The terminology you're using is inexact.

Assuming you're referring to using the transistor as a switch, the base current should be 10 times one tenth of the collector current to guarantee that the transistor will be saturated.

EDIT for brain fade.

May I give the correct terminology?
What is the DEFINITION for saturation ?
Answer: The BJT is in saturation when both, the B-E pn juntion as well as the B-C pn junction are forward biased. This happens when the voltage drop across the collector resistor is so large that the base-collector voltage Vbc>0.
As a consequence, the base curent Ib consists of two componenets - and it is much larger than predicted by the specified B-value (Ib=Ic/B).

Hence, such a large base current does not cause the saturation state - in contrary, it is the RESULT of (and a safe INDICATION for) saturation.
(In this context, one should remember and realize again that the BJT is NOT a current-controlled device.)

 

realize again that the BJT is NOT a current-controlled device

Also realize that for switching applications, it is usually more convenient to use a black-box, current-controlled model for the BJT's, since the voltage-controlled model is of little help for those circuit calculations.

 

Also realize that for switching applications, it is usually more convenient to use a black-box, current-controlled model for the BJT's, since the voltage-controlled model is of little help for those circuit calculations.

I think everyone can decide for themselves which model they want to use - provided they need a model at all for their understanding and/or design of a circuit.
(I also did not speak of a model, but primarily of the definition of the saturation state).

 

I think everyone can decide for themselves which model they want to use

Certainly we all have free will.
But for a newbie, that could be a confusing decision.

 

Certainly we all have free will.
But for a newbie, that could be a confusing decision.

May ask : What do you mean with "that" ?
According to my (teaching) experience, the most confusing thing in this regard is the following:
Some books and other (simplifying) contributions claim the BJT would be current-controlled (without any evidence) - however, at the same time they present and apply design formulas/rules which are derived from the voltage-control function of the BJT.
Apparently they do not recognize the contradiction .
I think, THIS is the most confusing situation.

 

May ask : What do you mean with "that" ?
According to my (teaching) experience, the most confusing thing in this regard is the following:
Some books and other (simplifying) contributions claim the BJT would be current-controlled (without any evidence) - however, at the same time they present and apply design formulas/rules which are derived from the voltage-control function of the BJT.
Apparently they do not recognize the contradiction .
I think, THIS is the most confusing situation.

Now I am curious. Can you show how you teach your students to determine the base resistor for a saturated switch?

 

...
Some books and other (simplifying) contributions claim the BJT would be current-controlled (without any evidence) - however, at the same time they present and apply design formulas/rules which are derived from the voltage-control function of the BJT.
Apparently they do not recognize the contradiction .
...

Which books are you referring to? I'd like to fact check this claim presented without evidence or even information.

 

Now I am curious. Can you show how you teach your students to determine the base resistor for a saturated switch?

No problem at all.
1) Let me start with the classical common-emitter amp.
We all know about the large uncertainty regarding the collector current Ic and the corresponding voltage Vbe. This is the main reason for providing DC negative feedback. In this case, the current Ic is primarily defined by the emitter resistor Re (provided the base divider ensures a - more or less - stiff voltage at the base. Due to the feedback effect, the "uncertainty" of the necessary voltage - 0.6 volts or 0.65 volts above the emitter - play no more an important role).

2) Lets assume we want to drive a BJT-switch with a control signal Vs=6 volts (of course, without a resistor Re in the emitter leg). So we need a base-emitter voltage of app Vbe=0.65...0.7 volts. The "exact" Vbe value for the desired (necessary) collector current Ic is unknown, of course. Hence, we need a series resistor Rs to drop the control voltage Vs voltage by (Vs-Vbe) volts.
And the corresponding resistor Rs is of course: Rs=(Vs-Vbe)/Ib.
I know that saturation is defined by Vbc>0 with a drastic increase of the current Ib into the base node (both pn-junctions are open). Therefore, for this calculations I use (rule of thumb) a base current Ib that is app. 10 times the current which follows from Ib=Ic/B.
With this empirical value, I am on the safe side and the uncertainties (tolerances) of Vbe and Ib (resp. B) practically no longer play a role.
Hence, it is of less importance if I insert Vbe=0.65 or 0.7 volts into the above equation (same situation as in the above case 1).

In this context, the following diagram can explain WHY the mentioned uncertainty (and temperature dependence) of the voltage-control characteristic does not play a big role in both cases: Negative DC feedback (Re) resp. series resistor Rs. (Second diagram: Replace Vo with Vs and RB with Rs)

 

Which books are you referring to? I'd like to fact check this claim presented without evidence or even information.

I don't usually answer a question with a counter question. I am sorry for that.
But in this case, isn't it easier if a forum member would tell me a source of knowledge where a proof for current control can be found ?
(I am from Germany and find most of the English language books only in the library. )

However, in the above context, the following article may be of interest: "Barrie Gilbert, Fellow IEEE: Introduction To The Transistor - A New Semiconductor Amplifier, (Proceedings of the IEEE 1999)"

In this paper, the late Barrie Gilbert explains why the „older beta-view Ic=B*Ib is still taught in many textbooks“ (historical reasons) and why „the BJT is ...being decidedly a transconductance device“.

 

hi,
Is it really fair that a number of User Threads asking a simple question are being hi-jacked in order to argue about Current versus Voltage transistor Base control.?

E

 

hi,
Is it really fair that a number of User Threads asking a simple question are being hi-jacked in order to argue about Current versus Voltage transistor Base control.?

