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Please could anybody explain the CE configuration, the graphs of \$I_b\$ vs. \$V_{be}\$ and \$I_c\$ vs. \$V_{ce}\$. I can't understand which current remains fixed and how can one regulate \$V_{ce}\$ when we know that \$V_{cc}\$ is the applied voltage on the output side. Please help.

Stefan Wyss
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Alex
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    Would [electronics.se] be a better home for this question? – Qmechanic Jan 02 '20 at 23:11
  • I know some schools don't have engineering departments and teach basic transistor circuits as part of a physics course. Considering, however, there is an EE SE, it should migrate there even if some of our stellar physics people here could answer. – Bill N Jan 02 '20 at 23:29
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    Also, this question is probably too broad to get a satisfactory answer. @Saddy should reduce the scope of the question. The good answer is a whole chapter in circuits textbooks. – Bill N Jan 02 '20 at 23:31
  • By "current [that] remain fixed" you are probably referring to Ic not changing when Vce changes (that is by neglecting the Early effect). If you look at the BJT as a reverse biased diode (collector junction) whose inverse saturation current is controlled by the carriers injected by the emitter, all you need to explain is why the reverse saturation current of a diode does not change with the reverse voltage applied to it. (In reality it does change, but not much as is the case with exponential tails). – Sredni Vashtar Jan 03 '20 at 13:25

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There's no current that must remain fixed (Why would you think that?).

Most important feature of a bipolar transistor is that \$I_{c}\$ is controlled by \$I_{b}\$ (or \$V_{be}\$).

\$V_{ce}\$ is regulated indirectly through \$I_{c}\$ (and thus indirectly through \$I_{b}\$ or \$V_{be}\$). Because \$V_{cc}\$ is fixed, and because there is a resistor from \$V_{cc}\$ to the collector, varying \$I_{c}\$ creates a voltage drop on the resistor which leads to voltage variation on the output side.

So in the end: Variation of \$I_{b}\$ (or \$V_{be}\$) leads to a variation of the output voltage.

Stefan Wyss
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  • The fixed current, I believe, is referred to the output characteristics Ic vs. Vce. In the active region, and negleting the Early effect, the characteristics are essentially flat. So Ic only depends on Ib (or Vbe if you prefer that) and this 'decoupling' between input and output is what allow for transistor action. – Sredni Vashtar Jan 03 '20 at 13:29
  • @StefanWyss...I am really surprised - you even do not mention the base-emitter voltage Vbe. Do you consider it as unimportant? In contrary - it is the most important input parameter [Ic=Io*exp(Vbe/Vt]. The base current is nothing else than an unwanted (but unavoidable) by-product. This is obvious because the beta-factor Ic/Ib has no influence at all on voltage gain; only the transconductance gm=d(Ic)/d(Vbe) matters. – LvW Jan 03 '20 at 13:53
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    @LvW You are right, I added the reference to V_be. But I would not consider the base current as a by-product, because a bipolar transistor only works because of drift- and diffusion currents over the PN-regions. From a technical view, V_be might be more important, but from a physical view, it's the charge carriers that make the music. – Stefan Wyss Jan 03 '20 at 14:20
  • Stefan...no - it is just the other way round: From the technical/practical aspect we can - in some cases !! - consider the BJT to be current-controlled, but from the physical point of view we have voltage-control. There is not one single proof for current-control - however, many observations and effects show that the BJT is voltage-controlled. This was extensively discussed in the past - also in this forum! Do you need examples? – LvW Jan 03 '20 at 14:40
  • @LvW you keep ignoring all the evidence that it is equally possible to consider a BJT as current controlled, and then you state there is no single proof. Did you read the books I have suggested here https://electronics.stackexchange.com/questions/470000/ce-amplifier-non-sinusoidal-signals (Streetman's textbook on semiconductor devices and Levinstein and Shimin's "Transistors - from crystals to ICs")? – Sredni Vashtar Jan 03 '20 at 18:47
  • My answer can be rather short: Please, convince me and present one single example of "all the evidence" for considering the BJT as current-controlled. Can you? However, do not simply refer to a book or any other publication - I know that several books are wrong in this respect. I repeat my proposal: Do you need examples and proofs for voltage control? But you can convince yourself if you are really interested: Consult relevant publications from Berkeley, Stanford, MIT,...or from one of the best-known electronic engineer Barrie Gilbert. – LvW Jan 04 '20 at 10:41
  • @LvW The Barrie Gilbert who did not give you the answer you wanted here https://www.researchgate.net/post/How_should_one_present_the_essential_and_fundamental_properties_of_a_bipolar_junction_transistor ? (Kind of elusive in the end, eh?) – Sredni Vashtar Jan 04 '20 at 15:44
  • No - there are many other papers and contributions from him...(He says: The base current is a "defect" and a "nuisance") – LvW Jan 04 '20 at 15:55
  • Other contributions like "A diode (or junction device in general -- or ANY electrical device, come to that) has no idea how it is being applied, whether using a (variable) source of either current or voltage, or in the case of active devices, a combination of both. But, obviously, some methods, in selected circumstances, are better than others."? Of course he privileged the voltage control model: he made a name designing translinear amplifiers! But that does not mean he is right in believing voltage causes current (as in the "Are there causal relationships in Ohms law" on the same site). – Sredni Vashtar Jan 04 '20 at 16:06
  • @Sredni Vashtar, with all respect - why did you not respond to my request (...please, present one single example of "all the evidence" for considering the BJT as current-controlled...). Such an example would help to bring the discussion back to the core of the problem and would make the discussion more technical! – LvW Jan 05 '20 at 10:30
  • @LvW I did, here https://electronics.stackexchange.com/questions/470000/ce-amplifier-non-sinusoidal-signals?noredirect=1&lq=1, here (sorry, I cannot locate the chat but the contents was as above), here https://electronics.stackexchange.com/questions/419153/the-collector-current-of-bjt#comment1158272_419153 and in other comments I cannot locate how, like when I pointed out that the very textbook you suggested (Hu) had a whole section on charge control showing how Ic is controlled by Ib. And again: you can't have Vbe withouth Ib and vice-versa. – Sredni Vashtar Jan 05 '20 at 11:22
  • As it seems, it is hard to find a proof for current control. Both references mentioned by you contain just assertions - without any proof!! I can give you many proofs for voltage control (circuit examples, observations, theoretical explanations). And - sorry to say - your last sentence is irrelevant...nobody has denied that there will be a base current. Do you not see the contradiction: The classical approach to design a simple BJT gain stage follows the rules based on voltage control (VBE=0.65, RE feedback, voltage divider for a "quasi-stiff" base voltage) - but you stick on current-control! – LvW Jan 05 '20 at 11:36
  • https://www.researchgate.net/publication/338670662_Tim_Tom_and_Ben_try_to_unlock_the_secrets_of_the_transistor – LvW Jan 18 '20 at 09:46