Exactly this. Your BJT is in active region because Vce > 0.2V.

And what does that mean? Also what would change if Vce was less than 0.2 V? It would be in saturation, right?

 

And what does that mean? Also what would change if Vce was less than 0.2 V? It would be in saturation, right?

Hi,

Can you elaborate a little please? Like why you are so concerned about the Vce not being 0.2v or being 0.2v ?
Thanks.

 

Hi,

Can you elaborate a little please? Like why you are so concerned about the Vce not being 0.2v or being 0.2v ?
Thanks.

Because from what I understand, that voltage Vce changes what region the transistor is working in. I think that if Vce is less or greater than Vces that the transistor is in active mode. I don't understand what's the difference between active and saturated mode.

 

Because from what I understand, that voltage Vce changes what region the transistor is working in. I think that if Vce is less or greater than Vces that the transistor is in active mode. I don't understand what's the difference between active and saturated mode.

You are still talking like you think that which mode you are in is like a switch and that you are either in one mode or the other (leaving aside the possibility of cutoff and reverse operation).

It's not.

If you plot the collector current verses the collector-emitter voltage for a fixed base current what you will see is that as you increase the Vce the collector current increases. At first, the collector current increases pretty linearly with increasing voltage while increasing the base current have only a small effect (this is the saturation region), but as you go higher you get to a point where further increases in voltage only increase the current slightly, however changing the base current results in a nearly proportional change in the collector current (this is the active region). The transition between these two regions occurs smoothly over a range Vce values that typically, for silicon transistors, is in the range of a couple hundred millivolts.

You have a continuous, smooth c

 

Because from what I understand, that voltage Vce changes what region the transistor is working in. I think that if Vce is less or greater than Vces that the transistor is in active mode. I don't understand what's the difference between active and saturated mode.

Hi,

Active mode is when the model base collector diode is reverse biased.
Sat mode is when the model base collector diode is forward biased.
That's the standard, and it is incorporated into full Spice evaluations in simulators.

Why is it like this way?
It is because when the BC diode is reverse biased, the model current controlled current source is free to follow the base current times Beta definition of the collector current Ic=Ib*Beta, but when the BC diode is forward biased some of the external base current is directed away from the base emitter (BE) diode and instead flows through the BC diode and that makes it look like the Beta is lower. The simplified equation for this condition would look like this:
Ic=(Ibe-Ibc)*Beta
and so we see the effective Beta drop.
When this first starts to happen it may happen quickly or gradually, so we might only see a very very small Ibc current and so it still looks like we have Ic=Ibe*Beta, but then as the CE voltage drops the Ibc current increases so we have again Ic=(Ibe-Ibc)*Beta and depending on the severity of this condition we might have a lot of Ibc current and so there is much less Ibe current left to drive the base emitter and thus we need more base current than usual in order to get a larger collector current. So we might define the degree of saturation such as DS=Ibc/ibe or perhaps DS=Ibc/(Ibc+Ibe) or something like that.

As the collector voltage falls more and more, the Ibc current increases more and more, and eventually there is an equilibrium reached where we have just enough base current to drive the transistor collector voltage down to maybe 0.4v but because of the load resistance it wont go any lower with the given amount of base current.

This is actually a very simple concept but it appears to be more complicated because we are using diodes instead of resistors. But a simple way to look at it is that the base collector diode provides an internal negative feedback path for the transistor and so it acts to limit the gain. Replace the two diodes with small value resistors and keep the current controlled current source and you can get an idea how this works. However, with diodes we see a more abrupt point where the transistor enters saturation because diodes dont conduct much until their forward voltage reaches a certain level like 0.4 volts or so.

Also keep in mind that the input current to the transistor Iin is the sum of the BE current and the BC current, and so when some of the BE current is directed away through the BC diode the current actually driving the transistor BE junction drops and that's the beginning of saturation.