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# NPN working operation

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#### parth22

##### Member level 4
Hi all,
I have a general question and confused with datasheet values.
I am using an npn transistor as a low side switch. Below is the data sheet of this transistor.

In the below image also you can see.
I would like to draw 10ma collector current. so for 10ma currnet VCE(sat.) is 0.2V (from data sheet). let's suppose VCC = 10 so i can calculate RC = (VCC- VCE(Sat))/10ma.

And same time for the calculation of base current...we need ß (dc gain) so for 10ma ß = 50 to 150 (from the data sheet )...in this way IB = 0.06 ma to 0.2 ma
but as per the data sheet for Ic= 10ma and VCE (sat) = 0.2V.....IB should be 1mA....HoW???
Whether I am opting wrong values or something else...
can anyone explain?

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more base current is needed for saturation - the definition of saturation is when Vce is < Vbe

so for saturation - you need more Ib ....

more base current is needed for saturation - the definition of saturation is when Vce is < Vbe

so for saturation - you need more Ib ....
so in my case for saturation IB should be 1mA ...right?

Rule of thumb used by many is Ib = Ic / 10 to get the transistor fully
saturated. Its called forced beta. 2N3904 for example

Regards Dana.

There's no harm in putting in too much base current...long as you can spare the mA from your supply rail.
Just make sure you give at least Ic/beta for base current.

There's no harm in putting in too much base current..

Unless you don’t like damaged transistors...
long as you can spare the mA from your supply rail.
Just make sure you give at least Ic/beta for base current.
No. You will NOT drive the transistor into saturation. This was already explained.

Beta is used for calculating the linear biasing conditions of the transistor, it is not for determining the base current for transistor operation as a saturated switch, as many beginners try to do.
As has been stated, the base current for saturated operation is typically 1/10th of the maximum collector current.

Beta is used for calculating the linear biasing conditions of the transistor, it is not for determining the base current for transistor operation as a saturated switch, as many beginners try to do.
As has been stated, the base current for saturated operation is typically 1/10th of the maximum collector current.
Okah that'S nice,...I got it.
So for the above transistor which I mentioned, if I have to calculate Rc = (VCC-VCE(sat))/IC,,,,, where VCC is 10V and Ic, i want 10mA,,, now the VCE(sat) will be 0.2V or 1V(as per the datasheet) ....for VCE(sat)which value should I choose.....
or any other calculation to find VCE(Sat) and then RC.....
--- Updated ---

Rule of thumb used by many is Ib = Ic / 10 to get the transistor fully
saturated. Its called forced beta. 2N3904 for example

View attachment 175330

Regards Dana.
Ohhhkakkkk. It means if i want IC= 10mA,,,,then with the rule of thumb, IB =1mA,,,,so for IB=1mA ,,,i have to choose VCE(Sat).
am i right?
if yes, then VCE (sat) can be get graph from datasheet...
--- Updated ---

Okah that'S nice,...I got it.
So for the above transistor which I mentioned, if I have to calculate Rc = (VCC-VCE(sat))/IC,,,,, where VCC is 10V and Ic, i want 10mA,,, now the VCE(sat) will be 0.2V or 1V(as per the datasheet) ....for VCE(sat)which value should I choose.....
or any other calculation to find VCE(Sat) and then RC.....
--- Updated ---

Ohhhkakkkk. It means if i want IC= 10mA,,,,then with the rule of thumb, IB =1mA,,,,so for IB=1mA ,,,i have to choose VCE(Sat).
am i right?
if yes, then VCE (sat) can be get graph from datasheet...
Okah so I got it....
from the IB and VCE graph....i can get VCE(Sat) that will be in the range of 0.1 V to 0.2V.
Thanks to all.

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You are thinking well about it...a lot of people are lazy , like me, and just use a large ohmic value collector resistor, then Ib is always > 1/10th Ic, and it just goes into saturation for you, and you dont even think about it.

There are shedloads of circuits out there, where the designer didnt bother exactly finding out how much base current they have to use to garantee saturation under all condiutions of temperature and batch tolerance............occasionally they fail in the field, and whole products get thrown away.

The thing is beta is indeed for linear operation , not saturated operation.....but the great unknown of 2N3904 electronics is....exactly what level of Ib is needed to get a transistor into saturation....given say, its got a 10k collector resistor off a 5v rail...and the npn is connected common emitter.......to find that exact value of ib, which will just take the npn into saturation........you do , in fact, take as a guidance, the beta value.....even though beta is ultimately not relevant to saturation in real terms.....the rule of ib/10 works in this case...but in some other npn's , it wont work....sorry to be nit-picky and i know you know this..

So the OP hhas raised a good question, because i think its impossible to exactly say what exact ib is needed to just enter saturation in the case that i state as above, with the 10k collector resistor......it will depend on temp and batch for a start.

I had one circuit which was getting its BJTs turned on into saturation by the leakage current coming out of a microcontroller pin.....that ib was much less than ic/10, but still turned the bjt on into saturation.

