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BJT output I-V characteristic
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amir.razzaqi:
Hello guys
I had a question about the I-V curve of a transistor. According to the available documents about this experiment, the base current of the transistor should be constant (e.g., 10 microamps), while the collector-emitter voltage affects the base current. My question is, if the collector-emitter voltage changes and consequently the base current deviates from its initial constant value, should the base current be returned to its previous constant value? In that case, would the collector-emitter voltage remain unchanged?
Thanks
tom66:
Why do you think C-E voltage will change base current at all?  It is the other way around, base current changes C-E voltage. Now, if you increase C-E voltage whilst maintaining a steady base current, you would expect to see C-E current increase until the transistor hits saturation, which would be one way to measure Hfe.  Just changing C-E voltage on its own shouldn't change base current unless your circuit has some other stimulus, I suppose there's a small possibility as the transistor gets hotter you can see a change but unless you're talking about a significant C-E current that effect should be fairly negligible.
T3sl4co1l:
That's a contradiction; did you mean the collector current changes with Vce?

Tim
magic:
The "output characteristic" of a BJT is the variation of collector current with collector voltage. Collector current is fairly constant with voltage, except at very low voltage where it approaches zero and very high voltage where it increases due to breakdown.

Output characteristic is measured for a particular value (or a few values) of base current. Base current is held constant throughout the tested range of collector voltage. It's not hard to drive constant current into the base, because base voltage varies very little.


--- Quote from: tom66 on September 30, 2023, 12:09:24 pm ---Why do you think C-E voltage will change base current at all?  It is the other way around, base current changes C-E voltage. Now, if you increase C-E voltage whilst maintaining a steady base current, you would expect to see C-E current increase until the transistor hits saturation, which would be one way to measure Hfe.
--- End quote ---
Base current doesn't affect collector voltage in this measurement, because collector is driven by a voltage source.
Saturation is the condition when collector voltage is close to zero.
TimFox:
The "characteristic curves" is a set of curves for IC vs VCE, where each curve is measured at a constant value of IB.
In normal operation, for VCE above "saturation", the current varies slowly with voltage on each of these curves.
You can take the same data and re-graph them to see, for example, the base current required for a given collector current as a function of VCE.
In that graph, the base current changes slightly as the collector voltage changes:  this is due to the "Early effect", which you can find in transistor textbooks.
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