If I lower the operating voltage from +-60V to +-45V, then the quiescent current changes from about 50mA to about 30mA. Is this a normal behavior? I thought Vbe multiplier (bias) keeps the quiescent current constant. Am I wrong?
the Vbe multiplier is a shunt regulator. It tries to maintain a constant voltage (Vbias).
What quiescent current are you referring to?
What quiescent current are you referring to?
No, show full scheme.
Output stage quiescent current must not depend of supply voltage.
Vbe multiplier design is always a compromise. The divider resistors will either steal current from the transistor or resist transistor base current.
Yes, "show full scheme.".
Yes, "show full scheme.".
Yes it is normal behavior.
The Vbe multiplier implements a base spreader, it' s purpose is to set a defined voltage between bjt bases. This voltage does set the quiescent current.
This voltage is quite constant versus rails voltage, but it is wrong to consider that it makes the quiescent current constant too.
Ohm law is simple and does apply in all cases.
However it must be applied correctly. It is wrong to apply it once over a complex circuitry, especially where there are non linear components like transistors.
Spice does apply the Ohm law correctly.
With LTspice one can see that a change on rail voltage will change very little on the vbe multiplier voltage but a lot on the quiescent current.
It can be seen too, that an increase of Vbe voltage induces an increase of quiescent current which is NOT proportional.
The Early effect is internal NFB in BJT. If Vce decreases, then Vbe increases. In this way :
(Vbias-Vbe)/Re=Ibias decreases.
(Vbias-Vbe)/Re=Ibias decreases.
Hi mchambin,
That is my observation too, I also use LTSpice and I never question the sudden changes in voltages and currents whenever an adjustment is made to a certain circuit, also my first thought when the OP mentioned "operating voltage."
Ohm's Law is simple, but only applies to ohmic resistances. Ohmic resistances are resistances to which Ohm's Law applies. All other resistances, semiconductors etc. ignore Ohm's Law, because it is not a law of nature (unlike, for example, Kirchoff's laws) but simply a description of what is often approximately true.mchambin said:
Early effect is what causes collector current to vary with collector-emitter voltage; a BJT does not have infinite collector impedance. What happens to Vbe depends on the bias arrangements.
I do not agree with your view of Ohm' s law.
It does apply to non linear circuit. For instance, a diode has a dynamic resistance. It applies piecewise, this is how spice handles it.
It does apply to AC when calculating with instant voltages and currents. That is how spice handles non linear caps for instance.
Remember that Ohms' law implies that if you double the voltage, the current MUST double. And vice versa. This does not apply to semiconductors and other non-linear elements.
You seem to interprete Ohms law as : for any voltage, you can find a current and the quotient is the impedance. That's not a law at all.
Jan
You seem to interprete Ohms law as : for any voltage, you can find a current and the quotient is the impedance. That's not a law at all.
Jan
Ohm's Law is V=IR. It only applies to linear devices.
dV/dx=R(x)dI/dx, where x is some parameter, is not Ohm's Law. It merely defines a function R(x), which is sometimes called slope resistance. It is not the same as resistance, in fact it usually has a different value from that calculated from I and V. That is, for a nonlinear device:
dV/dI = R(x) is not equal to V/I.
Note that for a nonlinear device V/I is not equal to R because R is not defined, although sometimes people may call V/I the 'DC resistance'.
dV/dx=R(x)dI/dx, where x is some parameter, is not Ohm's Law. It merely defines a function R(x), which is sometimes called slope resistance. It is not the same as resistance, in fact it usually has a different value from that calculated from I and V. That is, for a nonlinear device:
dV/dI = R(x) is not equal to V/I.
Note that for a nonlinear device V/I is not equal to R because R is not defined, although sometimes people may call V/I the 'DC resistance'.
dv/di is a resistance, what else could it be, it's unit is Ohm.
Sure, dv/di is a constant equal to V/I for a linear device. This is actually in the definition of linearity.
When dv/di is variable it is a dynamic resistance whereas when it is constant it is like a resistor where R is V/I.
This is the same with speed.
dl/dt is a speed in meter per second as well as L/T.
Differential calculus from Leibnitz.
Before speed was understood, they were enable to explain that Achille could overpass the turtle.
Sure, dv/di is a constant equal to V/I for a linear device. This is actually in the definition of linearity.
When dv/di is variable it is a dynamic resistance whereas when it is constant it is like a resistor where R is V/I.
This is the same with speed.
dl/dt is a speed in meter per second as well as L/T.
Differential calculus from Leibnitz.
Before speed was understood, they were enable to explain that Achille could overpass the turtle.
The units of action and angular momentum are the same, but they are not the same thing.mchambin said:
Ohm's Law is for linear devices only. That is a fact, whether you accept it or not. R(x)=dV/dI is not Ohm's Law.
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