Some would even say, just use a 2N7002 signal FET and be done with it......but then there are those like me who feel it needs a gate source resistor otherwise ESD damage may ensue to the fet......also, i just generally feel that BJTs are more ESD immune than FETs.......though the purveyors of modern ic's, which have done away with Bipolar tech, would slam me down for that.

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Remember the graph is "typical", usually at room temp. So don't depend, one
transistor to another, that you can predict with any accuracy the Vcesat value
you are going to get. Its just a guideline.

But it does show support for the "rule of thumb" to achieve Vcesat, as discussed
earlier.

Note, if in future you have a requirement I need the lowest possible Vcesat, some
transistors, when you reverse connection of emitter and collector, will give you
even lower Vcesat results.

Regards, Dana.

If it's not clear, the saturation collector current is determined by the collector load impedance, not the transistor itself.
The transistor is being used as the contacts of a switch so, just like the contacts of a relay, the current is determined by the load, not the contacts.

If it's not clear, the saturation collector current is determined by the collector load impedance, not the transistor itself.
The transistor is being used as the contacts of a switch so, just like the contacts of a relay, the current is determined by the load, not the contacts.
Yes excatly. The current can be regulated by the Rc value. transistor is just as a switch.

Yes i agree.
What would be interesting to know would be, for the case of a 2N3904 (in common emitter connection) with 10k collector resistor to a 5v rail.....what exact base current would need to flow to just about give saturation?.....just right on the edge of saturation?

I actually think that this current is not exactly knowable.......the OP has raised an excellent question....its simply unknown

Yes i agree.
What would be interesting to know would be, for the case of a 2N3904 (in common emitter connection) with 10k collector resistor to a 5v rail.....what exact base current would need to flow to just about give saturation?.....just right on the edge of saturation?

I actually think that this current is not exactly knowable.......the OP has raised an excellent question....its simply unknown
Well as we earlier discussed,,,
you atleast should know one parameter....either base current or collector current. In most or I will say almost cases,,,we already know the collector current(that will go to our load,,,so that we know our requirement like we need 10mA or 50mA or 1A whatever it is..)
once you will have Ic,,,...from the datasheet you can find VCE(sat),,,,if you find VCE(sat),,,you can calculate Rc....
if you have IC then from the datasheet you will have ß also...so you can calculate IB also...the further RB also..
Thanks.

@cupoftea @parth22 You guys are TOTALLY ignoring everything that people have been TRYING to tell you.

1) Beta has nothing to do with saturation voltage. Again, BETA HAS NOTHING TO DO WITH SATURATION VOLTAGE. By definition, when a BJT is in saturation an increase in base current causes no change in collector current.
2) If you read the data sheet you will see that the value for beta (hfe) has a HUGE range. In fact, they only specify minimum and typical; there’s no maximum. Knowing Ic will not enable you to calculate Ib ’exactly’.
3) This is why transistors use feedback to set gain.
4) Reading the data sheet will also tell you typical Ib for typical Vce-sat at a particular Ic.

if you have IC then from the datasheet you will have ß also...so you can calculate IB
How often do we need to say that Beta has little to do with saturation voltage, until you understand?

This is beginning to seem like a troll.

@cupoftea @parth22 You guys are TOTALLY ignoring everything that people have been TRYING to tell you.

1) Beta has nothing to do with saturation voltage. Again, BETA HAS NOTHING TO DO WITH SATURATION VOLTAGE. By definition, when a BJT is in saturation an increase in base current causes no change in collector current.
2) If you read the data sheet you will see that the value for beta (hfe) has a HUGE range. In fact, they only specify minimum and typical; there’s no maximum. Knowing Ic will not enable you to calculate Ib ’exactly’.
3) This is why transistors use feedback to set gain.
4) Reading the data sheet will also tell you typical Ib for typical Vce-sat at a particular Ic.
Yes I got it. I read some more notes. Basically once the transistor is in saturation, no matter you how far base current you inject after this.

Although a lot of thanks.

How often do we need to say that Beta has little to do with saturation voltage, until you understand?

This is beginning to seem like a troll.
This is cupoftea, formerly known as treez, always known as a willfully ignorant poster.

I believe we are talking at crossed purposes...i agree that beta doesnt apply when you are in saturation....

The OP (forgive me if i am wrong), was wanting to know exactly what ib will take it "just" into saturation, and i believe it is a fact that it is not possible to know this figure exactly. ....Considering the case of a common emitter NPN with a 10k resistor to a 5V rail................the question being, exactly what ib takes it just into saturation?........and the answer, i think we all agree, is not knowable?

However, if we are saying that as ib is raised from zero...at first its in the linear region........then it eventually goes into saturation..........wont the beta (if known, and i dont think it is exactly), give us a guide as to that exact point that it eventually goes into saturation?.....in which case.......the beta does have some role in helping us to find what exact ib leads us "just" into saturation.......i do agree that when you are in saturation, beta does not apply.

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Hi,

In post#1 the OP asks about BJT used as low side switch.
And "switch" for me personally is not the "edge to saturation" but " safely saturated with some overdrive".

Klaus

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