How a BJT Transistor works (base current version)

[cabraham]

Gentlemen - I must admit that I am not a specialist in charged carrier physics. Hence, I cannot comment on most of your last statements.
I was educated as an engineer - therefore, I think as an engineer and I have worked as an engineer for many years.
For this reason, I primarily rely on measurements and observations - followed by calculations, evaluations and interpretations..
And I am still waiting for a single example (based on measurements, observations) which could clearly demonstrate if the BJT is current-controlled.

On the other hand, in my various posts I have mentioned many (at least 10) phenomena which can be explained only assuming that the current Ic is controlled/determined by the voltage Vbe (transconductance model). See experiment description below.

Regarding Claude`s statement (applying a voltage Vbe would destroy the device): Did you never record the characteristic of a pn diode or a BJT ?
(This can be done with an xy recorder in seconds, without any thermal problems). I did it - resp. my students did it - in lab experiments rather often..

In this context, here is the description of a very simple but instructive experiment:
1.) Grounded emitter E and grounded collector C : Connect the base terminal B to a varying dc voltage (0.1...0.6 volts) and record the input characteristic Ib=f(Vbe).
Note that only Vbe can be the cause of the currents Ib, Ic and Ie.
2.) Keep the B-E voltage at at a fixed level and measure Ib (within the allowed current range, uncritical with respect to temperature) ;
3.) Now - connect C to a voltage source and slowly increase the C-E voltage from zero up to some volts. During this procedure record the base current Ib and the collector current Ic.
4.) You will notice the following: For rising voltages Vce the current Ib will decrease below the initial value and - at the same time - the current Ic will increase, thus fulfilling the known relation Ie=Ic+Ib.
5.) This measurement/simulation can be repeated for another initial voltage Vbe (see 2))
6.) Evaluation: Which electric quantity determines the final currents Ib and Ic for voltages above Vce,sat (normal BJT operation) ? Claude, what is your answer?

That is a self fulfilling prophesy. If a CVS is attached to the base w/ emitter grounded, then of course the CVS value, which equals external Vbe, will determine or "control the current". But such a connection is thermally stable MAYBE up to 0.60 volts, likely less. In real world applications, a current source driving the b-e junction, or CVS plus series resistor, can be stable well beyond 0.80 volts or even more. But what does driving a bjt with a CVS prove? I can drive the junction w/ a CCS, and plot Vbe as a logarithmic function of Ie. Just as I can force Vbe, I can also force Ie, or Ib. What is proved here? Likewise, if a drive the gate source terminals of a FET with an ac current source, as gate current Ig is varied, drain current Id varies accordingly. If we force gate current, then drain current will be controlled by Ig as well as Vgs will be controlled by Ig. That does not "prove" thaty a FET is current controlled.

Maybe if time permits I can repeat my sims using a FET instead of bjt. In the "free mode", i.e. not directly forced by a CCS nor CVS, we can see that the gate current does indeed precede gate-source voltage, but drain current Id does not respond to the immediate change in Ig, but to the delayed change in Vgs. The same sim plot that demonstrates Ie control over Ic in a bjt, also shows Vgs control over Id in a FET. Only in the free mode can any device, bjt, FET, LED, etc., be understood.

Driving the base-emitter w/ a voltage source only shows the exponential relation of Ie/Ib/Ic with Vbe. It cannot show which is in control. By forcing Vbe to a specific fixed value, the other variables Ie/Ib/Ic are forced to a value determined by the Shockley relation. By forcing Vbe to be the independent variable, Ie/Ib/Ic become dependent on Vbe. Self fulfilling prophecy.

We could force Ie, then Vbe = Vt*ln((Ie/Ies)+1), and Ib = Ie/(beta+1). If Ib was forced, then Ie = Ib*(beta+1), and Vbe is as above. Your forcing Vbe proves nothing.

Just curious LvW, what course did you teach? Is this an engineering college where students are awarded BSEE, MSEE? I never hear this kind of talk in the unis I studied at, nor where I taught.

Claude

 

[cabraham]

I am still waiting for Claude to explain why this anomaly is actually happening. Please excuse me if I have missed it in the 100 or so posts. And I hope he can explain why a similar simulation using only a voltage controlled current source with BJT parasitic and associated components as in his original simulation of the BJT produce the same results. No transistor, emitter current, base current or collector current and no p-n junction.
Adam

What are you asking? Please be specific.

