I think that some of us here have already accepted that the professor is using a hypothetical case where β is constant at 150.
He is using a simplified model and not using real world data.

As you can see the picture was acquired on an oscilloscope using a transistor characteristics curve tracer and a real 2N3904 transistor. I did not have time to try a sample of 2N3904 transistors.

 

When they ask questions like this they should provide all the details or at least links to the data sheet they used otherwise it is impossible to be sure. Perhaps the instructor has little real world work experience

And so when a Physics I instructor asks how car a cannon shell will travel when fired from a cannon at a certain muzzle velocity and barrel elevation, should they provide links to ballistics tables and air friction coefficients as a function of velocity, temperature, air pressure, and atmospheric humidity and also tell where on the Earth the cannon is located and what direction is it pointing so that they can take into account the Earth's rotation?

Or is it perhaps reasonable to start off with a very simplified model that assumes that the cannon is on a perfectly flat plane with a uniform gravitational field and that air resistance is negligible, even though none of those things are true and all of those are taken into account in long range artillery.

 

I alluded to this in post #13 but got no response.
What is the max Ic at Vcc = 10V?
What is the max Ic at Vcc = 15V?
Given Ic = β x IB
What can we say about the saturation/non-saturation state of the common emitter circuit?
End of Question

Since then it has intrigued me as to how can we calculate VCE and at what supply voltage Vcc can we say the transistor is just borderline in-out of saturation?

You can only calculate something according to a mathematical model. Even then, before you can begin to answer that question you have to define what it means to be borderline in-out of saturation? Look at your curve traces. Saturation is best described as that quite linear (at this scale) line to the left of the curves. But from a practical standpoint would you really call any of those curves at a Vce of 0.3 V to be in the active region? Even if you did, if you zoom in on that saturation line you see that there isn't a definite point at which each curve departs the line. It is a smooth transition. Even at this scale it is quite evident that each trace has pronounced curvature as Vce falls.

So if you want to calculate the borderline in-out point, you have to first develop a mathematical model that reproduces those curves with sufficient fidelity to provide an answer that is accurate enough for your purposes. One common way to do this would be to have two curves, one that models the active region and that assumed that there was no such thing as saturation, and one that modeled the saturation region that assume there was no such thing as the active region. Where the two curves cross would determine the borderline. But that is going to result in a sharp transition that is not going to agree with the real device which is going to have a smoother transition. For the overwhelming majority of cases, that disagreement doesn't matter.

 

And so when a Physics I instructor asks how car a cannon shell will travel when fired from a cannon at a certain muzzle velocity and barrel elevation, should they provide links to ballistics tables and air friction coefficients as a function of velocity, temperature, air pressure, and atmospheric humidity and also tell where on the Earth the cannon is located and what direction is it pointing so that they can take into account the Earth's rotation?

Or is it perhaps reasonable to start off with a very simplified model that assumes that the cannon is on a perfectly flat plane with a uniform gravitational field and that air resistance is negligible, even though none of those things are true and all of those are taken into account in long range artillery.

Hi there,

I agree in part. We did see some confusion come up among people here so it also seemed reasonable to ask for more information.
Later i saw a difference in models, where one model might not quite fit the question while the other did.
Also note that my notes from my own experience and simplified conclusions were always that it was in sat to start, then came out of sat with the higher voltage, but i would have preferred more info too.
I also think that a transistor question as well as a physics question deserves it's place in the context of the course work. If the student is in physics 101 they could most likely get away with just a few details for the cannon example, but if they try that in an advanced course in physics the instructor will certainly have a good laugh cause he's going to get paid twice for that student when they come back next semester to take the same course again
So i partly agree, with a just few small reserves. I believe your way of thinking about this is good too...take it for what it says and dont question it too much.

You might also want to note that in spice the transition between sat and non sat is not the difference between a straight line and a curve. That i believe seems true because the line is never really straight for one thing, and even if part of it is, the sat point is defined mathematically with an inequality based on the base emitter voltage and the collector emitter voltage and as we know the base emitter voltage vs base current is never really straight. This also means that i believe that the curve in question was a simplified approximation of the real response. Feel free to add more though to this discussion on saturation.

The difference between saturation and non saturation is a very important facet of switching circuits using bipolar transistors. If we had to design a fast switching circuit, we'd have to define this more accurately. Lucky here we dont have to do that

 

Please help me with the following assignment question. A certain 2N3904 dc basis circuit with the following values is in saturation. Ib = 500 uA Vcc = 10V and Rc = 180 ohm and hfe = 150. If you increase Vcc to 15V, does the transistor come out of saturation?

My attempt at a solution:

Ic (sat) = (Vcc - Vce (sat))/Rc but Vce (sat) then work out whethere Ib is capable of producing Ic (sat) but Vce (sat) isn't given. It's only a two mark question. Please help.

Here's how I'm looking at this
Using a grounded emitter CE config, with constant IB of 500 uA, assuming 0V for VCE sat.

1). I would calculate IC under VCC of 10 volts, with VCE @ 0v.

2). Then calculate IC under 15 volts with VCE @ 0v.

3). And then calculate IB (500uA) times Beta constraint (150) to give max. current under VCE sat.

4). After that multiply this sat. current times collector load resistor 180 ohms to give voltage drop across the load under this assumed condition.

5). And subtract the voltage dropped across the load resistor, from the increased supply of 15 volts. and the difference then becomes the drop across the transistor.

From the problem, it seems the assumptions are that under VCE sat. the beta is 150 rather than the nom.10 used for real world design purposes. And it's OK to assume 0v. for sat.

I think the fact there constraining IB makes it possible to be able to do this problem, because with IB constrained and Beta given, it doesn't matter what the VCC value is to attain a calculated value for IC sat. IC sat is constrained as (500uA times 150).

That's how I would approach this problem, maybe it helps.

 

I think it's been pretty well beaten out that the beta of 150 is for when it is NOT in saturation.