Is it possible for the base current to exceed the collector current for a BJT in saturation mode? (If the base current was increased without the collector current increasing at all)
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Certainly possible. At some point, the Vce(sat) will stop decreasing and will begin to increase as the base current increases (for a given collector current), so it's not something that is necessarily useful. – Spehro Pefhany Feb 19 '15 at 18:45
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@SpehroPefhany How does it happen? – Undertherainbow Feb 19 '15 at 19:24
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The base-collector junction is forward biased in saturation- so current does not just flow to the emitter, but also to the collector. – Spehro Pefhany Feb 19 '15 at 19:40
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It is possible. Consider the figure below.
The collector current at saturation will be $$I_{Csat} = \frac{V_{CC}-V_{CEsat}}{R_C} \approx \frac{V_{CC}}{R_C}$$
The base current is given by, $$I_B = \frac{V_{CC} - V_{BE}}{R_B} \approx \frac{V_{CC}}{R_B}$$
So it is clear that the currents at saturation are entirely decided by the resistors and hence base current can be greater than the collector current at saturation.
nidhin
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Yes, it is obvious that the base current can exceed the collector current if you consider the limiting case where the collector is open-circuit. The collector current will be 0, but you can still drive from 0 to the maximum allowed base current thru the base. With the collector open, base to emitter just looks like a diode.
Olin Lathrop
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