BJT output I-V characteristic - Page 1
Forum section: electronics / general

author: amir.razzaqi
author_rank: Contributor
timestamp: September 30, 2023, 08:20:50 am
content: Hello guysI 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

author: tom66
author_rank: Super Contributor
timestamp: September 30, 2023, 12:09:24 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 30, 2023, 12:36:58 pm
content: That's a contradiction; did you mean the collector current changes with Vce?Tim

author: magic
author_rank: Super Contributor
timestamp: September 30, 2023, 01:02:07 pm
content: 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 pmBase 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.

author: TimFox
author_rank: Super Contributor
timestamp: September 30, 2023, 02:06:14 pm
content: The "characteristic curves" is aset of curvesforICvsVCE, where each curve is measured at a constant value ofIB.In normal operation, forVCEabove "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 ofVCE.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.

author: David Hess
author_rank: Super Contributor
timestamp: October 01, 2023, 12:00:33 am
content: Quote from: TimFox on September 30, 2023, 02:06:14 pmAnd if you plot the slope of that line back to where it intersects the voltage axis, that is the Early Voltage which reflects how high the collector resistance is.

author: amir.razzaqi
author_rank: Contributor
timestamp: October 01, 2023, 05:50:26 am
content: Quote from: tom66 on September 30, 2023, 12:09:24 pmIn theory, such an event should not occur. However, both in simulation and in practical experiments with a real transistors, a slight deviation in the base-emitter current occurs by changing in the collector-base voltage.

author: amir.razzaqi
author_rank: Contributor
timestamp: October 01, 2023, 06:42:44 am
content: Quote from: T3sl4co1l on September 30, 2023, 12:36:58 pmI have attached 2 images and a simulation file in Proteus software to clarify my question.

author: magic
author_rank: Super Contributor
timestamp: October 01, 2023, 07:11:40 am
content: You will get the plots you want if you replace VBB and R1 with a constant current source. That being said, what you have now may already be a close approximation. Your base current varies because base-emitter voltage varies, and it probably only varies significantly with near-zero collector voltage. I suspect that difference between 15V and 1.5V will be almost nothing.Also, what's the deal with 4.5mA going into the collector and zero emitter current?

author: amir.razzaqi
author_rank: Contributor
timestamp: October 01, 2023, 07:23:32 am
content: Quote from: TimFox on September 30, 2023, 02:06:14 pmThank you dear TimFox.Most likely, this issue is related to the phenomenon you mentioned earlier (Early effect) However, Although these changes are minor, do you think curve tracer devices correct these slight variations or not? Does this correction actually create noticeable changes in the final shape of the I_V curve, or is it generally negligible and can be overlooked?

author: amir.razzaqi
author_rank: Contributor
timestamp: October 01, 2023, 07:42:40 am
content: Quote from: magic on October 01, 2023, 07:11:40 amI don't have a current source that can maintain stability in the microampere range for (REAL) experimentation.Regarding to "zero current value at the emitter output", I should mention that sometimes my Proteus software gets confused. However, I managed to resolve the issue by removing and re-adding the ammeter.

author: magic
author_rank: Super Contributor
timestamp: October 01, 2023, 08:17:57 am
content: You can easily make a variable current source by inserting a PNP between R1 and Q1 and connecting its base to a potentiometer adjustable between 0V and VBB.This will produce current which is highly insensitive to small changes in Q1 base voltage, although it may change with ambient temperature somewhat.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 01, 2023, 01:35:52 pm
content: Ib is not a constant at all. It's from a fixed voltage and resistor.The question you were looking for was: "does Vbe vary with Vce?" And indeed, it does: Vbe(sat) < Vbe(lin). You can think of this as the B-C junction becoming forward-biased, thus sinking additional current through the base, reducing its voltage drop somewhat.The base current increasing is only a consequence of the reduced Vbe and Thevenin source (voltage + resistance).Tim

author: amir.razzaqi
author_rank: Contributor
timestamp: October 02, 2023, 08:13:52 am
content: Quote from: T3sl4co1l on October 01, 2023, 01:35:52 pmI did not understand the reason why increasing the Vcc leads to an decrease in the current in the base-emitter path. The Base-Collector junction is in reverse polarity.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2023, 03:13:46 pm
content: I don't see in your screenshots where Ib increased from increased Vcc.Tim

author: amir.razzaqi
author_rank: Contributor
timestamp: October 02, 2023, 08:37:42 pm
content: Quote from: T3sl4co1l on October 02, 2023, 03:13:46 pmSorry, it was a typo. I mean "decrease"

author: Sredni
author_rank: Frequent Contributor
timestamp: October 03, 2023, 02:06:07 am
content: To add confusion, let me say that in certain conditions you can actually see base current change with VCE.If you do not pull the base toward a fixed voltage, VBE can be seen changing appreciably with VCE. You need a very big resistance in base, so some might argue that VBE actually changes when IB=0, but if I can measure VBE with an ordinary voltmeterhttps://www.eevblog.com/forum/testgear/what-is-the-internal-resistance-of-the-uni-t-ut139c-%28on-the-mv-range%29/(see page 2. the last pictures)...then we must conclude that a base current proportional to the base-emitter voltage is actually flowing due to the applied VCE.