# Some questions about BJT in saturation region

In below circuit I sweep Vb from zero to 5V Vcc. And I obtain the following plots for the currents:

I have a bit problem with understanding two things in saturation region:

1-) Why/how does the Ic decreases with Vb after the saturation point?

2-) How can I quantify/formulate Ib at saturation region?

• Once Vce aproaches 0V Vc rises with Ve and Vb thus V(Rc) drops with rising Vb and Ic=V(Rc)/Rc so you expect Ic to drop with such a large Re. It is neither a switch or a linear amp during this entire sweep, but both. Sep 24 '17 at 17:09

The collector current in your circuit decreases because, when the BJT is in saturation, by definition, the junction B-C is forward biased.

Therefore you'll have that the collector voltage will be just $V_C=V_B-V_{BC}$.

As a result, the current in the resistor Rc will be:

$$I_C=\frac {V_{CC}-V_C}{R_C} = \frac{V_{CC}-(V_B-V_{BC})}{R_C}$$

Therefore the higher $V_B$, the smaller $I_C$.

To answer the second question, simply use KCL at the BJT. You know that $I_C+I_B-I_E=0$ (assuming base and collector currents entering the BJT. Emitter current exits the BJT). Hence $I_B=I_E-I_C$. In fact in your plot, $I_B$ is the difference of the other two currents.

As a first order approximation, you can assume constant $V_{BC}$ and $V_{BE}$. This allows you to calculate $I_C$ and $I_E$ easily.