op amp Find Vo in an op-amp transistor circuit

Posted on Feb 4, 2014

WHY is the voltage at 12V after the emitter If it`s coming out of the op amp at +15V, then that would put a 3volt drop across the transistor. and why isn`t the gain 2, shouldn`t there be 8 volts coming out to the transistor In that case wouldn`t a 4volt drop across R3 work just as well (You begin to see why I spent 12 hours on this today. ) Qprime Apr 1 `13 at 8:28

op amp Find Vo in an op-amp transistor circuit
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But if there`s negative feedback, then it wouldn`t be coming out at 15 volts. but then. GOD THIS STUFF IS SO CONFUSING. I`m having a serious chicken and the egg complex with this circuit. Qprime Apr 1 `13 at 8:30 O_O before anyone even thinks it, I want to stay FAR away from what`s inside the 741 chip. If you have to go there. just say "because of the magic black box. " Qprime Apr 1 `13 at 8:34 the gain equation for a negative feedback would be R3/R2, which is 100/50. which is 2. So why is the voltage coming out as 3x the input I`m not saying you`re wrong, I just want to know that I`m missing here. I built this thing in the simulator and it said the same thing - 12V. what I need to know is why, god almighty, WHY Qprime Apr 1 `13 at 8:39 @Qprime - concentrate on V+ and V- inputs HAVE to be the same voltage. For this circuit it`s 4V. Imagine you had a variable PSU connected across R3 and R2 (in series), what voltage would you need from the PSU to see 4V across R2 12V is the answer. The Op-amp (via the transistor) cannot "live" with its inputs not at the same voltage so, it has to make them the same and "manufactures" a voltage at the emitter that makes V(R2) = 4V. Does this help A non-inverting op-amp circuit gain is 1 + R3/R2 and it is this because of it needing to maintain the same volts on both inputs. Andy aka Apr 1 `13 at 8:53

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