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Assistance with Transistor Theory

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davenn

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I think I brought up some new points, and I am willing to defend them.

Ratch

Hi Ratch
and Merry Christmas to you and your family :)

yes you did and it seemed a less biased (haha pun intended) approach than some :)
 

hevans1944

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Hows about we discuss common-base RF linear power amplifiers? Those are pretty cool. And you can show off your acumen concerning decibel power-gain computations! I could use a little education in this area too.

Merry Christmas, each and every one of you. I am in Virginia Beach with a house-full of grandchildren until Friday, but will keep an eye on the forum anyway.
 
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Ratch

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Hows about we discuss common-base RF linear power amplifiers? Those are pretty cool. And you can show off your acumen concerning decibel power-gain computations! I could use a little education in this area too.

Merry Christmas, each and every one of you. I am in Virginia Beach with a house-full of grand children until Friday, but will keep an eye on the forum anyway.

That would be off topic with respect to this thread, which is about how BJT's work. I would have to study up on your suggstion, because I don't pretend to be knowledgeable with every application using a BJT. But if you start another thread, I will see what I can do. However, I don't think a BJT is the best device for RF amplifiers. Some companies make good off the shelf amplifiers for that kind of thing that are easier and probably cheaper than you can do from scratch.

Ratch
 

chopnhack

Apr 28, 2014
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Hows about we discuss common-base RF linear power amplifiers? Those are pretty cool. And you can show off your acumen concerning decibel power-gain computations! I could use a little education in this area too.

Merry Christmas, each and every one of you. I am in Virginia Beach with a house-full of grandchildren until Friday, but will keep an eye on the forum anyway.
Stay there Hevans, its sounds safer than this thread right now ;):D
Enjoy and Merry Christmas!
 

hevans1944

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... I don't think a BJT is the best device for RF amplifiers.
Gee, someone tell the cell phone manufacturers, who have been using GaAs heterojunction BJT power transistors for quite some time now. I used to do ion implant isolation on gallium arsenide wafers patterned with thousands of 'em as microwave RF power integrated circuits (MMICs}.

Some companies make good off the shelf amplifiers for that kind of thing that are easier and probably cheaper than you can do from scratch.
Did you not notice that electronics experimenters sometimes gather here? We do build things... just for fun, mostly.

As a licensed radio amateur (AC8NS) I would buy off-the-shelf stuff only if and when I could afford it. I bought factory-assembled radio gear for my current ham radio rig, although no-solder kit versions are available for less money . Many hams, myself included, home-brew or modify existing equipment to serve our purposes, even when commercial versions are available. It isn't always about "easier and probably cheaper" although I usually do prefer it that way. It's all about learning what you can do with what you can scrounge, beg, trade, or (as a last resort) buy.
 

Ratch

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hevans1944,

I was not thinking about exotic BJT's like hetrojunctions--just silicon. I hate scrounging and home-brewing, although I used to do it when I had to. No more. Fortunately nowadays, I am in a better financial position where I don't need to do that anymore. There is plenty to learn from textbooks and documentation without building everything from the ground up. And if you are going into high frequency electronics whole hog, that takes some very expensive and exotic equipment. Like maybe a screen room? But we wander off the reservation talking about this. The OP wanted to know how transistors work.

Ratch
 

davenn

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Gee, someone tell the cell phone manufacturers, who have been using GaAs heterojunction BJT power transistors for quite some time now. I used to do ion implant isolation on gallium arsenide wafers patterned with thousands of 'em as microwave RF power integrated circuits (MMICs}.

I have been working with bjt transistors in RF gear since "Adam was a boy" --- Just silicon ----
There's no problems there
Love the MMIC's with the 50E in and out, makes matching a breeze
Tho the one I use on 10GHz are FET based (FMM5061VF). And there is a plethora of FET amplifier devices out there these days

Dave
 

LvW

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To all participants of this thread: Merry Christmas and a healthy New Year.

Again I like to mention my hope that somebody is willing and able to follow Adams recommendation in post#35 (Maybe someone else can have ago using their own method and we can see how it compares.).
I think, it would be really helpful if someone who is following the "current-control model" can show - in contrast to the steps I have listed in post# 33 (voltage control) - how he would design the same BJT amplifier stage. (I remember Adams comment in post#32: ...while the voltage-model is not usable in practice).
Adam - what about YOU?
I am waiting....
 
