Transistor as a Switch

The switch has two distinct states - on and off. In electronics, you need to apply an input to some device in the form of two distinct voltage levels. When switch is on, one voltage level is applied but when switch is off, the other one is applied. Such voltage levels are used in computers, where digital signals are employed. This is done by using a transistor in the non-linear region of its operation.

In the transistor characteristics, there are two extreme regions: cut-off region and saturation region.

The (jagged) region below the zero base (I_B = 0) signifies the cut off regions. The transistor does not conduct and entire supply voltage V_CC appears across the transistor between the collector and the emitter (V_CE). The output voltage at the collector is V_CC.

When the base current I_B is greater than its saturation value, the transistor conducts fully and collector-emitter voltage V_CE is almost zero. In such a case, the output voltage obtained between collector and ground is zero and entire voltage drop appears across R_L. The collector current I_C =V_CC/R_L.

The control signal for switching the transistor on or off is given in the form of V_BB. For the input loop,

I_BR_B + V_BE – V_BB = 0

When V_BB = 0,

I_B = – V_BE/R_B

Since I_B is less than zero, the transistor is cut off, and V₀ = V_CC.

If V_BB = 5V, and V_BE = 0.7 V,

I_B (100 kΩ) + 0.7 V – 5 V = 0

I_B = 43 µΑ

For normal transistors, this value of base current is enough to drive the transistor to full saturation. 

This kind of switch can also be used as an indicator in displays.