I don`t think that this is the case. We cannot "forget about Vbe" . For calculation of the resistor necessary to allow a certain current Ib you need the voltage difference V(source)-V(base). And V(base) dpends on the desired Vbe.
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You are correct in that a "overvoltage" of 0.7 volts has to be applied to the base in order to reach the emitter resistor. But you don't have to worry about the few millivolts that the Vbe will vary in order to control large amounts of Ic. That takes care of itself.
Ratch
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I am only trying to portray where I think this base current control idea came from. I think I am doing a pretty bad job 
Others haven't come up with any other ideas, it was the best I could think off. For me I don't think about the method I am using but if I was to explain it to someone else then I think I might word things differently now. I would have followed the text book approach even though I know this is not how they work.
Adam
Others haven't come up with any other ideas, it was the best I could think off. For me I don't think about the method I am using but if I was to explain it to someone else then I think I might word things differently now. I would have followed the text book approach even though I know this is not how they work.Adam
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I am only trying to portray where I think this base current control idea came from. I think I am doing a pretty bad job
Adam
In a BJT operating in the active region, the Vbe is used to control the charge carriers diffusing into the thin base area where they are whisked away into the collector circuit by the collector voltage. Unfortunately, not all the charge carriers are captured. A relatively small number of charge carriers make it into the base circuit where they become waste current because they do not contribute to the collector current. This waste current is a relatively constant proportion of the collector current and is roughly the inverse of BETA. The base bias circuit has to be designed so that this waste current does not affect its ability to deliver constant bias voltage.
Ratch
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Thanks Ratch.
Adam
No one yet has written the magical words, 'small-signal analysis' where an operating curve is constrained to a small incremental range. If the increment is relatively small, then curves will approximate a straight line in that range. The BJT exhibits linear transconductance in that small range. In practice, keeping ΔVbe<10mV allows BJT transconductance analysis to be accurate with less than 10% error. However if delta Vbe exceeds this small range, then the BJT no longer exhibits transconductance -- there is no name for it other than possibly 'distortion'. For large-signal analysis there is the BJT large-signal model focused on the transistor beta (current gain).
The BJT in the active region always exhibits transconductance. However its transconductance is not linear. Yes, Sedra and Smith in their book Microelectric Circuits advise a maximum 20 mV P-P swing of Vbe to stay within an approximate linear region. They solve a demonstration problem where the peak input signal voltage turns out to 0.79 volts peak due to internal base and emitter resistances. This voltage is for the specified problem and will be different for another transistor with a different bias and configuration. This should not be a problem because that restriction is on the input voltage. A respectable output gain can still be had with one or two BJT's if the circuit gain is set high enough to use the collector voltage available.
Ratch
Non-linear transconductance is not transconductance at all, it is just distortion.
Gray, Hurst, Lewis and Meyer in their book Analysis and Design of Analog Integrated Circuits advise a maximum delta Vbe of 10mV to stay within an approximate linear region.
Which textbook are you referring to? Different textbooks offer different descriptions.
By the way - here is a slide created by the Barrie Gilbert:
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Page 63 of "The Art Of Electronics"
"Property 4 gives the transistor its usefulness: A small current flowing into the base controls a much larger current flowing into the collector"
"Property 4 gives the transistor its usefulness: A small current flowing into the base controls a much larger current flowing into the collector"
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This is a really interesting discussion. I'm especially interested in what Ratch said here:
Laplace and LvW, do you agree with that interpretation of the relationship between IB and IC?
That interpretation is likely referring to the definition of DC current gain: hFE=Ic/Ib. So Ib = Ic x (inverse BETA). I would not use the words 'waste current' to describe it.
BJT nonlinearity in the active region is both transconductance and distortion. If a hypothetical voltage amplifier is not linear, you would still not hesitate to call it a voltage amplifier, would you? Same with transconductance. So keep the signal input small to approximate linearity and crank up the circuit amplification to compensate.
Ratch
Shame on Winfield Hill for allowing that misstatement to creep into his book. That is certainly not what he is saying now. Ib is an indication of the collector current when a BJT is in the active region, not a control of Ic. When my bedside clock says 5:30 AM, the sun rises. However, I do not assume that my clock is controlling the sunrise. Same with the relationship of Ic and Ib.
Ratch
Well, Ib is not going to the collector where it can do some good. It does not control Ic, the physics of the BJT prove that. It is a nuisance that complicates the bias circuit design. So if it does not do any good and just causes complications, by definition it is a waste.
Ratch
Yes - I agree, of course. In particular the term "waste current". That`s what it really is: A current that exist and cannot be avoided.
(And which is the source of a lot of misinterpretations because of the misnomer "current gain").
However, please note that on page 79 we read: "Clearly, our transistor model is incomplete and needs to be modified ......Our ...transconductance model will be accurate enough for the remainder of this book."
And on the same page (chapter 2.10): "But to understand diff. amplifiers ................you must think of the transistor as a transconductance device-collector current is determined by the base-to-emitter voltage."
OK -but. in fact, it is a wasted current (see Barrie Gilberts term "defect" current)
It could also be said that the base current is a defect that demolishes the hopes and dreams of those who fervently believe in the BJT as a transconductance device, leaving them a mere ten millivolt slice of base-emitter voltage as their linear refuge.
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