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Old 01-27-2016, 10:48 AM   #1
brandonb916
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Default 2 transistor circuit input output question

Let me start by saying thank you for reading this. I hate signing up just to ask a question, but I would really appreciate any help. I am studying for an employment test and it is on stuff I learned in the Marine Corps more than a decade ago, so I'm a little rusty.

I actually have the answers to the problem I have, but I don't quite understand how they came about the 8 volts. I get that the transister lowers voltage by .7 and that's why the second answer is 7.3. I know this is probably fairly simple, but I just can't make sense of it. I have written the answers from the key in the blanks. I am more than willing to put in the work myself and I don't expect to be spoon fed. So if any of you have a suggestion on where I can look, I'd very much appreciate it.
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Old 01-28-2016, 11:35 AM   #2
symbol
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If you do a Tevenin equivalent of the 5k and 20K bias resistors, you will get a voltage Vbb =8V and a series resistor of 4k.



i10K=VE/10k=(VB-0.7)/10K
ib=(8-VB)/4k
i10k=2*iE=2*(beta+1)*ib=2*(beta+1)*(8-VB)/4k

thus
2*(beta+1)*(8-VB)/4k=(VB-0.7)/10K
solve for VB and make an assumption on beta(if not given)

for beta=30 we get VB=7.95v
for beta=100 we get VB=7.98v
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Old 01-29-2016, 02:33 PM   #3
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Thank you very much for the detailed answer. Certainly would not have come up with that on my own. Now to practice. Thanks again!
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