Transistors / hfe / Active region conflicts

[LvW]

The value of Vbe is totally irrelevant to the design process of setting the transistor bias current, which is why Vbe is assumed to be a standard value. If Vbe controlled the base current, it would lead to perdition.

Hi Laplace - with all respect, a simple experiment (hardware or simulation) can show that this assertion is false!

 

[Ratch]

Exactly! No matter what the device does, nothing changes. It's as though the device were connected to nothing.

Not so, if the device were connect to nothing, then no input current would be present.

Damnable Physics!

One can do many things which are forbidden by common sense, but that leads to perdition.

When the diode is driven by a current source the current is set by the current source, and Vbe becomes whatever value it must to accommodate that current because the current source controls the base current.

It is the current source that is supplying the Vbe voltage. It is the Vbe that is controlling the current according to the physics of the depletion region and the bulk resistance of the base-emitter diode.

The value of Vbe is totally irrelevant to the design process of setting the transistor bias current, which is why Vbe is assumed to be a standard value. If Vbe controlled the base current, it would lead to perdition.

We are not talking about design, we are talking about how a BJT works. Vbe does control both the base and collector currents in a BJT. When designing circuts, we try to minimize the exponential response of Vbe by swamping out its effect with lots of resistance. However, the transistor still follows its Vbe.

Ratch

 

[LvW]

We are not talking about design, we are talking about how a BJT works.

Even during the design of BJT stages the (assumed) voltage VBE plays an important role.
Everybody who ever has designed a common emitter stage (without feedback - just to identify the critical steps) with a biasing voltage divider knows about the influence of VBE during calculation of the resistors. (Example: Ic=0.7mA for calculating with VBE=0.6V and Ic=3.4mA for calculating with VBE=0.7V; data based on Q2N2222). Note: In this example, tolerances of beta resp B have not yet taken into account!

Only the knowledge about this huge influence of VBE on the resulting current Ic has created the necessity for providing DC feedback.

 

[Laplace]

...a simple experiment (hardware or simulation) can show that this assertion is false!

If my statement were taken in isolation then your objection could have merit. However, the assertion was in the context of base current being set by a current source (or a biasing network that approximates a current source). In that context, the exact value of Vbe is not relevant although it will have some value corresponding to the exact current being driven through the diode of the base-emitter junction.

 

[Laplace]

Only the knowledge about this huge influence of VBE on the resulting current Ic has created the necessity for providing DC feedback.

Even in this example Vbe is some assumed constant value (that may drift over a small range), not an exact value of Vbe that sets the base current.

 

[Ratch]

Even in this example Vbe is some assumed constant value (that may drift over a small range), not an exact value of Vbe that sets the base current.

I aver that at the same temperature, the base current, Vbe, and collector current of a BJT will be the same in a circuit as it would be if the transistor were isolated from the circuit and run only on voltage sources.

Ratch

 

[Laplace]

Not so, if the device were connect to nothing, then no input current would be present.

But again you misunderstand. I never claimed there was no connection. This is where the magic happens.

We are not talking about design, we are talking about how a BJT works.

That particular stance only goes so far. Transistors do not work just by themselves, they eventually end up being used in a circuit. And one objective of good circuit design is to make the value of Vbe irrelevant.

 

[Laplace]

I aver that at the same temperature, the base current, Vbe, and collector current of a BJT will be the same in a circuit...

No doubt true, but the value of Vbe is irrelevant, and the base current as set by the bias network controls the operation of the transistor in-circuit.

 

[Ratch]

But again you misunderstand. I never claimed there was no connection. This is where the magic happens.

I don't believe in magic. The current is supplied by the current source.

That particular stance only goes so far. Transistors do not work just by themselves, they eventually end up being used in a circuit. And one objective of good circuit design is to make the value of Vbe irrelevant.

Even so, transistors work independently of the circuit. Their contribution along with the rest of the circuit make the final output response. Yes, desired response requires external components. What better way to increase the external resistance of the base than to use a current source.

Ratch

 

[Ratch]

No doubt true, but the value of Vbe is irrelevant, and the base current as set by the bias network controls the operation of the transistor in-circuit.

Vbe does control the base current according to the physics of the transistor, so it is relevant. Just because its particular value can be ignored when it is forced to comply with a desired current does not mean it is not controlling the base current along with the collector current. The base biasing circuit cannot set any current that is not in tune with what Vbe dictates.

Ratch

 

[LvW]

If my statement were taken in isolation then your objection could have merit. However, the assertion was in the context of base current being set by a current source (or a biasing network that approximates a current source). In that context, the exact value of Vbe is not relevant although it will have some value corresponding to the exact current being driven through the diode of the base-emitter junction.

OK - agreed. It was not clear to me that you comment was restricted to that what we call "current injection". Indeed, in this case - and if the supply voltage Vcc is much larger than VBE (at least 12 V or so) - the choice VBE=0.6V or VBE=0.7V is not too important for calculating the resistor between Vcc and base.
Finally, I strongly support you sentence "So there is the full story: base current controls Vbe, and Vbe controls collector current."
Sorry for the misunderstanding
LvW

 

[LvW]

No doubt true, but the value of Vbe is irrelevant, and the base current as set by the bias network controls the operation of the transistor in-circuit.

Instead of "irrelevant" I would prefer "less important". OK?

