In the following circuit, what voltage should I use to bias the transistor reliably into saturation without damage to the transistor?
Now replace V1 with a current source and tell me what current should I use to bias the transistor reliably into saturation without damage to the transistor?
Hi Steve, I am afraid, there is a deep misunderstanding between us. Nevertheless, at first, I will answer your question:
1.) As you know, the voltage V1 should be in the range 0.6...0.7 Volts. However, this biasing scheme makes absolutely no sense because the transfer function Ic=f(Vbe) has very large tolerances. More than that, there is no temperature stabilization and the stage cannot be used as an amplifier because of zero input resistance.
2.) I think, replacing V1 with a current source will not much improve the situation because the relation Ib=Ic/beta also is equipped with large tolerances - nevertheless, I confess that the danger of damage may be smaller in this case. This is not surprising because the series resistor Rb used to convert a voltage into a current acts as a protection resistor (similar to the series resistor in a simple Z-diode stabilization circuit). But remember: Rb is caculated using the difference between the external voltage and the assumed Vbe value. Hence, the calculated Rb value ensures that the selected current Ib causes such a voltage drop across Rb so that the remaining voltage (Vbe) is as initially assumed. This means (in short): At first we have the voltages and the current is the result.
3.) Now - regarding the mentioned misunderstanding: For my opinion, both of your examples cannot say anything about the main question we are discussing in this thread:
Which electrical quantity controls the collector current Ic (Vbe or Ib) ?
You have shown two extremely different methods for biasing the transistor - and everybody knows, that both alternatives cannot be used in practice.
But the misunderstanding - as far as I can see - is the following:
Your circuit No. 2 (current source instead of v1)
looks as if we would inject a current into the base which then would control Ic. And indeed - doubling Ib will double Ic. And therefore you are using the term "current control model". And I agree: This works as a
model!
However, my understanding of "control" is not based on a model but rather on physical phenomena (and this seems to be the only difference between our understandings).
4.) Earlier I have mentioned the thickness of the depletion layer which
undoubtly is altered due to the changing VOLTAGE across the junction.
As another physical parameter I like to mention the temperature dependence of the collector current:
We all know the value of app. (dVbe/dT) = -2mV/K (for Ic=const). This is another clear indication that Ic depends on Vbe.
Question: Is there any equivalent figure (dIb/dT) which tells us how to reduce Ib for a 1K temperature increase to keep Ic constant?