(NPN/PNP Transistor

Thread Starter

sharanbr123

Joined Sep 29, 2014
49
Hello All,

I think I am a little confused with the operation of a transistor. Here is my doubt.

In NPN transistors, is it true that current flows into emitter node from the collector node?

Also, in case of NPN transistor, the overall effect is two diodes with Anodes connected, when both tha diodes are in active state (saturation region), in which direction does the current flow?

Again, when transistor is in acive region, BE diode is forward biased and BC diode is reverse biased.
In this case, does current flow from base to Emitter?
If so, I am assuming that emitter is the source of current but it is actually sinking current?

As you can see, I am a lot confused. Can I get comments please ...
 

alfacliff

Joined Dec 13, 2013
2,458
the "two diodes " are not seperate, the base to emittter current controls the collector to emitter current. just tieing two diodes together wont work. the materials are in molecular contact, usually difused on eachother. the base region is very thin, and allows carriers through when he b to e is foreward biased.
 

wayneh

Joined Sep 9, 2010
17,496
The convention is current flow from base and collector towards emitter, higher positive voltage toward lower. (Electrons go the opposite direction but it is not conventional to equate that with current flow.)
 

Thread Starter

sharanbr123

Joined Sep 29, 2014
49
Thanks, everyone. I did get some clarity ... I adding a follow-up questions below ...

In case of NPN transistor working in Saturation mode, base-collector junction is forward biased. Similarly, base-emitter junction is forward biased.
This means that current flows from base to collector and from base-emitter. Is this correct?

If above is correct, then for the Vce supply, current is flowing both into the -ve terminal and into +ve terminal.
How is this possible?Capture.PNG

Please see the snapshot that I am referring to ...

Thanks in advance ...
 

wayneh

Joined Sep 9, 2010
17,496
This means that current flows from base to collector and from base-emitter. Is this correct?
The collector is normally at a higher voltage than the base and the emitter, so current does not flow uphill. I believe transistors usually have a spec for how far the base voltage can exceed the collector voltage. I suppose current can flow and damage the transistor in that scenario.
 

Thread Starter

sharanbr123

Joined Sep 29, 2014
49
Thank you very much. A couple of questions on the same topic ...

Why the transistor symbol shown as current flow from base to emitter or emitter to base even though current flow from base to collector and vice-versa (when transistor is in saturation mode)?

For base to collector junction to be forward biased (or base to emitter), is it necessary that there is positive voltage at base terminal and negative voltage at collector terminal. Typical trasistor schemes show positive voltage connected to collector terminal
 
Last edited:

ScottWang

Joined Aug 23, 2012
7,397
In the practical circuit, the B needs a Rb resistor to limiting the Ib current, and the C needs a Rc or Load to limiting the Ic current.

Using the voltage to explain how the bjt working is more easily to understand.

When transistor is in liner mode:

C voltage level is the highest pole, it will be Vc>0.2V and some more, lets say Vc=1.5V
B voltage level is the middle pole, lets say Vb=0.7V
E voltage level is the lowest pole, and it is connecting to ground, lets say Ve=0V.

So
Vbe = 0.7V,
Vce = Vc-Ve = 1.5V,
Vbc = Vb-Vc = 0.7-1.5V = -0.8V, the Vb(0.7V) is less than Vb(0.7V) -0.8V, so the B and C is reverse, there is no current flows through B to C.

When transistor is in saturation mode:

C voltage level is the middle pole, it will be about Vc=0.2V.
B voltage level is the highest pole, lets say Vb=0.7V
E voltage level is the lowest pole, and it is connecting to ground, lets say Ve=0V.

So
Vbe = 0.7V,
Vce = 0.2V,
Vbc = Vb-Vc = 0.7V-0.2V = 0.5V, the Vb(0.7V) is great than Vb(0.7V) 0.5V, so the B and C is forward, but the diode of Vbc needs 0.7V to active and the current start flows through B to C, but 0.5V can't active the diode of Vbc, so still there is no current flows through B to C.

I hope there is nothing wrong.
 

Thread Starter

sharanbr123

Joined Sep 29, 2014
49
I would like to get some more clarification from members here ...

I am using NPN transistor operating in saturation mode as an example ...

1) Base-Emitter junction is forward biased
2) Base-Collector junction is forward biased
In this case, I am making the following observations,

a) Current flows from Base to Emitter
b) Current cannot flow from Base to Collector since Collector is connected to positive source of voltage. This terminal can source current but cannot sink it
c) Current cannot flow from collector to emitter since Base-Collector junction is forward biased and current cannot flow from N (collector) to P (Base).
In other words, current cannot flow from Cathode to Anode.

In many articles describing transistor operation, collector to emitter current is shown but somehow I am unable to appreciate how this is possible either in active region or in saturation region.
Hoping for comments ...
 

alfacliff

Joined Dec 13, 2013
2,458
since the base to collector voltage or current does nothing, why are you worrying about it? the current through the transistor is controlled by the base to emitter voltage or current.
 

Papabravo

Joined Feb 24, 2006
21,158
1. One of your assumptions is WRONG. In an NPN transistor the base collector junction is normally reverse biased.
2. Ie + Ib + Ic = 0
 

Thread Starter

sharanbr123

Joined Sep 29, 2014
49
@alfacliff
no, actually, the fundamental doubt I am having is how current is flowing across a reverse biased junction.
That is from, emitter to base (forward biased) and from base to collector (reverse biased) ...

Also, for a given voltage Vce, how Ic increases with increasing Ib?
 
Top