# BJT in Saturation

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)

• 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. Feb 19, 2015 at 18:45
• @SpehroPefhany How does it happen? Feb 19, 2015 at 19:24
• The base-collector junction is forward biased in saturation- so current does not just flow to the emitter, but also to the collector. Feb 19, 2015 at 19:40

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.

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.

• Didn`t the questioner ask for saturation mode conditions?
– LvW
Feb 20, 2015 at 9:37