What does the chart tell us about VCE_SAT

Thread Starter

MikeJacobs

Joined Dec 7, 2019
95
Trying to interpret these graphs correctly.

So for a given IC current and a known base current, I can tell what the VCE sat should be.

So does this mean

Choice A: that if my VCE is less then this found number I am in saturation
or
Choice B: That if my VCE is greater then this found number I am in saturation.

I am thinking it should be choice A right?

I would need to have a VCE smaller then the graph shows to be in full saturation?


Also, what voltage might I be missing to calculate VCE if you ground your emitter?
Obviously your emitter is at zero volts and theoretically your VC is also zero
We know its not really, there is some small voltage at VC. This assumes a current limiting resistor on the collector
So assuming when the BJT is saturated and the emitter is grounded of VCE is very very close to zero correct?

Thanks!
 

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AlbertHall

Joined Jun 4, 2014
9,326
The Vcesat shown is a typical value. Individual devices will exhibit values around those shown in the datasheet. You cannot exactly determine what voltage you will get.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
95
yes I do recall that VCE sat is essential about as good of a number as Hfe or Beta.
However, does that mean saturation is below or above this number ?

So like I said about. using the graph, if we have a known IC and a known IB and follow it to a VCE, does our real VCE need to be above or below that number to be considered in saturation?
 

AlbertHall

Joined Jun 4, 2014
9,326
So like I said about. using the graph, if we have a known IC and a known IB and follow it to a VCE, does our real VCE need to be above or below that number to be considered in saturation?
The values are typical. An actual device could be above or below the datasheet value.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
95
The values are typical. An actual device could be above or below the datasheet value.
right, I get that the value is a guess
But lets say that its known for a second. that the sheet is 100% acurate

I don't understand which way the graph goes when talking in terms of the characteristic curve of a BJT
If you found the VCE sat and it was 100% accurate, does that mean all VCE's below that number mean the device are in saturation, or all VCE values above that number mean its in saturation
 

Audioguru again

Joined Oct 21, 2019
918
Design your circuit so that it works with all transistors of the part number you selected, even the weakest ones. Then all your projects will work. The graphs for a transistor are only for typical or better transistors, weakest but still passing transistors do not work as well as typical or better. But you can design the circuit so it works fine with them.

You did not say the part number or written maximum Vce saturation spec for your transistor.
A BC337 datasheet shows a graph with a typical Vce saturation voltage of 0.18V when the collector current is 500mA and the base current is 50mA. But the written saturation spec for 500mA collector current and 50mA base current shows a maximum Vce saturation voltage of 0.7V which is much worse than the 0.18V typical shown on the graph. If this transistor drives a 500ma load plus is needed to turn off another transistor then the weakest transistors will not turn off another transistor unless you design a voltage divider to help a weak transistor do it.
 

Papabravo

Joined Feb 24, 2006
12,954
All device parameters can be considered as normally distributed random variable with a mean and a variance. This guarantees that for any given population mean and non-zero variance there will be devices above it and devices below it.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
95
I understand all this thus far and appreciate everyones help
However, I am asking an extremely specific question

Is the value less then or greater then.....

Like I said in post #5
I am trying to figure out one simple thing.

Does it mean the transistor is in saturation of VCE is greater then or less then the value found in the chart.

I fully understand the values are a distribution

but lets say that its known for a second. that the sheet is 100% acurate

I don't understand which way the graph goes when talking in terms of the characteristic curve of a BJT
If you found the VCE sat and it was 100% accurate, does that mean all VCE's below that number mean the device are in saturation, or all VCE values above that number mean its in saturation
 

TeeKay6

Joined Apr 20, 2019
572
I understand all this thus far and appreciate everyones help
However, I am asking an extremely specific question

Is the value less then or greater then.....

Like I said in post #5
I am trying to figure out one simple thing.

Does it mean the transistor is in saturation of VCE is greater then or less then the value found in the chart.

I fully understand the values are a distribution

but lets say that its known for a second. that the sheet is 100% acurate

I don't understand which way the graph goes when talking in terms of the characteristic curve of a BJT
If you found the VCE sat and it was 100% accurate, does that mean all VCE's below that number mean the device are in saturation, or all VCE values above that number mean its in saturation
If the chart is 100% accurate for the transistor you have in your hand, then Vce(sat) is exactly as the graph shows. Since the chart is 100% accurate, Vce(sat) can be neither above nor below the line in the chart.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
95
I do not understand this then
The characteristic curve of a BJT shows a range of VCE saturation
So how can you say its exactly as the graph shows

I don't understand why I a failing to ask this questions. I must be missing something.
Look at the characteristic curve of a BJT, the saturation region gets large as IC goes up and IB goes up.

Therefore, VCE has a range

If you are at the top end of the characteristic curve, VCE for saturation is very wide. VCE of 1.0 and less means in saturations and 1.0 and right puts you in linear
 

TeeKay6

Joined Apr 20, 2019
572
I do not understand this then
The characteristic curve of a BJT shows a range of VCE saturation
So how can you say its exactly as the graph shows

I don't understand why I a failing to ask this questions. I must be missing something.
Look at the characteristic curve of a BJT, the saturation region gets large as IC goes up and IB goes up.

Therefore, VCE has a range

If you are at the top end of the characteristic curve, VCE for saturation is very wide. VCE of 1.0 and less means in saturations and 1.0 and right puts you in linear
Of course Vce(sat) varies with transistor type and other parameters. However, you said in your post#8: "but lets say that its known for a second. that the sheet is 100% acurate." YOU are the one who said to "consider the graph 100% accurate." Now when I do so, you are upset.

