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I'm trying to answer the question that has been given here. I think I understand which transistors are on and which are not under the two different logic states i and ii. However, what I really struggle with is understanding the voltages at the nodes.

enter image description here

For instance, for condition (i) :

T1:ON(forward), T2:ON, T3:ON, T4:ON(cut-in), T5:OFF, T6:OFF

  • I'm not completely certain about these states of the transistors, particularly T4. I think it might be in cut-in because it appears to be in a similar question I have looked at, but I'm not sure why -

And so the diagram can be re-drawn without the transistors that are off.

like: enter image description here

From looking at a similar example in a book I can see that

N1 = V BE Sat (of T1), N2 = V CE Sat (of T5) and N3 = 0V

But I'm not exactly sure why this is and how I would go about calculating the voltage at the remaining nodes.

Any insights or help would be greatly appreciated. Many thanks in advance

Peter
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    You might consider reading [this](https://electronics.stackexchange.com/a/304722/38098) for perhaps some of what you are looking for. – jonk Aug 25 '19 at 22:50
  • Hi @jonk , I read through your answer on that question. I have a few questions relating to it: 1. "it is worth considering. If =5V, then it roughly follows that ≤5V−2⋅≈3.6V" why is this the case? what two base-emitter voltages are you referring to here? 2. "≤2⋅≈1.4V ... there are just two junctions between and ground" which two base-emitter junctions are you referring to here? Q2 and Q4 or Q2 and Q1, because is it not a base-collector junction at Q1? – Peter Aug 26 '19 at 10:07
  • @jonk 3. I don't understand what it would mean for the input to "pull down" on the output. "They can source current but they really can't sink current." could you explain this please? 4. Generally speaking, why don't the resistors come into the equations when calculating the voltages at different nodes? 5.When you refer to the base voltage of about 700mV, is this supposed to be Vbe sat or Vbe on? and why does this "pull" Va to <= 200mV? and what does this 200mV represent? – Peter Aug 26 '19 at 10:09
  • I went to some careful detail in that answer. Your first question means tells me that you didn't follow other important details I discussed there, which worries me a little. It means there's something missing that I need to point you at, but I don't know what that is. But let me provide the direct answer and hope it helps some. (I won't bother with the rest until I feel comfortable you got this one.) (1) NPN \$Q_3\$ is *ON*, so one \$V_\text{BE}\$ drop from base to emitter. Also, the drop across \$D_1\$. The base can be near \$V_\text{CC}\$. Cathode of \$D_1\$ must be 2 drops below it or more. – jonk Aug 26 '19 at 17:47
  • And in case it also helps, I did write [this](https://electronics.stackexchange.com/a/368467/38098) a while back as well. It its a closer schematic to one you provided above in (b). – jonk Aug 26 '19 at 17:53

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