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Why transistor Vbe drops when is in active region? BC547 ib=constant

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Trying to measure H parameters with a curve tracer, I have seen that the Vbe drops when transistor is active region, BC547. I dont find any explantion looking the  Ebers-Moll transistor model,equations and theory, and Gummel-Poon.

Anyone can explain why it drops?

First photo is Ic vs Vce, second Ic vs Vbe, and third Ic vs Vbe x10

I have tried the same with other transistors and others curve tracers with the same result.


Heating perhaps? 10mV decrease can be explained by 5°C junction temperature rise.

What's the Vce in those tests?

Hi, 10v is the first photo. Is easy to see thermal effects at curve tracer, but they are slowly and affect at the whole curve, not only the edge

I think the Ic vs Vbe traces are the same as the Ic vs Vce except that Vbe replaces Vce. If that's the case then you have constant base input current and stepped Vce voltage for a number of different base currents. If you notice the first trace you will see that for higher currents the Vce voltage cuts short earlier, that is because BJT breakdown voltage is current dependent, and when the tester detects this problem it ceases that test. For the Vbe vs Ic you have the same condition except it is a bit more pronounced. There is special overcurrent protection built into the tester it might be a side effect of that.

The voltage Vbe definitely dropped a few millivolts in the active zone, I tested it today with two multimeters, measuring the base current to verify that it is constant, and another measuring Vbe
The only explanation that I find is hre

Unigor multimeter is for check Ib (constant)

Someone making the same answer 6 years ago


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