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LvW

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So how do we know that a BJT is a transconductance device? We know from its physics, not by any one of many models that can be conjured up.
Ratch
Not only from physics! It is not a problem to show by test&measurement that it is the voltage Vbe only that determines the current Ic (see my reply#17)
 

Arouse1973

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I'll do an example after xmas of how I do it.
All the best Adam
 

Arouse1973

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Here is my attempt, it maybe a simplistic method but it has worked for me many times. This is how I do it and it gets me close enough that all I would need to do is replace the one of the bias resistors with a variable resistor once common values had been chosen which allows me to trim anything out. I don't consider this the voltage control method, but I might be wrong.
Adam

CE_AMP.PNG

CE_AMP1.PNG

CE_AMP2.PNG
 

LvW

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Adam - thank you providing this numeric example.
What I do not understand is the following: Why you don`t consider this as the "voltage-control" method?
This the main question: At which design step did you implement any idea of current-control?
In contrary, you follow exactly the list of steps I have mentioned in reply#33:

You fix the voltage across RE and you design a voltage divider which - as good as possible - produces a "stiff" voltage at the base node (therefore, the factor "10").
As a result you have a fixed voltage VBE which determines the current Ic (which is stabilized against temperature using RE feedback).

I repeat again: That is the "surprising and funny" situation I spoke about earlier:
All persons who claim to believe in current-control make - in reality - use of voltage-control because there is no other way because of semiconductor physics.

Finally, here is an answer from one of the leading developers in the world of electronics: Barrie Gilbert (Analogue Devices):

"The old current-in, current-out seems view simple at first, but that's about as far as it goes.
We clearly agree that the BJT should be seen in the same way as an MOS device, explaining that the DC base current of the BJT is actually due to a defect (of sorts) and only a nuisance.
At Analog Devices we have made BJTs (under special conditions) having a DC beta of over 25,000."




 

BobK

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Where is the current control?

Um, the equation for Ib in terms of Ic perhaps?

To me, voltage control would mean that Ic would be related to Vbe in some equation. None of Adams equations does that. The only place Vbe comes into these equations is using an approximate value of 0.65V in order to determine the voltage at the base provided by the R1 R2 voltage divider. He is NOT setting Vbe to control Ic, he is assuming a typical value of Vbe aid in setting Ib and hence controlling Ic. In fact, if you build Adams circuit, Vbe will not be 0.65V it will be something else in that neighborhood.

Bob
 

Laplace

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If one peers deeply into the crystal lattice around the PN junctions, it will be seen that processes like minority carrier injection are influenced by electric fields. But that is not sufficient to make the transistor work as a transconductance device. A transistor is just a three-terminal device, and to make it work as a transconductance device a voltage would be applied at one terminal in order to control the current at another terminal. However, it would be lunacy to apply a voltage to the base of a transistor.

In order to make the transistor work, a current is injected into the base with the expectation of a current gain at the collector. The base current causes a voltage to appear across the base-emitter junction, but the actual value of Vbe is unimportant except that it is equivalent to one diode voltage drop. The simple physical reality is that in order to make the transistor work, a current is injected into the base to control the current at the collector. That is why the transistor works as a current amplifier.
 

Ratch

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To All,
This question will never get straighted out until all of you realize that when you hook up a transistor like Adam did above, you are talking about a CIRCUIT. You cannot argue about whether the circuit is a current amplifier, voltage amplifier, or whatever, and then impute that the internal transistor device within is a current amplifier, voltage amplifier or whatever. In the active region, if you drive the transistor with only a voltage and no other components, you will see a change of current. That makes it transconductance device. If you drive the transistor with a current, then you have a circuit again because a current source contains a resistance.external to the transistor. The only truly current amplifier devices I can think of are a gas discharge tube and a magnetic amplifier.

Modifying the collector current by changing the base current is not how the transistor works. It is what that particular circuit does. The physics of a BJT are that it is a diffusion device and Vbe that controls the diffusion, which in turn controls the collector current. That relationship between Vbe and Ic determines what kind of device (transconductance) a transistor is. If a current amplifier circuit is using a BJT, that transistor's Vbe will change the same amount to control the Ic as it does without being in a circuit.

Ratch
 

LvW

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Where is the current control?
Um, the equation for Ib in terms of Ic perhaps?

To me, voltage control would mean that Ic would be related to Vbe in some equation. None of Adams equations does that.
Bob

Hi Bob - I am afraid you have overlooked something.
For calculation of the gain, Adam has used the quantitty re which is nothing else than the inverse transconductance gm=1/re.
Do you know the meaning of gm? It is the inverse slope of the curve Ic=f(Vbe).

Are you aware what happens without feedback? For RE=0 the gain is Acl=-RC/re=-gm*Rc.
Hence, the transconductance gm directly determines zhe voltage gain.
Does this clarify your doubts?
 

LvW

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In order to make the transistor work, a current is injected into the base with the expectation of a current gain at the collector. The base current causes a voltage to appear across the base-emitter junction, but the actual value of Vbe is unimportant except that it is equivalent to one diode voltage drop. The simple physical reality is that in order to make the transistor work, a current is injected into the base to control the current at the collector. That is why the transistor works as a current amplifier.

Hi Laplace,
sorry - but contains your contribution more than just a claim without any justification?
In my post#17 I have presented two effects which can be measured/evaluated and which - I am convinced - can explain WHY the BJT is voltage-controlled.
Why don`t you comment on this? If I am wrong it shoud be possible to explain these effects using the current-control approach.
 
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