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I just started reading about transistors from book "Microelectronics circuits"by Sedra/Smith,

This is written in book which I don't understand, any help would be much appreciated

For forward active mode -

"Collector current(Ic) is independent of Vcb(reverse voltage between collector and common base )"

But in next paragraph it is also written that

"saturation current is inversely proportional(Is) to width of base (W) "

And "saturation current (Is) is Directly proportional to collector current (Ic)"

So I concluded that -

collector current (Ic) is inversely proportional to width of base(W)

But how it is possible that collector current (Ic) depends on width of base (W) but not on
Vcb?

Isn't by changing Vcb we can change the size of junction (between base and collector) and if size of base-collector junction changes then effective width of base (W) also changes And hence collector current changes?

So why there is contradiction between my conclusion and what is written (as I explained above) in book?

Where did I go wrong?

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  • \$\begingroup\$ In common base, collector current is substantially whatever you feed into the emitter, minus base current of course. (Except, as analog... points out, when the transistor is saturated.) \$\endgroup\$
    – user16324
    Commented Oct 18, 2020 at 21:05
  • \$\begingroup\$ @Brian Drummond ,so collector current depends on emmiter current and base current but isn't base current also depends on width of base (W)?And we can change its value by changing Vbc? \$\endgroup\$
    – user215805
    Commented Oct 18, 2020 at 21:22
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    \$\begingroup\$ Saturation current (Is) as in the Eber-Moll equation is an intrinsic property of the device. It is not affected by the working point. \$\endgroup\$ Commented Oct 19, 2020 at 0:53
  • \$\begingroup\$ @user215805 In the forward-active region, basewidth modulation affects the saturation current, the forward beta, and the forward transit time (which models the excess charge storage, aka the emitter diffusion capacitance.) I specifically recommend Ian Getreu's "Modeling the Bipolar Transistor" as perhaps the better resource for anyone wanting to get a good engineering perspective on the BJT, including methods for testing and measuring parameter values as well as understanding their relevance. The book provides extensive references, as well. So you can go back to primary sources, too. \$\endgroup\$
    – jonk
    Commented Oct 19, 2020 at 5:56
  • \$\begingroup\$ @jonk, I agree, of course, to everything you have written. However, I would suggest to write "collector current" instead of "forward beta". Both is correct, but mentioning "beta" might give some inexperienced readers the impression that "beta" is a very important parameter for the common base configuration. In fact, it is the current Ic which is influenced by base width modulation... \$\endgroup\$
    – LvW
    Commented Oct 19, 2020 at 8:08

3 Answers 3

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But how it is possible that collector current (Ic) depends on width of base (W) but not on Vcb?

Very short (and simple) answer:

The collector current depends (somewhat...to a small extent) on the voltage Vcb because this voltage influences W.

For a first and rough description of the transistor function this dependence is often neglected (introductory chapters of books). However, in a detailed treatment of the transistor effect (further chapters) the influence of Vcb (resp. W) is explained (in most cases, see EARLY effect)

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  • \$\begingroup\$ This is exactly how answer should be! \$\endgroup\$
    – user215805
    Commented Oct 19, 2020 at 15:47
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You are discussing TWO DIFFERENT REGIONS of operation.

Get an I_V plot for the bipolar device.

The Saturation is at far_left, with little Vce to cause "collection" to succeed.

The region where Ic ~ Ie uses substantial Vce, and "collection" of "emitter" will be nearly 100%.

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  • \$\begingroup\$ Thanks for your efforts , But how does this answer my question ? \$\endgroup\$
    – user215805
    Commented Oct 18, 2020 at 21:25
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Ic~Ie for high hFE, and Ic depends on Vbe bias so from a 0Vdc emitter source, e.g. Ic=1.0mA for Vbe=0.60V and increases exponentially with Rce bulk resistance drop approaching 0.7V and up.

E.g. 2N5906/7 ~ 8 Ohms and PN2222 ~4 ohms

Then Req for Thevenin Voltage bias, Req = is loaded by base=hFE*Re + Vbe which reduces Veq (Thev.) perhaps 10% by choice then add Cb to gnd for T=hFE Req’ Cb.

Req’ = new base impedance with bias Req// Rin(=hFe*Re )

E.g. for 50ohm gen on emitter. Rin= hFe(typ)*50

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  • \$\begingroup\$ Finger trouble, needs edits \$\endgroup\$ Commented Oct 19, 2020 at 14:16

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