E

Is it really a kind of "hi-jack"?
Remember, in his post#5 the thread opener has asked:
"Thanks! You brought something up that I am unsure of. How can you tell how much current a BJT needs sent to it's base from the control signal to make it conduct?"

 

I don't usually answer a question with a counter question. I am sorry for that.
But in this case, isn't it easier if a forum member would tell me a source of knowledge where a proof for current control can be found ?
(I am from Germany and find most of the English language books only in the library. )

However, in the above context, the following article may be of interest: "Barrie Gilbert, Fellow IEEE: Introduction To The Transistor - A New Semiconductor Amplifier, (Proceedings of the IEEE 1999)"

In this paper, the late Barrie Gilbert explains why the „older beta-view Ic=B*Ib is still taught in many textbooks“ (historical reasons) and why „the BJT is ...being decidedly a transconductance device“.

It is quite impossible for a non IEEE member to access IEEE materials. So I guess my efforts will not amount to much.

 

May ask : What do you mean with "that" ?

Simple.
"That" is how to select between the voltage-control model and the black-box current control model when designing with a BJT (after all, much of the information in a BJT data sheet more or less uses the current-control model. I've never seen much about its transconductance.).
My answer, for a newbie, is that the current-control model is most useable for switching and bias-current calculations, and the voltage control-model seems more suitable for small-signal AC amplifier gain calculations.

 

My answer, for a newbie, is that the current-control model is most useable for switching and bias-current calculations, and the voltage control-model seems more suitable for small-signal AC amplifier gain calculations.

OK - why not? When somebody is happy (satisfied) with a situation where he must switch between two models.
But I did not speak about "models" - it was my primary intention to underline the correct definition of saturation (see post#21 and #23) because this question is directly related to the original question (post#1).
And- not very surprising - in this context, the question came up if this state of saturation is caused by a large base current or if such a large base current is the RESULT of (resp. an indication for) saturation.
And this question can be answered without making use of any model (post#21 and #28) - its just physics.

(Answer to papabravo: Barrie Gilberts IEEE article is available also via ResearchGate:
https://www.researchgate.net/public...The_Transistor-_A_New_Semiconductor_Amplifier)

 

(Answer to papabravo: Barrie Gilberts IEEE article is available also via ResearchGate:
https://www.researchgate.net/public...The_Transistor-_A_New_Semiconductor_Amplifier)

The site says you have to request a copy from the author. That will be difficult.

AND - I always thought that he developed the Gilbert Cell while at Analog Devices. Turns out, he developed it while at Tek.

ak

 

The site says you have to request a copy from the author. That will be difficult.

Download it here:
Introduction to “The Transistor—A New Semiconductor Amplifier”
https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=775421

 

But I did not speak about "models"

No, but you stated that

(In this context, one should remember and realize again that the BJT is NOT a current-controlled device.)

Which infers that that the BJT is a voltage-controlled device, as is technically correct from the physics (I will avoid the word "model" since you somehow seem to think that is not appropriate here), and that just would seem to muddy the water when discussing a switching application and transistor saturation with a newbie.

Your long and somewhat contorted explanation in post #28 does not really show that a BJT is a voltage-controlled device, only that it has a Vbe voltage that varies with base-emitter current, which can be viewed basically as the diode forward voltage due to that current.

 

Your long and somewhat contorted explanation in post #28 does not really show that a BJT is a voltage-controlled device, only that it has a Vbe voltage that varies with base-emitter current, which can be viewed basically as the diode forward voltage due to that current.

In my post#28 I have answered a question from BobTPH. He was interested to see my explanation for a saturate switch when the base current is not the controlling quantity but the result of saturation.
So - it was not my primary purpose to "show that a BJT is a voltage-controlled device".
(This was shown elsewhere several times).
Regarding your last sentence ("Vbe voltage that varies with base-emitter current"):
Actually, it is the other way around: The current varies with the voltage (i.e. as a result of the voltage variation). The voltage opens the pn junction and makes a current possible. No current can flow through a "barrier" and generate thereby a voltage.

 

Which makes no difference in applying the math. Mathematically, the current is a function of the voltage. And the voltage is a function (the inverse if the other function) of the current. The same math applies no matter which is the cause and effect. It just so happens that, in designing circuits, controlling the current is the simpler way to approach it. Using this approach does not imply that one thinks the current physically controls the voltage.

 

Which makes no difference in applying the math. Mathematically, the current is a function of the voltage. And the voltage is a function (the inverse if the other function) of the current. The same math applies no matter which is the cause and effect. It just so happens that, in designing circuits, controlling the current is the simpler way to approach it. Using this approach does not imply that one thinks the current physically controls the voltage.

Yes - mathematically you can do everything. But this is not the question - as far as I understand the subject of dicussion.

And I do not agree that in BJT-based circuits "controlling the current is the simpler way to approach it. "
I think, this view primarily might apply to persons who design circuits based on well-established formulas and rule of thumbs which are readily available (without deeper thinking).
They use, perhaps, the gain formula A=-gm*Rc/(1+gm*Re) without realizing that the transconductance is derived from the voltage-control function and that the resistor Re provides a feedback signal to the emitter which is a voltage.
More than that, they have perhaps never analyzed the meaning of the tempco d(Vbe)/d(T)=-2mV/K for Ic=const.
And - are they curious why the resistors forming the voltage divider at the base should be of relatively small resistors?
And can they explain a simple current-mirror or the principle of class-B-operation (without using voltage-control)?

However, when you try to UNDERSTAND the background or when you even try to invent some new circuit ideas, the current-control view does not
work.
(By the way: I have answered your question in my post'28. I must admit that I have expexted any reply from your side.)