How to know a transistor is carrying out its function ?


by Outrageous
Tags: carrying, function, transistor
Outrageous
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#1
Nov21-12, 03:37 AM
P: 375
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.
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Studiot
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#2
Nov21-12, 06:37 AM
P: 5,462
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 Rc 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
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#3
Nov21-12, 09:25 AM
P: 375
Quote Quote by Studiot View Post
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 Rc 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
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#4
Nov21-12, 09:54 AM
P: 5,462

How to know a transistor is carrying out its function ?


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 Rb = 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.
Attached Thumbnails
bias1.jpg  
Outrageous
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#5
Nov21-12, 11:13 AM
P: 375
Quote Quote by Studiot View Post
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?

Quote Quote by Studiot View Post
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
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#6
Nov21-12, 12:07 PM
P: 5,462
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 Vcb 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
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#7
Nov21-12, 05:22 PM
P: 375
Quote Quote by Studiot View Post
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?


Quote Quote by Studiot View Post
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?


Quote Quote by Studiot View Post
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
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#8
Nov21-12, 08:11 PM
P: 2,452
I think we need to know the configuration before we go farther. Schematic please?
Outrageous
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#9
Nov21-12, 10:08 PM
P: 375
Given Vbe=0.7V
Vb=0.7V
Ve=0
Vc=5.0V
Ib=0.45μA
Ic=100Ib
Attached Thumbnails
titi.png  
NascentOxygen
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#10
Nov22-12, 12:03 AM
HW Helper
P: 4,716
Quote Quote by Outrageous View Post
Given Vbe=0.7V
Vb=0.7V
Ve=0
Vc=5.0V
Ib=0.45μA
Ic=100Ib
You appear to be saying that IC = β. IB

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 VC = 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
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#11
Nov22-12, 01:16 AM
P: 375
So do you mean that the transistor is operating in active mode for the question post #9 ?
NascentOxygen
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#12
Nov22-12, 04:28 AM
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P: 4,716
Quote Quote by Outrageous View Post
So do you mean that the transistor is operating in active mode for the question post #9 ?
I am, for the reason outlined. With VCE=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
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#13
Nov22-12, 05:19 AM
P: 375
5V . because of I want to ask what will happen when it is highly reverse biased .
Thank for replying.
Studiot
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#14
Nov22-12, 05:31 AM
P: 5,462
I do not believe that the circuit arrangement you showed in post#9 will lead to the voltage values etc you listed.
Outrageous
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#15
Nov24-12, 02:45 AM
P: 375
Quote Quote by Studiot View Post
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
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#16
Nov24-12, 03:11 AM
P: 5,462
Can you see the difference between your arrangement and my post#4?
Outrageous
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#17
Nov24-12, 03:15 AM
P: 375
Quote Quote by Studiot View Post
Can you see the difference between your arrangement and my post#4?
Voltage supplied?
Studiot
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#18
Nov24-12, 11:58 AM
P: 5,462
Well what you have drawn is a transistor with zero base emitter voltage, regardless of your list.


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