# NPN Bipolar junction Transistors.

#### Tobias12

Joined May 25, 2019
8
Hello everyone ! How can I solve this problem ? I don't have any idea.

#### ericgibbs

Joined Jan 29, 2010
9,112
hi Tobias,
Welcome to AAC.
As this appears to be Homework, I will move to the Homework Forum.
E

#### Tobias12

Joined May 25, 2019
8
hi Tobias,
Welcome to AAC.
As this appears to be Homework, I will move to the Homework Forum.
E
Yes, it's a homework.

#### ericgibbs

Joined Jan 29, 2010
9,112
hi T,
You will get more help if you post your best shot at answering the questions, others will then guide you.
E

#### Tobias12

Joined May 25, 2019
8
Af means active forward conduction (amplifier region) and exc means extreme conduction (saturation region).
I know that in Active forward Ic=βIb, but i don't know how to find Ib because i don't know Rb and also i can't find Ic because i don't know Vce.
In saturation regiom Ic<βIb, Vce = 0.2 V. I can find Ic, but i don't know how can find Ib.

#### MrAl

Joined Jun 17, 2014
6,817
Hello everyone ! How can I solve this problem ? I don't have any idea.
Hello,

Ok then to figure out the values you would assume that in the active mode Vce is greater than Vbe, and in the saturation mode Vce is less than Vbe. You then only have to solve for the base resistor values given your Beta values.

#### Tobias12

Joined May 25, 2019
8
Hello,

Ok then to figure out the values you would assume that in the active mode Vce is greater than Vbe, and in the saturation mode Vce is less than Vbe. You then only have to solve for the base resistor values given your Beta values.
But for active mode Vce> Vbe gives me Ib<5,65/β. I don't know what value choose for ib to find the range of Rb that's my problem.

#### Tobias12

Joined May 25, 2019
8
For Rb, I am calculating the values between 11.25kΩ and 90kΩ
Anyone agree or disagree?
How did you calculate ?

#### Ford Prefect

Joined Jun 14, 2010
173
How did you calculate ?
Hi Tobias12,
Sorry, but I am not going to disclose how I arrived at these results. It is up to you to do the work and discover how I arrived at these results, after all, it is your homework... also the results I gave may not be accurate.
People can give you hints and solutions to work out the results (as most people do) or they can give you the answer and for you to work out the solutions to that answer - I did the latter.

• ericgibbs

#### MrAl

Joined Jun 17, 2014
6,817
But for active mode Vce> Vbe gives me Ib<5,65/β. I don't know what value choose for ib to find the range of Rb that's my problem.
Hi,

Well no, this is not to calculate the Beta the idea is to take the known values 25 and 200 and calculate the values of Rb which satisfies the criteria given. For the active mode i would think these would be the minimum values of Rb, and for the sat mode i would think it would be the maximum values.
To choose Rb for any given current, just calculate the current through Rb knowing the input voltage and the Vbe voltage and the current. So it is the difference divided by the current.

#### WBahn

Joined Mar 31, 2012
24,974
Hi Tobias12,
Sorry, but I am not going to disclose how I arrived at these results. It is up to you to do the work and discover how I arrived at these results, after all, it is your homework... also the results I gave may not be accurate.
People can give you hints and solutions to work out the results (as most people do) or they can give you the answer and for you to work out the solutions to that answer - I did the latter.
Please do NOT do the latter. Remember, increasingly homework is done via online tools that just want the answer. If you give the answer then the student doesn't need to work out how you do it because you've given them what they need in order to claim credit for solving the problem and move on -- which is quite detrimental in the end. Also, intentionally giving inaccurate answers is almost always counterproductive. Consider what you have just stated yourself -- your approach is to give them the answer and let them work out solutions to that answer; so if you give them inaccurate answers you can only be expecting them to spend a bunch of time trying to figure out some way to work out a solution that leads to that wrong answer. So even if they work the problem correctly, they will assume that they've done something wrong because they didn't get the solution that you are expecting them to work out a solution to.