Claude

 

[Arouse1973]

Please explain why this same anomaly can be see by using a behavioural voltage controlled current source with the same components used in your amplifier. Then please explain why the same anomaly is happening as in your original simulation. There is no current controlling factor what so ever.
Adam

 

[Arouse1973]

Hello Claude. Can you please answer these questions. Just Yes or No will be fine.

1) Do you believe that an ideal voltage controlled current source exist in a simulation program.

2) Do you also believe that this ideal model can in no way be effected or controlled to produce a different current than the applied voltage on it's voltage control pin.

3) Do you also believe that the model is dependant on the A / V setting of this model and it will produce this current voltage ratio no matter what is attached to it two output terminal.

Thanks
Adam

 

[LvW]

Hi Claude, some comments to your reply.

"If a CVS is attached to the base w/ emitter grounded, then of course the CVS value, which equals external Vbe, will determine or "control the current"."

Ohh - some new words from your side. Sounds good.

"But such a connection is thermally stable MAYBE up to 0.60 volts, likely less. In real world applications, a current source driving the b-e junction, or CVS plus series resistor, can be stable well beyond 0.80 volts or even more. "

The main subject was not how to design a circuit. From the beginning you are trying to deviate from the main question by telling us well-known facts. Why?

"Likewise, if a drive the gate source terminals of a FET with an ac current source, as gate current Ig is varied, drain current Id varies accordingly. If we force gate current, then drain current will be controlled by Ig as well as Vgs will be controlled by Ig. "

Claude - this statement is far below your niveau, sorry.

"We could force Ie, .."

Yes I remember your recommendation "it is best to inject the current Ie" - and I am still waiting for a corresponding circuit. Up to now... just an assertion from your side..

"Just curious LvW, what course did you teach? Is this an engineering college where students are awarded BSEE, MSEE? I never hear this kind of talk in the unis I studied at, nor where I taught."

May be that - up to now - you didn`t hear "this kind of talk". But - is it therefore false? If you like, you can have it in a written form from the universities
in Berkeley, Stanford, Colorado, Columbia, Georgia and from the MIT (just to mention some of them). If requested, I can give you the corrsponding links.

LvW
____________________________________________________________________________________________________
“I think it's much more interesting to live not knowing than to have answers which might be wrong.” ― Richard P. Feynman

 

[Ratch]

To All,

I got an idea. I don't have an electrolab, but someone here does. Why not take a common transistor like a 2N3904 or something similar. Hook it up to operate in the active region without any resistors and bias the collector with some midrange voltage. Then carefully increase the Vbe voltage with a voltage source until the collector current is 1 ma or thereabouts. Carefully measure the Vbe voltage needed to make this collector current happen. Then hook up a current source to the base, or put a high value resistor in series with the base voltage source, and crank it up until the collector current is 1 ma once again. Measure the Vbe voltage for the second time. If the Vbe voltage is not the same as before, then I am proved wrong.

Ratch

 

[(*steve*)]

Hi Ratch,

Is this supposed to show correlation or causation?

 

[Ratch]

Hi Ratch,

Is this supposed to show correlation or causation?

It should show correlation. It won't show causation because the base currents will be the same in both cases, and it could be said that current is controlling the transistor. This demonstrates that control cannot be determined by experimenting, models, or simulations. One has to get involved with the physics of the device, which in this case shows that Vbe is what is controlling Ic.

Ratch

 

[(*steve*)]

It should show correlation.

That's what I thought.

