Anyway, for those who would like to see an emitter resistor in the circuit i've included that in this next circuit. The values are:
..........................
If you would like to throw a few calculations for this circuit in, we can take a look.
Hello to all.
Just for information:
Some comments from H&H (The art of Electronics) regarding this circuit:
"Bad biasing"
"Disaster"
"Don`t do this".
"By using voltage biasing with a stiff voltage divider,.., the quiescent point is insensitive to variations in transistor beta."
W.
Remark: Voltage feedback and current biasing (at the same time) cannot work properly.
hi W,
With respect you are missing the point that 'Al' is making and you are now moving the 'goal posts'
Eric
Now for the circuit with Re resistor and 2N2222 and the beta spread between 50 and 300.Anyway, for those who would like to see an emitter resistor in the circuit i've included that in this next circuit. The values are:
Rc=1k as before,
Re=100 ohms
Rb=497k
Vbe=0.65 as before.
If you would like to throw a few calculations for this circuit in, we can take a look.
I'm a little surprised because I don't see any improvement compared to previous circuit without Re resistor.
Interesting question arise. For example if we use a Shockley equation (Ic=Is*(e^(Vbe/VT)-1)) to find ( determined) the collector current for a constant Vbe = 0.65V the collector current should also be constant? Or maybe I missing something in Shockley equation?
But i dont understand yet what you mean by 'control' vs 'determine'.
As i said, we call the technique "voltage control" but we want to use
it to determine the bias operating conditions, if possible, and it should
be possible somehow, some way.
For my own comparative analysis, i repeat the two formulas we are looking at
here:
1. Ic=Beta*Ib
2. Ic=Is*(e^(Vbe/VT)-1) [perhaps modified later for non ideal device]
Not explicitly. But you stated thatHi crutschow,
it seems I have expressed myself not clear enough.
Of course, you are right that B (resp beta) has an effect on the bias point. Did I state there would be no influence?
which, to me, means that your are assuming Beta has no significant influence. But if you assume Beta is infinite and there is no base current, then you could design a base bias network using very high value resistors, and that obviously could give an incorrect bias point with a real transistor that does draw base current. My point is that, implicitly or explicitly, you need to account for base current when you design a BJT bias network.* B=infinite (and that is the background for ignoring Ib)
OK so for RB = 518KΩ and Vbe = 0.65V and I assume Is = 10x10^-15 and VT = 26mV and β = 50.
Ib = (Vcc - Vbe)/RB (1)
Vbe = VT*ln(Ib/Ise) where
Ise = Is/β = 200x10^-18
So in first iteration I get this
Ib = (10V - 0.65V)/518kΩ = 18.05μA
And the new Vbe value
Vbe = VT*ln(Ib/Ise) = 26mv *ln *(18.05μA/200x10^-18 ) ≈ 0.65587V
Second iteration
Ib = (10V - 0.65587V)/518kΩ = 18.0388μA
Vbe = 0.655856V
third iteration
Ib = 18.0388μA
Vbe = 0.655856V
So I end iteration here and Ic current for β = 50 is equal to
Ic = 0.901944mA and Vbe = 0.655856V
And if we do the same for β = 300 I end up with this solution
Vbe = 0.702312V and Ic = 5.38476mA
So in this case I get almost the same result as before.
But the first method is much simpler to use. All we need is assume Vbe value and use a "current control" view to determined Ib and Ic.
I'm aware that "current control" wins here because Vbe<<Vcc.
Why doesn't the discussed transistor have a voltage divider providing the base bias voltage? If it did then the emitter resistor provides bias stability.
..................
your are assuming Beta has no significant influence. But if you assume Beta is infinite and there is no base current, then you could design a base bias network using very high value resistors, and that obviously could give an incorrect bias point with a real transistor that does draw base current. My point is that, implicitly or explicitly, you need to account for base current when you design a BJT bias network.
Hi MrAl,
you will remember that I`ve tried to explain the difference (more than once) in a forgoing thread, however, I`ll do it again:
1.) According to Shockley`s equation, the collector current is determined by three quantities (Vt, Vbe, Is), however, there is only one external parameter that can control (modify, vary)
the current Ic - and that is undoubtly the base-emitter voltage Vbe.
2.) A particular value for Ic can be calculated only if all parameters that determine this value are known. In particular, this applies to the saturation current Is. However, this is a theoretical statement without practical background because of the large parameter uncertainties (tolerances) for each transistor type.
3.) By the way - this parameter uncertainty is the only reason for providing DC feedback with the aim to meet the desired Ic value (approximately) and to stabilize Ic against tolerances and environmental influences.
4.) Here is a very illustrative example: You can control the speed of a car with a corresponding pedal position - but you cannot derive from this information (position) the momentary speed of the car.
The speed is determined - in addition - by some other environmental parameters.
I appreciate your desire to use Shockley`s formula to "determine the bias operating conditions".
However, as said above, I am afraid this will be not possible in practice.
W.
I use iterative method.Jony,
what is the method you used here?
Jony, i don't see the second method that you mentioned. Can you show the other method to figure out Vbe?So in this case I get almost the same result as before.
But the first method is much simpler to use. All we need is assume Vbe value and use a "current control" view to determined Ib and Ic.
I use iterative method.
https://en.wikipedia.org/wiki/Diode_modelling#Iterative_solution
That's the first time i ever saw anyone other than myself use the Lambert W function to solve a diode circuit.
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