• crutschow

#### WBahn

Joined Mar 31, 2012
24,974
But for active mode Vce> Vbe gives me Ib<5,65/β. I don't know what value choose for ib to find the range of Rb that's my problem.
I suspect that there is some side-channel information you are expected to know and use. For instance, a common practical definition of "saturation" is that the β has dropped to the point where it is no more than 10. This is an arbitrary threshold and is not universally used, but it is extremely common for small-signal transistors.

Another thing to keep in mind is that there is no hard distinction between active region and saturation -- the transistor continuous and smoothly transitions from one to the other, so any thresholds that are set are arbitrary. It is not unreasonable to set two thresholds, one that marks the end of "official" saturation and one that marks the beginning of "official" active-region operation. For people that choose this route, a common threshold between transition and active-region is when the base-collector junction is positive (i.e., when Vce < Vbe) while the other threshold is when β = 10 and Vce = Vcesat (which is typically taken to be 200 mV, give or take, if you lack other specific information upon which to base a better value). The gap between is then just a region where you can't really say for sure that it is in either of those two regions -- it's in transition.

Another thing that makes it difficult to tell for sure what they are asking (for us -- you might know since the problem is written in the same style as whatever text you have been learning from) is when they say that β is in the range of 25 to 200 are they saying that you should consider the transistor to be in the active region if the beta is within that range, or are they saying that the manufacturer is merely saying that this particular model of transistor may have active-region β values that are anywhere within that range. I would play it safe and assume it is the latter and run the numbers for both limiting values; then choose your range of resistance values that satisfy both cases.

Let's tackle the saturation case first (part b). What is the collector current when the transistor is at Vcesat? If you don't know what value you are supposed to use for Vcesat, simply pick a reasonable one and state it as an assumption at the start of your work? What, then, is the minimum base current that is consistent with the transistor being in saturation? What range of base resistor values are consistent with establishing at least that minimum base current?

#### Tobias12

Joined May 25, 2019
8
Hi,

Well no, this is not to calculate the Beta the idea is to take the known values 25 and 200 and calculate the values of Rb which satisfies the criteria given. For the active mode i would think these would be the minimum values of Rb, and for the sat mode i would think it would be the maximum values.
To choose Rb for any given current, just calculate the current through Rb knowing the input voltage and the Vbe voltage and the current. So it is the difference divided by the current.
You wanna say that Rb = (Vc0 - Vbe)/Ib, right ? If i succeed to find Ib i can solve the problem easy, But my problem didn't give me a certain value for the current base Ib. I don't know how to find first the value of base current and then, use the above formula to find Rb.

#### MrAl

Joined Jun 17, 2014
6,817
You wanna say that Rb = (Vc0 - Vbe)/Ib, right ? If i succeed to find Ib i can solve the problem easy, But my problem didn't give me a certain value for the current base Ib. I don't know how to find first the value of base current and then, use the above formula to find Rb.
Hi,

Without given the whole solution, the iB base current is based on the collector current required to bring the Vce down to the target voltage given the value of the collector resistor and the chosen Beta. So if you are using Vbe=0.7v for example then the BC diode gets reverse biased when Vce becomes lower than that, or for active mode just above that as you know. So your target Vce is either 0.7 or 0.8 volts.
Do you understand this concept?

#### Tobias12

Joined May 25, 2019
8
Hi,

Without given the whole solution, the iB base current is based on the collector current required to bring the Vce down to the target voltage given the value of the collector resistor and the chosen Beta. So if you are using Vbe=0.7v for example then the BC diode gets reverse biased when Vce becomes lower than that, or for active mode just above that as you know. So your target Vce is either 0.7 or 0.8 volts.
Do you understand this concept?
What I have to do is choose Vce= 0.7 V or Vce = 0.8 V and find the current Ic ? Sorry but i don't clearly understand this concept.

