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Question about transistor

electronic123

Oct 21, 2017
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Hi
i have a question about transistors

how can i understand that how much a transistor amplifie current ?

thank you
 

Minder

Apr 24, 2015
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How much? is the current gain,
Mosfet is a transconductance amplifier, similar to a valve (tube).
M.
 

LvW

Apr 12, 2014
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To understand the procees of amplification (voltage or current) you have to realize that it is possible only to amplify VARIATIONS of the input quantity (voltage or current).
 

LvW

Apr 12, 2014
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??? a fixed base current (in a BJT) results in a fixed but AMPLIFIED current in the c-e circuit.

It is a matter of definition if the term "results" is equivalent to the term "amplify".
To me, there is - of course - a relation between both currents (Ib is a small part of Ic), however I would not use the term "amplification" for these DC quantities.
As another example: The drain current Id (of a FET) is the result of a corresponding voltage Vgs - but it certainy is not an amplified version of Vgs.
 

LvW

Apr 12, 2014
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I spoke about DEFINITIONS and the terms we are using during design of transistor-based circuits and corresponding discussions.
So - a definition NEVER can be "wrong". It can be suitable or not.
I think, for the subject under discussion there is a common agreement to say:

(1) We select the desired collector DC current Ic and calculate the corresponding bias network - taking into account the DC base current Ib=Ic/B.
In this context, there is no necessity (and - physically spoken - not correct) to say that the DC current Ic is an amplified version of Ib.

(2) For calculating the voltage amplification of such a stage, we can (CAN !) say that any signal input vin (voltage variation) causes a corresponding signal base current variation (ib=vin/rin) that is amplified by a factor beta and will result in a signal current ic (variation of Ic).

(3) As a result we get:
ic=ib*beta/rin=ib*gm (gm=transconductance beta/rin=hfe/hie=h21/h11).

But again: We are using parameter definitions!
 
Last edited:

Ratch

Mar 10, 2013
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Ah, my favorite subject. A BJT, FET, and thermionic vacuum tubes are all transconductance devices (voltage controls current). True, you can incorporate them into voltage amplifying, current amplifying, etc circuits, but by themselves, they are all transconductance devices. The base current of BJT does not control the collector in the BJT's active mode. It is only a useful indicator the the collector current. It is also waste current that has to be taken into consideration. The bias voltages applied to a device are not considered to be a DC amplification. Much of the BJT literature avers that a BJT is a current amplifier, but that is not true. A BJT by itself is essentially a current source controlled by a voltage. I await questions and comments.

Ratch
 

LvW

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Yes, Ratch - as you know, I am completely with you.
It is funny and surprising, that even today two different function descriptions can be found in textbooks (current-controlled vs. voltage-controlled).
May be it is interesting for you that - as far as I know - the famous Barrie Gilbert has found an explanation for this surprising fact. For his opinion, the whole story has to do with corresponding patent applications in the late 40th of the 19th century.
 

Minder

Apr 24, 2015
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Being raised in the thermionic valve era, I took an immediate liking to the Mosfet when it was introduced.
I always make sure I have a supply of 2n7000 on hand!
M.
 

kellys_eye

Jun 25, 2010
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I spoke about DEFINITIONS and the terms....
D'oh. As much as I read your posts I read OVER the use of 'definitions'..... trying to keep it too simple. I take your point.
 

LvW

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D'oh. As much as I read your posts I read OVER the use of 'definitions'..... trying to keep it too simple. I take your point.
OK - no problem.
I also was too fast in writing my comment in post#10.
I did mention the 19th century - but, of course, I mean the 20th. century.
 

AnalogKid

Jun 10, 2015
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Because all language is subjective, all definitions rely on other definitions. Calling something a definition is not a license to misuse or misapply those that came before.

ak
 

LvW

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Because all language is subjective, all definitions rely on other definitions. Calling something a definition is not a license to misuse or misapply those that came before.
ak
For my opinion, this subject could lead us to a - more or less - semantic/philosophical discussion.
Just an example: The -3dB point on a lowpass magnitude response can be (and very often is) "defined" as cut-off frequency.
Is this correct or is it wrong ? I think, it is suitable and makes sense for some cases - but not always.
But it is neither "correct" nor "wrong".
 
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