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Re: [funwithtransistors] Why are BJTs not symmetric?


Feb 9, 2012

 


----------------------------

#580 Feb 9, 2012

My first post to the transistors group, although I bet it's the

samepeople as on the tubes list....

I'm stumped on something that should be very basic. . I connected up

aBJT transistor with the collector and emitter swapped and it sort

ofworked. . I think I simply read the data sheet wrong.

So now I'm re-reading introductory level "how a transistor works"

(Ikept my freshman level EE books and that is all the further I

gotbefore switched majors to computers and AI) and they show a

transistoras three blocks of semiconductor in a stack, either NPN or

PNP.It's clear why current flows or not depending on how it is biased

butwhat's not clear and my question is "how do you decide to call

oneblock the "collector" the the other block the "emitter". .From

thediagrams and equations the transistor is symmetric and one could

flip a coin

Likely real transistors are not symmetric and there is a reason

forpicking one side or the other as the collector. . Next, if there is

adifference then there should be a way to detect which is which in

thelab. I can figure out if a random transitor is PNP or NPN and

find the base lead, how to tell which of the other two is the

collector? and why does it matter?

--



Chris Albertson

Redondo Beach, California



----------------------------

#581 Feb 10, 2012

It's because the emitter is doped more heavily than the collector. I've used BJTs backwards by accident as a simple square-wave oscillator. When they were backward the square was pretty warped. When I switched them to the way they were supposed to be, the square wave shaped up. I think they will work in the reverse direction, just not very well.

Ed

On Thu, Feb 9, 2012 at 2:49 PM, Chris Albertson albertson.chris@...> wrote:

.My first post to the transistors group, although I bet it's the

samepeople as on the tubes list....

I'm stumped on something that should be very basic. . I connected up

aBJT transistor with the collector and emitter swapped and it sort

ofworked. . I think I simply read the data sheet wrong.

So now I'm re-reading introductory level "how a transistor works"

(Ikept my freshman level EE books and that is all the further I

gotbefore switched majors to computers and AI) and they show a

transistoras three blocks of semiconductor in a stack, either NPN or

PNP.It's clear why current flows or not depending on how it is biased

butwhat's not clear and my question is "how do you decide to call

oneblock the "collector" the the other block the "emitter". .From

thediagrams and equations the transistor is symmetric and one could

flip a coin

Likely real transistors are not symmetric and there is a reason

forpicking one side or the other as the collector. . Next, if there is

adifference then there should be a way to detect which is which in

thelab. I can figure out if a random transitor is PNP or NPN and

find the base lead, how to tell which of the other two is the

collector? and why does it matter?

--



Chris Albertson

Redondo Beach, California



----------------------------

#582 Feb 10, 2012

What Ed says is correct and also there is a matter of geometry. The

collector as viewed from the emitter subtends a larger angle than does the

emitter when viewed from the collector. In other words the emitter may be a

dot with the base making a semicircle around it and the collector making a

larger semicircle around the base. I have even dissected power transistors

in which the base and collector were complete circles surrounding the

emitter. The electrons or holes don't travel in nice parallel lines but

tend to diverge as they get farther away from the emitter.



Regards.



Max. K 4 O D S.



Email: max@...



Transistor site www.funwithtransistors.net

Vacuum tube site: www.funwithtubes.net

Music site: www.maxsmusicplace.com



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----- Original Message -----

From: "Chris Albertson" albertson.chris@...>

To: funwithtransistors@yahoogroups.com>

Sent: Thursday, February 09, 2012 4:49 PM

Subject: [funwithtransistors] Why are BJTs not symmetric?





My first post to the transistors group, although I bet it's the

samepeople as on the tubes list....

I'm stumped on something that should be very basic. I connected up

aBJT transistor with the collector and emitter swapped and it sort

ofworked. I think I simply read the data sheet wrong.

So now I'm re-reading introductory level "how a transistor works"

(Ikept my freshman level EE books and that is all the further I

gotbefore switched majors to computers and AI) and they show a

transistoras three blocks of semiconductor in a stack, either NPN or

PNP.It's clear why current flows or not depending on how it is biased

butwhat's not clear and my question is "how do you decide to call

oneblock the "collector" the the other block the "emitter". From

thediagrams and equations the transistor is symmetric and one could

flip a coin

Likely real transistors are not symmetric and there is a reason

forpicking one side or the other as the collector. Next, if there is

adifference then there should be a way to detect which is which in

thelab. I can figure out if a random transitor is PNP or NPN and

find the base lead, how to tell which of the other two is the

collector? and why does it matter?

--



Chris Albertson

Redondo Beach, California





---------------



Always keep electrons and holes flowing in opposite directions.Yahoo! Groups

Links







----------------------------

#583 Feb 10, 2012

--- On Thu, 2/9/12, Max Robinson max@...> wrote:

> From: Max Robinson max@...>

> Subject: Re: [funwithtransistors] Why are BJTs not symmetric?

> To: funwithtransistors@yahoogroups.com

> Date: Thursday, February 9, 2012, 11:28 PM

>

> What Ed says is correct and also there is a matter of geometry..

> The collector as viewed from the emitter subtends a larger angle

> than does the emitter when viewed from the collector.. In other

> words the emitter may be a dot with the base making a semicircle

> around it and the collector making a larger semicircle around the

> base.. I have even dissected power transistors in which the base

> and collector were complete circles surrounding the emitter.. The

> electrons or holes don't travel in nice parallel lines but

> tend to diverge as they get farther away from the emitter.