#### MrAl

Joined Jun 17, 2014
6,817
What I have to do is choose Vce= 0.7 V or Vce = 0.8 V and find the current Ic ? Sorry but i don't clearly understand this concept.
Hi,

Let's look at one case where we make Vbe=0.7v.

Now when it goes into sat, Vce will be less than Vbe even by a tiny amount here. So we have
Vce<Vbe

but because it is a tiny amount we can solve for:
Vce=Vbe

and go from there.

Now what does it take to get Vce the same as Vbe?
It takes a certain base current which develops a collector current which in turn develops a collector voltage which is the voltage Vce in this case.

So knowing any Beta, find the base current required for that in order to pull the collector voltage down to the same level as Vbe. Once you have the base current, you can calculate the base resistor value.

You know that the Beta acts as a current controlled current source right?

#### WBahn

Joined Mar 31, 2012
24,974
Hi,

Let's look at one case where we make Vbe=0.7v.

Now when it goes into sat, Vce will be less than Vbe even by a tiny amount here. So we have
Vce<Vbe

but because it is a tiny amount we can solve for:
Vce=Vbe

and go from there.

Now what does it take to get Vce the same as Vbe?
It takes a certain base current which develops a collector current which in turn develops a collector voltage which is the voltage Vce in this case.

So knowing any Beta, find the base current required for that in order to pull the collector voltage down to the same level as Vbe. Once you have the base current, you can calculate the base resistor value.

You know that the Beta acts as a current controlled current source right?
But what beta are you going to use?

If it is in saturation, then it does NOT act like a current-controlled current source. It only behaves that way in the active region. As you approach saturation, the beta starts falling. So what is it when Vce = Vbe?

#### MrAl

Joined Jun 17, 2014
6,817
But what beta are you going to use?

If it is in saturation, then it does NOT act like a current-controlled current source. It only behaves that way in the active region. As you approach saturation, the beta starts falling. So what is it when Vce = Vbe?
Hi,

It's not like that. It is always a current controlled current source. The trick is that the BC diode steals base current so the effective Beta looks less. The current controlled current source still pumps out B*ib, but ib falls due to the now forward bias of the BC diode.

Now when Vce=Vbe the BC diode is not conducting yet, although with a real diode it would be. But this simplified model has to be handled in a more idealized way. We have to set some thresholds even though they are not the same with a real diode.

So if the Beta is 25 then we will see one result, and if the Beta is 200 we will see another result. This will happen for both modes of operation -

#### WBahn

Joined Mar 31, 2012
24,974
Hi,

It's not like that. It is always a current controlled current source. The trick is that the BC diode steals base current so the effective Beta looks less. The current controlled current source still pumps out B*ib, but ib falls due to the now forward bias of the BC diode.

Now when Vce=Vbe the BC diode is not conducting yet, although with a real diode it would be. But this simplified model has to be handled in a more idealized way. We have to set some thresholds even though they are not the same with a real diode.

So if the Beta is 25 then we will see one result, and if the Beta is 200 we will see another result. This will happen for both modes of operation -
Even if you model it that way, you haven't answered the question. You told the TS what to do if they known the beta. So, what beta do they "know"? The datasheet gives the range of betas when the device is well into the active region and it gives the Vce at a beta that is well into saturation. How does the TS use the datasheet to determine what beta they should use when it is operated at the point where Vce = Vbe?

#### MrAl

Joined Jun 17, 2014
6,817
Even if you model it that way, you haven't answered the question. You told the TS what to do if they known the beta. So, what beta do they "know"? The datasheet gives the range of betas when the device is well into the active region and it gives the Vce at a beta that is well into saturation. How does the TS use the datasheet to determine what beta they should use when it is operated at the point where Vce = Vbe?
Hi,

Not sure what you are asking.

If i said the Beta was 100 what would you calculate?
Then if i said it was 200 what would you calculate?
So you get two results.