How to know a transistor is carrying out its function ?

[Outrageous]

For a npn transistor, Vb= +0.7V ,Vc=+1.0V , Ve=0
Then my teacher says it is not operating, why is that so?


Thank you.

 

[Studiot]

I see no reason to believe the transistor as specified is not operating.

Let us suppose a 5 volt supply and a 3k9 collector resistor, then with the figures stated the current through the collector resistor R_c is (5-1)/3.9 milliamps which is a litle over 1 milliamp.
This is also the collector current and perfectly acceptable.

I wonder if there is not a typo with the figures, for example is there a missing negative sign or has someone mixed up Vc and Ve?

 

[Outrageous]

I see no reason to believe the transistor as specified is not operating.

Let us suppose a 5 volt supply and a 3k9 collector resistor, then with the figures stated the current through the collector resistor R_c is (5-1)/3.9 milliamps which is a litle over 1 milliamp.
This is also the collector current and perfectly acceptable.

I wonder if there is not a typo with the figures, for example is there a missing negative sign or has someone mixed up Vc and Ve?

Why (5-1) instead of 5 ?
No typo, my teacher said Vb is +0.7V
Ve=0
Vc=+1.0V
My teacher said although this is in active mode, the transistor is now off( not carrying out its function as in active mode) as Vb-Vc is less than 0.5V
I can't understand why is that so?

 

[Studiot]

Attached is the simplest arrangement to setup your conditions.

Say the collector resistor is 3k9 (as before) and the beta of the transistor is 100 then the base will require 1/100 milliamps current (10 microamps) as in the diagram there is 4.3 volts across the base bias resistor so R_b = 4.3 / (1/100) k ohms = 430 k say 390k as nearest preferred value.

Your teacher is somehow mistaken. There is nothing whatsoever wrong with the collector being only 0.3 volts above the base. In fact in some circuits in saturation the collector could be taken below the base (ie negative with respect to the base). This condition is called saturation and is what is required in a switching transistor.

The collector - emitter voltage of the type of transistor I talk about here will be about 0.2 volts ie the collector will be at +0.2volts. The base will still be at +0.7 volts.

 

[Outrageous]

There is nothing whatsoever wrong with the collector being only 0.3 volts above the base.

What if it is collector is 4V above the base? Or I should say the base-collector is highly reverse biased? Any thing will happen?

In fact in some circuits in saturation the collector could be taken below the base (ie negative with respect to the base). This condition is called saturation and is what is required in a switching transistor.

The collector - emitter voltage of the type of transistor I talk about here will be about 0.2 volts ie the collector will be at +0.2volts. The base will still be at +0.7 volts.

When collector is negative with respect to base mean that we forward base-collector?

So do you mean that 'Vb-Vc is less than 0.5V' does not mean anything in an active mode of transistor?

 

[Studiot]

What if it is collector is 4V above the base? Or I should say the base-collector is highly reverse biased? Any thing will happen?

What if it is?

This is bound to happen in proper normal operation as an amplifier.

One of the manufacturers published parameters is Maximum Collector-base voltage V_cb which is usually several tens of volts.

When collector is negative with respect to base mean that we forward base-collector?

Yes, but a transistor works differently in saturation from active mode.

So do you mean that 'Vb-Vc is less than 0.5V' does not mean anything in an active mode of transistor?

No it means that 'Vb-Vc is less than -0.5V' since the collector is at a higher voltage than the base.

You have posted this in the electrical engineering section.

Physicists are interested in how a transistor works from the point of view of junctions, cariers, holes elctrons etc.

Electrical engineers are not really interested in this but want a model that allows them to predict quantities of interest to create useful circuit configurations.

Which are you interested in?

 

[Outrageous]

What if it is?

I mean Vb=+0.7V, Vc=+100V, and Ve=0
In npn, Now the base-collector is highly reversed biased. So now the transistor is still carrying out active mode?

No it means that 'Vb-Vc is less than -0.5V' since the collector is at a higher voltage than the base.

Vb-Vc is less than +0.5V mean the collector is at a lower voltage than the base, correct?

Physicists are interested in how a transistor works from the point of view of junctions, cariers, holes elctrons etc.

Electrical engineers are not really interested in this but want a model that allows them to predict quantities of interest to create useful circuit configurations.

Which are you interested in?

Physicist. Then where should I pose this thread?

Thank you.

 

[Averagesupernova]

I think we need to know the configuration before we go farther. Schematic please?

 

[Outrageous]

Given Vbe=0.7V
Vb=0.7V
Ve=0
Vc=5.0V
Ib=0.45μA
Ic=100Ib

 

[NascentOxygen]

Given Vbe=0.7V
Vb=0.7V
Ve=0
Vc=5.0V
Ib=0.45μA
Ic=100Ib

You appear to be saying that I_C = β. I_B

If that's what you are saying, then the transistor is certainly in the active region. That's how the active region is defined: collector current is controlled by and directly proportional to base current. Yes, even if V_C = 0.5V.

(N.B. Even when a transistor is operating in saturation, we still say it is "working" and "carrying out its function", etc. That's the function of a transistor switch.)

