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[MOVED] BJT Common-emitter transistor input characteristics

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lonevolf

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I cannot understand why the current at a particular emitter-base voltage decreases as we increase emitter-collector voltage.

You'll observe, at a particular Vbe, the Ib is decreasing as we increase the Vce. What is the mechanism that is causing this to happen?

I hope I have made myself clear.
 

BradtheRad

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From another standpoint, the graph shows that...

with increased supply voltage, you must increase the bias voltage slightly in order to maintain the same bias current.

Probably something to do with electrons (or holes) getting more concentrated at the border of a PN junction in reponse to higher electromotive force. Inhibiting the migration of electrons (or holes).

This is an unconventional graph. As for me I've never seen this. Informative, yes... although the horizontal scale is disjointed.

Nevertheless it may be the first time I have seen such close detail of bias turn-on characteristics. (With the exception of my own experiments carried out on real diodes.)

Notice that the bias operates over a range of several tenths of a volt. Yet why is it we usually see tutorials talk as though the bias operating voltage is a constant? One article might quote 0.6V, another 0.7V, but a constant nonetheless?
 

boob_fet

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I cannot understand why the current at a particular emitter-base voltage decreases as we increase emitter-collector voltage.

You'll observe, at a particular Vbe, the Ib is decreasing as we increase the Vce. What is the mechanism that is causing this to happen?

I hope I have made myself clear.
I guess it is EARLY effect, the base region is becoming smaller and tighter because BC junction is widen
 

lonevolf

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I understood how the Early effect works for Common-base junction but common-emitter is quite confusing!

In the common-base configuration the bias voltage(Vcb), when increased, causes the depletion region to penetrate into the base region thus reducing the probability of recombination of e- and h+ in the base and hence we get an increased output current. This is easily understood because the collector junction is a pn junction itself and the bias voltage for it is analogous to that of a diode. But in common-emitter, the bias voltage is across a p-p or n-n region and not across a p-n region. That's why I can't understand how Early effect might be involved with it...

---------- Post added at 00:29 ---------- Previous post was at 00:14 ----------

Well, what I know about it is that until the knee voltage is achieved (0.7V for Si), as the bias voltage of the diode increases, it keeps on reducing the thickness of the depletion region. Before the knee voltage is achieved, only a negligible current due to minority charge carrier flow is obtained. As soon as the barrier is overcome by the bias voltage, there is a flood of majority charge carries that contribute to the near linear relationship between I and V...
 

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