 

[LvW]

It should show correlation. It won't show causation because the base currents will be the same in both cases, and it could be said that current is controlling the transistor. This demonstrates that control cannot be determined by experimenting, models, or simulations. One has to get involved with the physics of the device, which in this case shows that Vbe is what is controlling Ic.
Ratch

Ratch - in this respect, I am not with you.
For my opinion, we do not necessarily need the device physics. I have mentioned already some tests (supported by corresponding formulas) which could reveal the "secrets" of Ic control.
Here again is a short survey:
* We can measure the form of the transfer characteristic for a diff. amplifier (long-tailed pair), which follows a tanh-curve. This can be theoretically verified only based on Shockleys exponential equation.
* We observe a temperature dependence for the Ic=f(Vbe) relation, which results in the known value of -2mV/K. And this value can be theoretically derived.
That means: It was calculated (based on carrier physics) that Vbe must be reduced by 2mV/K in order to keep Ic constant; and this theoretical prediction was verified in practice.
* Each class-B amplifier shows cross-over distortions on the scope which exactly mirror (represent) the exponential relationship Ic=f(Vbe).
* And again my feedback example: The measurable increase of the input resistance of a common emitter stage due to Re feedback can be explained only assuming voltage feedback.
* Integrated circuits have been realized working in the log-modus. These circuits exploit the "translinear principles" introduced by Barrie Gilbert already in 1975.
This principle of "translinearity" uses exclusively the fact that Ic is controlled by Vbe (equal Vbe gives equal currents). Otherwise it wouldn`t work at all.
* Finally - in my view - the clearest indication for voltage control: The EARLY effect is caused by base width modulation (Ic increase due to Vce increase). Which other quantity also effects the base width? Of course the voltage across the base region (and not the current through it).

Regards
LvW

Final question to Ratch and Steve (just for my understanding what we are talking about):
Are you now talking about a pn junction in general or only about the B-E path of the BJT?

 

[Arouse1973]

To All,

I got an idea. I don't have an electrolab, but someone here does. Why not take a common transistor like a 2N3904 or something similar. Hook it up to operate in the active region without any resistors and bias the collector with some midrange voltage. Then carefully increase the Vbe voltage with a voltage source until the collector current is 1 ma or thereabouts. Carefully measure the Vbe voltage needed to make this collector current happen. Then hook up a current source to the base, or put a high value resistor in series with the base voltage source, and crank it up until the collector current is 1 ma once again. Measure the Vbe voltage for the second time. If the Vbe voltage is not the same as before, then I am proved wrong.

Ratch

BC109 on bench with two power supplies. One set to 6 Volts one variable for base voltage. Base with 5M resistor and collector drawing 1.00 mA Vbe reading of 0.627 Volts power supply reading14.27 Volts.

No base resistor Vbe reading for 1 mA was 0.630 Volts. The small error I put down to my hand on the dials and meter fluctuation I only had a basic setup.

Adam

 

[Ratch]

Ratch - in this respect, I am not with you.
For my opinion, we do not necessarily need the device physics. I have mentioned already some tests (supported by corresponding formulas) which could reveal the "secrets" of Ic control.
Here again is a short survey:
* We can measure the form of the transfer characteristic for a diff. amplifier (long-tailed pair), which follows a tanh-curve. This can be theoretically verified only based on Shockleys exponential equation.
* We observe a temperature dependence for the Ic=f(Vbe) relation, which results in the known value of -2mV/K. And this value can be theoretically derived.
That means: It was calculated (based on carrier physics) that Vbe must be reduced by 2mV/K in order to keep Ic constant; and this theoretical prediction was verified in practice.
* Each class-B amplifier shows cross-over distortions on the scope which exactly mirror (represent) the exponential relationship Ic=f(Vbe).
* And again my feedback example: The measurable increase of the input resistance of a common emitter stage due to Re feedback can be explained only assuming voltage feedback.
* Integrated circuits have been realized working in the log-modus. These circuits exploit the "translinear principles" introduced by Barrie Gilbert already in 1975.
This principle of "translinearity" uses exclusively the fact that Ic is controlled by Vbe (equal Vbe gives equal currents). Otherwise it wouldn`t work at all.
* Finally - in my view - the clearest indication for voltage control: The EARLY effect is caused by base width modulation (Ic increase due to Vce increase). Which other quantity also effects the base width? Of course the voltage across the base region (and not the current through it).

Regards
LvW

Final question to Ratch and Steve (just for my understanding what we are talking about):
Are you now talking about a pn junction in general or only about the B-E path of the BJT?

I might also mention the current mirror, which depends on Vbe vs Ic. However, I am wary of models and experiments which can be misinterpreted. And, all those applications you listed above are circuits, not the device only. As for a BJT, the diffusion diode action brings the charges into the base region, and the electric field of the collector voltage sends them across the B-C junction into the collector circuit.

