Ib, Ic vs Vce (fixed Vbe)

[celebrevida]

Suppose I have an npn connected such that:
Ve=0V
Vbe=0.7V
Vce=1V

I measure Ib=1uA, Ic=100uA, Ie=101uA
So far so good.

Now let's say I lower Vce=0.2V driving the BJT into saturation but keep Vbe=0.7V.

Clearly Ic goes down. But what about Ib? Does it go up, down or stay the same and why?

 

[KlausST]

Hi,

Ve is what? Emitter voltage with respect to????

Currents look O.K.

Don´t drive the base with voltage, it´s far easier (to calculate) to drive it with current.
Voltage depends on temperature, tus when you drive Vb with constant voltage the current may be high, low, or zero....

Klaus

 

[Audioguru]

When you drive the transistor into saturation then the base voltage and base current go UP and the collector current also goes UP.

Why, because when you increase the base current then the base voltage increases a little and the collector current also increases as the collector voltage drops into saturation.

 

[celebrevida]

Hi,

Ve is what? Emitter voltage with respect to????

Currents look O.K.

Don´t drive the base with voltage, it´s far easier (to calculate) to drive it with current.
Voltage depends on temperature, tus when you drive Vb with constant voltage the current may be high, low, or zero....

Klaus

Ve=0V means the emitter is grounded.

As for temp, assume unchanged.

So if everything is unchanged, except Vc going from 1V to 0.2V, I was simply wondering what is expected to happen to base current?

--- Updated Sep 7, 2020 ---

When you drive the transistor into saturation then the base voltage and base current go UP and the collector current also goes UP.

Why, because when you increase the base current then the base voltage increases a little and the collector current also increases as the collector voltage drops into saturation.

I am not increasing base current. I am keeping Vbe constant at 0.7V and dropping Vce from 1V to 0.2V.

In that case, Ic surely decreases. But what about Ib?

I am guessing that if temperature is unchanged, I expect base current to remain at 1uA but collector current to drop from 100uA to a much smaller value. Is that right?

 

[Audioguru]

Usually the transistor turns on more and decreases its Vce to the 0.2V of saturation.
But since you have another way of reducing Vce (maybe by reducing the voltage of the collector load?) then the spec's in the datasheet shows that the beta decreases when the Vce is reduced by saturation.

 

[FvM]

Referring to the original question. Without considering transistor self-heating, Ib is expected to slightly increase in saturation (e.g. a few percent) because bc junction is forward biased and overtaking part of the base current. Reduced power dissipation and respective change of junction temperature and forward voltage with decreased Vce can partly reduce or even reverse the effect.

 

[celebrevida]

Referring to the original question. Without considering transistor self-heating, Ib is expected to slightly increase in saturation (e.g. a few percent) because bc junction is forward biased and overtaking part of the base current. Reduced power dissipation and respective change of junction temperature and forward voltage with decreased Vce can partly reduce or even reverse the effect.

Okay that makes sense, sort of.

But if the B-C forward bias results in reverse diffusion current adding to the B-E forward bias diffusion current, why only a few percent? Is it because collector not as heavily doped as emitter and Vbc=0.5V while Vbe=0.7V?

This leads me to think. What if I drove Vce=0V but kept Vbe=0.7V? Why doesn't that lead to current flowing into the base and out of both emitter and collector???

 

[celebrevida]

Okay I have decided to use circuitlab.com to simulate a 2N3904 npn. You can repeat this by going to circuitlab.com, grounding E, fixing Vbe=0.7V and sweeping Vce.

With Vbe=0.7V, I plotted Ib (into base), Ic(into collector), -Ie (out of emitter)
The plot is as follows:

When Vce=0V, according to circuitlab sim, there is actually current flowing out of both collector and emitter! (negative sign of Ic)
Vce needs to rise above 5mV before Ic current reverses to be INTO the collector!
This is what I thought should happen and sims show it is true.

Vce=0.3V Ib=32.6uA (At this point, Ib is fairly steady and we are well in active mode)
Vce=0.2V Ib=32.8uA (only a few percent higher)
But as we go below Vce=0.1V, Ib in fact increases exponentially.

So far this all makes sense. But that still leads me to one last question.

By fixing Vbe and lowering Vce, I see that both Ie and Ic go down.

Is it the case that as Vbc gets forward bias, it adds a reverse diffusion current that cancels out the Vbe forward bias diffusion current?
I've never heard of saturation described this way but it seems like this is what is happening.

Rather I've heard it described confusingly as base current is being "saturated" with carriers and can't hold any more. But clearly Ib is increasing so this isn't quite true.

But looking at it as two opposing diffusion currents (one from collector electrons flowing to base & the other from emitter flowing to base) makes a lot of sense. Is that what is happening or does it just look that way and it is the wrong way to think about saturation?