Let's reiterate what you have already been told by several posters. The graph of Vce(sat) vs any-other-parameter is not 100% accurate. The graph shows typical characteristics. If you tested 1000 transistors (of the same type), many would have Vce(sat) near (but not exactly at) what is shown in the graph, but only many, not all. If you want an upper or lower limit for a parameter you must look to the tabular data given in the datasheet; all the graphs show typical data, not min nor max.
 
Last edited:

crutschow

Joined Mar 14, 2008
24,397
Let's look at the basics:

Saturation of a BJT is generally defined as when the collector-emitter voltage is equal to or less than the base-emitter voltage, thus a collector-emitter voltage higher than that means the transistor is not saturated.

In general, a BJT is fully saturated when a further increase in the base-emitter current does not cause a significant reduction in Vce.

Note in the LTspice simulation below, the 2N3055 transistor reaches initial saturation [collector voltage (red trace) equals the base voltage (blue trace)] at a base current of about 350mA (yellow trace, horizontal scale) for a collector current of 5A (blue trace), with full saturation at about a 425mA base current, above which the collector voltage changes very little..

The typical way to insure saturation is to provide a base current that's at least 1/10th of the maximum collector current (forced β of 10).
The data sheet Beta value is not used for this purpose as that value pertains to operation in the linear region, not the saturated region.

1577088534796.png
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
95
ok I think I understand the issue here and why im so confused.
So the VCE Saturation in a data sheet is the expected VCE voltage drop!!
It is not the VCE voltage needed to be considered in saturation!

Correct?!! That is why I am so confused.

So my confusion stemmed from the characteristic curve of a BJT

As the IC goes up, the saturation region grows. So I thought that the chart was showing us what the vce RANGE was for a given IB and IC. AKA how big the range is in the characteristic curve.
I found a video now shows that is not what VCE sat means.

It should be the VCE VOLTAGE DROP. for a given IC and IB

Can someone correct me if im wrong!

Thanks for all the input and help
 

OBW0549

Joined Mar 2, 2015
3,209
So the VCE Saturation in a data sheet is the expected VCE voltage drop!!
It is not the VCE voltage needed to be considered in saturation!
You've got it!

That chart merely tells you what Vce(sat) to expect under the given conditions of Ic and Ib for a range of collector currents, with the ratio Ic/Ib held constant. This information is mainly of interest in situations where the transistor is switching considerable current, since Ic * Vce(sat) will determine how much power the transistor is dissipating and thus how hot it gets.
 

Papabravo

Joined Feb 24, 2006
12,954
To place a bookend on this discussion, the opposite of saturation is cutoff. In cutoff there is very little if any current flow, and a voltage at the collector equal to Vcc. Although you can measure the voltage difference between the collector and the emitter you have to realize that since there is no current flow there can be no voltage drop between the two. In between saturation and cutoff is called the linear region. In this region for the most part the output of an amplifier will look like the input. If this was instead a non-linear region then we would have new frequencies, sensitive dependence on initial conditions, and chaotic behavior.
 

Papabravo

Joined Feb 24, 2006
12,954
A shoutout to @crutschow for getting me to look back at my own Vce simulations that did not look like his for the 2N3055. My problem was that my graph for I(c) was concave up. It took more than a few nanoseconds to realize that his graphs have a linear scale for base current while mine had a logarithmic scale covering 3 decades. Thanks man!
 
Last edited:

TeeKay6

Joined Apr 20, 2019
572
To place a bookend on this discussion, the opposite of saturation is cutoff. In cutoff there is very little if any current flow, and a voltage at the collector equal to Vcc. Although you can measure the voltage difference between the collector and the emitter you have to realize that since there is no current flow there can be no voltage drop between the two. In between saturation and cutoff is called the linear region. In this region for the most part the output of an amplifier will look like the input. If this was instead a non-linear region then we would have new frequencies, sensitive dependence on initial conditions, and chaotic behavior.
"...you can measure the voltage difference between the collector and the emitter you have to realize that since there is no current flow there can be no voltage drop between the two" ??? Would you like to try that again? In cutoff there can be no voltage drop between collector and emitter?
 

MisterBill2

Joined Jan 23, 2018
5,229
Trying to interpret these graphs correctly.

So for a given IC current and a known base current, I can tell what the VCE sat should be.

So does this mean

Choice A: that if my VCE is less then this found number I am in saturation
or
Choice B: That if my VCE is greater then this found number I am in saturation.

I am thinking it should be choice A right?

I would need to have a VCE smaller then the graph shows to be in full saturation?


Also, what voltage might I be missing to calculate VCE if you ground your emitter?
Obviously your emitter is at zero volts and theoretically your VC is also zero
We know its not really, there is some small voltage at VC. This assumes a current limiting resistor on the collector
So assuming when the BJT is saturated and the emitter is grounded of VCE is very very close to zero correct?

Thanks!
The saturated VCE is below the curve for that particular device. So if the VCE is less than the stated typical then the device is in saturation. But the saturation voltage is dependent on both the base current and the collector current, and the junction temperature. And the relationship is not always simple. You can sometimes find a "max" value and that will apply to most of the devices with that type number.
 

Papabravo

Joined Feb 24, 2006
12,954
"...you can measure the voltage difference between the collector and the emitter you have to realize that since there is no current flow there can be no voltage drop between the two" ??? Would you like to try that again? In cutoff there can be no voltage drop between collector and emitter?
You can measure the voltage, with a DC voltmeter, between ANY two points in a circuit and there can be a difference. Just because there is a difference does not mean there is a voltage drop between those two points. I can measure a voltage difference across a capacitor when there is no DC current flowing
 
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