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BJT Circuit Problem I have already posted a similar problem in the link above. I am facing the same issue in this circuit. What will be the state of the 2 BJT's Cutoff, Saturated or Active ? Please suggest me a generalised approach to tackle such problems.

  • \$\begingroup\$ Seems to me that you are asking most of your homework, what have you tried? First look that the transistors are both in feedback loops, that should give you a hint, also, have you tried simulating this circuit?, if the input is 5V that means that the output of the first opamp should be around 5.7V to account for the 0.7V drop of the first transistor, this means that the emmiter of the first transistor is at 5V, the current of the first transistor is then 5/1k=5mA, you should try following the same line of reasoning with the second transistor. \$\endgroup\$ – S.s. Dec 5 '17 at 2:30
  • \$\begingroup\$ Do the same as you were taught for diodes or any other nonlinear device: Guess which operating regime each part is in, solve the circuit, check for contradictions. Once you've done a few, you'll be very likely to guess right the first time. \$\endgroup\$ – The Photon Dec 5 '17 at 3:21
  • \$\begingroup\$ To be fair, I'm at a loss to answer part ii, without knowing what the function of the circuit is supposed to be. As far as I can see, the 2nd op-amp has positive feedback, which doesn't lead to any useful behavior I can see. \$\endgroup\$ – The Photon Dec 5 '17 at 3:27
  • \$\begingroup\$ @ThePhoton We can just assume the op-amp magically drives the inputs to the same voltage can't we? /s \$\endgroup\$ – Spehro Pefhany Dec 5 '17 at 5:09

This is a bit of a wild guess indeed, but I'm going to suggest that the question is totally effed up and this is the actual circuit that it is supposed to be:


simulate this circuit – Schematic created using CircuitLab

Okay, as per Bruce's comment and a previous edit, this (rather major) change seems to agree with one of the answers.

It must be very frustrating when there are errors in the problem set statements.

  • \$\begingroup\$ If you want constant current output from that circuit then OA2 + and - inputs must be swapped. I suggest you change the circuit to match your edit (Q2 = NPN, Collector to R4) then it is closer to the original and AO2 inputs are correct. \$\endgroup\$ – Bruce Abbott Dec 5 '17 at 6:52
  • \$\begingroup\$ @BruceAbbott Yes, you're right. Then we have only one major change. Though imagining a purpose for this circuit (as opposed to Q2 PNP and +/- of OA2 flipped) is more difficult. \$\endgroup\$ – Spehro Pefhany Dec 5 '17 at 7:01
  • \$\begingroup\$ I have posted the problem again with the solution I have with me. Please have a look electronics.stackexchange.com/questions/343440/… \$\endgroup\$ – user170930 Dec 5 '17 at 14:17
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    \$\begingroup\$ @user3653788 - And as I pointed out in the other question, essentially Spehro is correct. The answer version of the circuit corresponds to Spehro's speculation. Stop messing around with the wrong circuit. \$\endgroup\$ – WhatRoughBeast Dec 5 '17 at 17:01

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