Semiconductor common emitter configuration

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.

• Would Electrical Engineering be a better home for this question? – Qmechanic Jan 2 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 2 at 23:29
• 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 2 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 3 at 13:25

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.

• 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 3 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 3 at 13:53
• @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 3 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 3 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 electronics.stackexchange.com/questions/470000/… (Streetman's textbook on semiconductor devices and Levinstein and Shimin's "Transistors - from crystals to ICs")? – Sredni Vashtar Jan 3 at 18:47