 

[Outrageous]

So do you mean that the transistor is operating in active mode for the question post #9 ?

 

[NascentOxygen]

So do you mean that the transistor is operating in active mode for the question post #9 ?

I am, for the reason outlined. With V_CE=5 V, it is almost a certainty that the transistor is not in the saturated region of operation.

(Did you intend to type 5 V there, or did you mean 0.5V?)

 

[Outrageous]

5V . because of I want to ask what will happen when it is highly reverse biased .
Thank for replying.

 

[Studiot]

I do not believe that the circuit arrangement you showed in post#9 will lead to the voltage values etc you listed.

 

[Outrageous]

I do not believe that the circuit arrangement you showed in post#9 will lead to the voltage values etc you listed.

Where do you think is wrong?

 

[Studiot]

Can you see the difference between your arrangement and my post#4?

 

[Outrageous]

Can you see the difference between your arrangement and my post#4?

Voltage supplied?

 

[Studiot]

Well what you have drawn is a transistor with zero base emitter voltage, regardless of your list.

 

[Outrageous]

Given Vbe=0.7V
Vb=0.7V
Ve=0
Vc=5.0V
Ib=0.45μA
Ic=100Ib

There is voltage supplied
Then change R1 to Vb
R2 to Vc
Will it look okay?

 

[Studiot]

Will it look okay?

No.

Look again at my circuit configuration and compare it with yours.

Where is the base resistor connected in each?

 

[Outrageous]

My base resistor is connected to the emitter, yours is connected to the committer.

Mine is common emitter, I don't know yours

This will affect voltage supplied to base and committer ?

 

[Studiot]

Yes indeed your base resistor as connected will have no voltage across it.

The bias resistors do not determine whether the configuration is common emitter or common collector.
That is determined by the terminal that is common between input and output.

 

[Jony130]

From your post we can tell one thing. You simply don't understand how NPN transistor work from engineers point of view.
How can any current to flow if the base is connect (short) with emitter?

 

[Outrageous]

Yes indeed your base resistor as connected will have no voltage across it.

The bias resistors do not determine whether the configuration is common emitter or common collector.
That is determined by the terminal that is common between input and output.

From your post we can tell one thing. You simply don't understand how NPN transistor work from engineers point of view.
How can any current to flow if the base is connect (short) with emitter?

So from my diagram, the current will not flow through base-emitter even though I connect Vb=+0.7V , instead , the current will straight flow to the earth, correct?

 

[Studiot]

So from my diagram, the current will not flow through base-emitter even though I connect Vb=+0.7V , instead , the current will straight flow to the earth, correct?

I seriously recommend that your learning process would be much enhanced and more efficient if you paid heed to what others who have gone before are telling you as well as simply asking question after question, many of which suggest you did not take notice of a previous answer.

The circuit you have just posted will simply burn out the transistor.

I am happy to discuss the perfectly satisfactory circuit I posted in post#4 , which has much to tell us, but this is already post #25 of this thread which seems to be going nowhere.

 

[Outrageous]

I asked because I still don't understand . So I must follow your diagram only can get true diagram? I don't know why resistor must connected between base and collector, why not base and emitter.

 

[jim hardy]

Two questions re sketch in post #9:

1. That line that connects R1 to R2 crosses the emitter line - is it connected there, or is it connected to a source of +5 voltage ? If the former, everything is tied to 0volts and those voltages are impossible.

2 What does teacher have in mind when (s)he says "operating"?
If that R1-R2 line is connected to positive 5V and the voltages are indeed what you posted,
the transistor is in, or very near, saturation, so it won't have a lot of AC voltage gain.
To an audio person that would appear not operating, but to a logic circuit person it's doing just fine.

I hope I'm on topic here - i tend to confuse easily so if off just ignore.

old jim

 

[sophiecentaur]

I asked because I still don't understand . So I must follow your diagram only can get true diagram? I don't know why resistor must connected between base and collector, why not base and emitter.

Just think What would your resistor achieve - it would merely be in parallel with the junction? It would not supply any base current and would merely lower the input resistance of the amplifier.
I think you should take to heart Studiot's earlier comment. Question and answer, just on your terms, will not achieve very much for you and it will just irritate people who know they are giving you good information. Just consider that you may be wrong about some things and Read Around to find things out. You have such a luxury these days, in the form of the WWW. Why not use it constructively and make a proper effort.

 

[Outrageous]

I am sorry for being so lazy to find out on my own.

I think this is the answer for this thread , I will try to read more first only ask.

2 What does teacher have in mind when (s)he says "operating"?
If that R1-R2 line is connected to positive 5V and the voltages are indeed what you posted,
the transistor is in, or very near, saturation, so it won't have a lot of AC voltage gain.
To an audio person that would appear not operating, but to a logic circuit person it's doing just fine.

Thanks :)

 

[Studiot]

Please take note that the 'off' condition is a valid operating condition for a transistor.

That is if Vce = 0.1 volts or 0 volts then a valid condition would be that a perfect transistor is not passing current.

Sometimes we want to switch th transistor off.
There is currently (pun intended) another thread here where a poster is trying to achieve exactly that with a light switch.