 

[Laplace]

Instead of "irrelevant" I would prefer "less important".

Perhaps "irrelevant" is too strong a word, but it was chosen to emphasize disregard for the nonsense that Vbe controls the base current when the base is driven by a current source. Nevertheless, biasing networks are seldom designed as perfect current sources so that voltage drift in the generated Vbe will thereby affect the base current. Revise to say that the exact value of Vbe is irrelevant, but the drift range must be accounted for.

 

[LvW]

Laplace - I see a kind of contradiction between your post'#33 (...nonsense that Vbe controls the base current when the base is driven by a current source) and your post#18 (So there is the full story: base current controls Vbe, and Vbe controls collector current.)

Please, can you clarify?
In my reply#31 I have expressed my "strong" support to the last quoted sentence; and the term "nonsense" is - in my view - not appropriate.

 

[Laplace]

Please, can you clarify?

It is just the simple realization that when the base is driven with a current source, Vbe does not control the current. It is the current source that controls the current which flows while Vbe becomes whatever value is necessary to reflect that exact current plus other drift factors. Once the transistor has been biased at a particular current, it can be modeled as a transconductance device for Vbe over a 10mV range at the bias point (or for a wider range if one is willing to accept greater distortion). Even so it is not the value of Vbe that is relevant, rather it is ΔVbe at the bias point that is the quantity of interest.

 

[LvW]

Sorry - but that`s not clear to me:

It is just the simple realization that when the base is driven with a current source, Vbe does not control the current. .

Which current are you speaking of (Ib or Ic)? Vbe does not control....?

Once the transistor has been biased at a particular current, it can be modeled as a transconductance device for Vbe over a 10mV range at the bias point (or for a wider range if one is willing to accept greater distortion). Even so it is not the value of Vbe that is relevant, rather it is ΔVbe at the bias point that is the quantity of interest.

So you think that it is NOT the Vbe value at the bias point that is of interest?
Did you forget that the this Vbe value determines the quiescent dc current Ic and - hence - the slope of the transfer curve Ic=f(Vbe) which is identical to the transconductance gm?
With other words: It is NOT ΔVbe that matters but the ratio ΔIc/ΔVbe that is of interest. And this ratio depends on Ic=f(Vbe).
Agreed?

 

[Laplace]

So you think that it is NOT the Vbe value at the bias point that is of interest?

What is the interest in knowing the exact value of Vbe when that value will drift?

 

[Ratch]

It is just the simple realization that when the base is driven with a current source, Vbe does not control the current. It is the current source that controls the current which flows while Vbe becomes whatever value is necessary to reflect that exact current plus other drift factors.

Before you can make that statement, you are obligated to show where my discourse in post #30 is in error. I have tried to explain to you how the physics of the BJT show the voltage Vbe lowers the barrier or back-voltage of the depletion region, thereby permitting more diffusion charge carriers (holes or electrons) to cross the depletion region. These charges mostly go to the collector circuit, but a small fixed fractional amount are shunted to the base circuit where they are waste current. Consider this. If you could increase the base current without increasing the voltage Vbe, You would not see an increase of collector current. BJT physics dictates that fact. For that reason, Vbe controls the collector current, not Ib, It does not matter if Ib is set by a current generator or not, current does not lower the barrier voltage so diffusion can increase. Base current is an indicator of collector current, not a control of it.

Before you again restate that base current from a current generator controls collector current, I would appreciate where and how my statements and reasons in this post and post #30 are in error.

Ratch

 

[Laplace]

Before you again restate that base current from a current generator controls collector current, I would appreciate where and how my statements and reasons in this post and post #30 are in error.

I never stated that "base current from a current generator controls collector current." Whereas your statement, "Vbe does control the base current according to the physics of the transistor {#30}" is nonsense when the base is driven by a current source because the current source will increase the applied voltage to any level necessary to force the specified current to flow into the base. The applied voltage is equal to Vbe at the base current controlled by the current source. This is why base current controls Vbe, and Vbe controls collector current as I have stated.

I agree that the BJT is a diffusion device. The transistor makes use of diffusion processes and some clever geometry to provide current amplification. And since the amplification is derived from the interaction of diffusion processes, the current amplification from base to collector is much more linear than the raw transconductance effect from base voltage to collector current.

 

[Ratch]

This is why base current controls Vbe, and Vbe controls collector current as I have stated.

You are wrong about that, and here is why. By your reasoning, the base driven by a current source controls the collector of a BJT through Vbe. But, Vbe acts directly on the internals of the BJT, specifically the depletion region to control the diffusion current. The forced base current from a current generator is a external input which does not physically affect the diffusion process directly like Vbe does. The important point is, what controls a BJT should not be defined by external circuitry. The control of a device, by definition, should be defined internally, regardless of how it is hooked up to external components. That means that what controls a BJT should not change if its base is connected to a voltage source or a current source. For that reason, the Vbe internally controls both the Ic and Ib.

I agree that the BJT is a diffusion device. The transistor makes use of diffusion processes and some clever geometry to provide current amplification. And since the amplification is derived from the interaction of diffusion processes, the current amplification from base to collector is much more linear than the raw transconductance effect from base voltage to collector current.

Since both Ib and Ic are both internally controlled by Vbe with the same exponential relationship, Ic and Ib are linear to each other within a certain range of values. That relationship is called beta.

Ratch