Be a little careful here, since some of this applies to the

manufacturing technique used to produce the transistor, and there

were quite a few different techniques.



en.wikipedia.org/wiki/Bipolar_junction_transistor#Early_manufacturing_techniques



For a counter example, consider the alloy junction technique, which

was used to produce some of the early Germanium transistors, by

alloying beads of Indium on each side of the Germanium slab.

> Regards.

> Max.. K 4 O D S.



Dave



----------------------------

#584 Feb 10, 2012

I think they will work in the reverse direction, just not very well.





Yep, I had an "100-in-one" electronics kit as a kid, and the transistor was installed incorrectly in it's plastic��block,��as I found out much later.�� The various setups worked, but were noisy and weak.��RegardsMark KB9VKE



-----Original Message-----

From: J Ed jedwardsat1@...>

To: funwithtransistors funwithtransistors@yahoogroups.com>

Sent: Thu, Feb 9, 2012 8:59 pm

Subject: Re: [funwithtransistors] Why are BJTs not symmetric?





It's because the emitter is doped more heavily than the collector. I've used BJTs backwards by accident as a simple square-wave oscillator. When they were backward the square was pretty warped. When I switched them to the way they were supposed to be, the square wave shaped up. I think they will work in the reverse direction, just not very well.



Ed

On Thu, Feb 9, 2012 at 2:49 PM, Chris Albertson albertson.chris@...> wrote:��My first post to the transistors group, although I bet it's the

samepeople as on the tubes list....

I'm stumped on something that should be very basic. �� I connected up

aBJT transistor with the collector and emitter swapped and it sort

ofworked. �� I think I simply read the data sheet wrong.

So now I'm re-reading introductory level "how a transistor works"

(Ikept my freshman level EE books and that is all the further I

gotbefore switched majors to computers and AI) and they show a

transistoras three blocks of semiconductor in a stack, either NPN or

PNP.It's clear why current flows or not depending on how it is biased

butwhat's not clear and my question is "how do you decide to call

oneblock the "collector" the the other block the "emitter". ��From

thediagrams and equations the transistor is symmetric and one could

flip a coin

Likely real transistors are not symmetric and there is a reason

forpicking one side or the other as the collector. �� Next, if there is

adifference then there should be a way to detect which is which in

thelab. I can figure out if a random transitor is PNP or NPN and

find the base lead, how to tell which of the other two is the

collector? and why does it matter?

--



Chris Albertson

Redondo Beach, California



----------------------------

#585 Feb 10, 2012

OK, the doping concestraition is different and the geometry too. . Does the forward voltage drop depend on doping? . What I'm getting at is a pratical way to measure a transistor and know which end is which.

What started this was that i needed a power transistor. .Then later I was tossing out a bunch of junk, in the pile was a 15 year old cell phone charger. .I opened it up and heated the PCB on the kitchen stove then pulled off most of the parts including two large looking power transistors. . I guessed wrong about connects and the performance was poor. . Finally I thought to swap collector and emitter and got much better performance. . Then I got to thinking "why does it matter" after all "PNP", like "kayak" is a palindrome.

Next, to find a way to measure them better

On Fri, Feb 10, 2012 at 4:30 AM, msalbert@...> wrote:

.

I think they will work in the reverse direction, just not very well.



Yep, I had an "100-in-one" electronics kit as a kid, and the transistor was installed incorrectly in it's plastic.block,.as I found out much later.. The various setups worked, but were noisy and weak..RegardsMark KB9VKE



-----Original Message-----

From: J Ed jedwardsat1@...>

To: funwithtransistors funwithtransistors@yahoogroups.com>

Sent: Thu, Feb 9, 2012 8:59 pm

Subject: Re: [funwithtransistors] Why are BJTs not symmetric?







It's because the emitter is doped more heavily than the collector. I've used BJTs backwards by accident as a simple square-wave oscillator. When they were backward the square was pretty warped. When I switched them to the way they were supposed to be, the square wave shaped up. I think they will work in the reverse direction, just not very well.

Ed

On Thu, Feb 9, 2012 at 2:49 PM, Chris Albertson albertson.chris@...> wrote:.My first post to the transistors group, although I bet it's the

samepeople as on the tubes list....

I'm stumped on something that should be very basic. . I connected up

aBJT transistor with the collector and emitter swapped and it sort

ofworked. . I think I simply read the data sheet wrong.

So now I'm re-reading introductory level "how a transistor works"

(Ikept my freshman level EE books and that is all the further I

gotbefore switched majors to computers and AI) and they show a

transistoras three blocks of semiconductor in a stack, either NPN or

PNP.It's clear why current flows or not depending on how it is biased

butwhat's not clear and my question is "how do you decide to call

oneblock the "collector" the the other block the "emitter". .From

thediagrams and equations the transistor is symmetric and one could

flip a coin

Likely real transistors are not symmetric and there is a reason

forpicking one side or the other as the collector. . Next, if there is

adifference then there should be a way to detect which is which in

thelab. I can figure out if a random transitor is PNP or NPN and

find the base lead, how to tell which of the other two is the

collector? and why does it matter?

--



Chris Albertson

Redondo Beach, California







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