors, and you don't have to worry about the exact open-loop gain of the "raw" op-amp. But, since there is so much tolerance in the transistor's gain, that's just a starting point. So, if the transistor has a gain of 100, we need 0.45mA through the resistor (and base) so that's about 11K. But, since the transistor's gain has such a wide range we don't need to be that exact and we can just assume 5V across the resistor. The base-emitter junction acts like a diode, so I'll estimate a voltage drop of 0.6V and that means with 5V applied you've got 4.4V across the base resistor. That's a wide range (typical for a transistor) so you either need a pot, or you can experiment and select a resistor, or design a more advanced circuit that depends on resistor values rather than the transistor itself.* ![]() According to the datasheet the PN2222 has a current gain (h fe) of between 50 and 375 depending on the conditions and the particular part. ![]()
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