Ratch

 

[LvW]

No base resistor Vbe reading for 1 mA was 0.630 Volts. The small error I put down to my hand on the dials and meter fluctuation I only had a basic setup.
Adam

Adam - did the transistor survive? (I suppose you know what I am referring to?).

 

[LvW]

I might also mention the current mirror, which depends on Vbe vs Ic. However, I am wary of models and experiments which can be misinterpreted. And, all those applications you listed above are circuits, not the device only. As for a BJT, the diffusion diode action brings the charges into the base region, and the electric field of the collector voltage sends them across the B-C junction into the collector circuit.
Ratch

Ratch - fully agreed. And I am happy that there are some guys who are able to explain things on charged carrier level.
But - in addition - I have collected some test, measurements, observations which can support your findings.
That`s because i think (feel) that that it must be possible to find some specific circuits which only can operate as desired because of Ic=f(Vbe).
And I think I`ve mentioned some of them.
Several times I have asked people from the "current party" to show one single application that can be explained with current-control only
(or, alternatively, cannot be explained with voltage-control). As you know - no response!

 

[cabraham]

Ratch - fully agreed. And I am happy that there are some guys who are able to explain things on charged carrier level.
But - in addition - I have collected some test, measurements, observations which can support your findings.
That`s because i think (feel) that that it must be possible to find some specific circuits which only can operate as desired because of Ic=f(Vbe).
And I think I`ve mentioned some of them.
Several times I have asked people from the "current party" to show one single application that can be explained with current-control only
(or, alternatively, cannot be explained with voltage-control). As you know - no response!

I've responded plenty. You ask for proof, I give it, ask for examples, I give them. Then you assert that nobody can give proof and claim victory. See my previous posts where I explain current mirror operation in terms of Ie, the emitter current. Ie is much more reliable to control current mirrors as opposed to Vbe. It is virtually impossible to expect 2 devices to have matched Vbe parameters. I covered this earlier. There is not one circuit, which cannot be explained in terms of Ic = alpha*Ie. Current mirror, diff amp, etc., all operate in accordance the above equation.

Claude

 

[cabraham]

I might also mention the current mirror, which depends on Vbe vs Ic. However, I am wary of models and experiments which can be misinterpreted. And, all those applications you listed above are circuits, not the device only. As for a BJT, the diffusion diode action brings the charges into the base region, and the electric field of the collector voltage sends them across the B-C junction into the collector circuit.

Ratch

Current mirror depends on Ie vs. Ic. Two mismatched devices w/ emitter degeneration will operate very well. The 2 Vbe parameters can be grossly mismatched but as long as the Ie values are close the Ic values will be very close. I always use Re, emitter degeneration resistors with discreet current mirrors. Relying on Vbe parameters to match and track w/ temp is not reliable.

Claude

 

[Arouse1973]

Adam - did the transistor survive? (I suppose you know what I am referring to?).

Yes Lvw of course it did I was so confident, I didn't even current limit the power supply. Maybe I was just lucky
Adam

 

[Arouse1973]

Hi Claude
Can you answer my questions please. Post #123 and #124
Cheers
Adam

 

[LvW]

"I've responded plenty. You ask for proof, I give it, ask for examples, I give them."

I cannot find in all your post´s any example or observation which can be explained with current control only - or which cannot be explained with Vbe-control. Please, help me to find it.

Claude, continuously you are speaking of Ie as a controlling quantity - but you totally ignore the fact that there must be voltage source driving such a current.
Or do you deny the old rule "No current without charge separation/charge imbalance (voltage)" ?

 

[Arouse1973]

"I've responded plenty. You ask for proof, I give it, ask for examples, I give them."

I cannot find in all your post´s any example or observation which can be explained with current control only - or which cannot be explained with Vbe-control. Please, help me to find it.

Claude, continuously you are speaking of Ie as a controlling quantity - but you totally ignore the fact that there must be voltage source driving such a current.
Or do you deny the old rule "No current without charge separation/charge imbalance (voltage)" ?

Yes
F = q.E
F = d(mv)/dt

Adam