General > General Technical Chat
BJT output I-V characteristic
amir.razzaqi:
--- Quote from: magic on October 01, 2023, 07:11:40 am ---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? :-//
--- End quote ---
I 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.
magic:
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.
T3sl4co1l:
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
amir.razzaqi:
--- Quote from: T3sl4co1l on October 01, 2023, 01:35:52 pm ---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
--- End quote ---
I 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.
T3sl4co1l:
I don't see in your screenshots where Ib increased from increased Vcc.
Tim
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