Electronics-Lab.com Community  # DaMoos

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1. I hope I'm not being aggrivatingly dense here, but I just want to fully understand this stuff. Does this form of biasing force a SPECIFIC current gain? I.E., i want a current gain of exactly 100, and I know the transistor can give me at least that. Does this form of biasing do this? What is the reason for making the emmiter resistor 1/10 to 1/20 the size of the collector ressistor? I always read this, but the reason is never really explained. This seems like the actual value is almost arbitrary. Same thing with making the current through the voltage divider being 10 times the current through the base. Why is this?
2. So the characteristics graph is really only academic, and useless for selecting specific resistors? Does this mean that ypu would never truley draw an actual loadline when designing a bjt amplifier? So basically, the combination of the voltage divider on the base, and the emmiter resistor form a "cap" on current gain. Say the actual transistor I have has a current gain of 250, but I want to make sure it realizes a current gain of only 100. is that the point of this form of biasing?
3. Say I have a 2n3904 transistor. I bias it using the voltage divider method described in virtually every book I have. The math all makes sense to me. Here's where I'm lost. A specific Beta is used in all the calculations in all the examples I read, but usually the same examples say that the current gain of even different transistors of the same part number can be quite different. So is the voltage divider method intended to force a specific current gain? Do you even really use the characteristics curves when biasing a transistor?
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