Bipolar Junction Transistor As a Switch

Switching:

The Bipolar junction transistors can also be used for switching in a circuit. It means that we can use BJT as for current controlling switch. The current in the base will decide whether the current will flow from emitter to collector or not, that’s how it can be used as switch. BJT in circuit used for switching the high voltage or high power circuit with the very low power control variable. We will try to figure it out with the help of example.

Example of BJT as Switch:

As shown in figure below simple lamp circuit with a battery. As switch is open then lamp is off as in fig (1), while when switch is closed, the circuit is closed and lamp is glowing in fig (2).

Fig (1)

Fig (2)

Now when we used bipolar junction transistor in place of switch in above circuit and connect a 1 K resistor with base to provide some current for the base. NPN type of Bipolar junction transistor is being used in this circuit. PNP could also be used, it is just difference of polarity in connections. Keep in mind that the direction of flow of current should be opposite to the direction of arrow on emitter of BJT. The direction of current flow is shown by red arrows in fig (3).

Fig (3)

The BJT do switching in between two state regions; cut off region and saturation region. When there will be no resistor connected with base or if there is open circuit at base of BJT, then the flow of current from emitter to collector will not occur and transistor would be in its “cut off” state. And thus lamp will not glow.

While when there will be resistance connected with the base, some current will flow through base (where we have studied before base current is controlling current). When there will be current through base then current will pass through collector and lamp will glow up and transistor would be in "saturated" state.

So we have seen that we can do switching of high level with the very small amount of current in base. That’s a big advantage of having a BJTs.

Applications:

Due to this switching property of BJT, it has many application in industry and smart production.

1. In a thermocouple circuit, we can use BJT as switch while taking heat as a source for base current. So that with heat on switch on.

2. In automatic door, with the presence of a person near the door, will initiate the base and switch will open.

Like these it has many application in this era.

Bi Polar Junction Transistor (BJT): Introduction, Construction and Working

Introduction:

BJT stands for Bipolar Junction transistor which means two polar junctions. The reason it is called Bipolar is that it uses both electrons and holes for carrying charge or to flow the current. While FET is known as a unipolar transistor.

Construction:

As we have discussed diode before, and how it is formed by the junction of two regions, P doped region, and n doped region. The BJT is formed by the combination of two back to back diodes. The Bipolar junction transistor or BJT has two junctions of the semiconductor. In one of the two types (PNP), a thin n-region is shared by two p-regions. And in another type (NPN), a thin p-region is shared by two n-regions, the figure below explains this.

          Fig (a)

                                                                                 Fig (b)

                                   

This complete structure of BJT has three terminals and has three layers. Each terminal for each layer. And the names of these layers are Emitter, Base, and collector shown in above figure. For NPN the arrow is going away from the Emitter terminal and for PNP the arrow is towards the emitter terminal. The reason is explained later.

 Functions of BJT:

BJT can be used for three purposes,

• Current controlling
• switching
• Amplification

Controlling Current:

BJTs are mostly used for controlling the amount of current flows from emitter to collector or collector to base. These are two cases for two different types of BJT (which are PNP and NPN). In PNP the current goes from Collector to emitter with the combination or addition of base current, see fig (a). While in NPN the current goes from Emitter to collector with separation or removal of base current see fig (b). In all this, the current which goes from emitter to collector or collector to emitter is the controlling current and the current which goes or comes from the base is controlled current. Now all this statement means that, the amount of current which have to go from emitter to collector in NPN transistor is decided by or controlled by the current flows towards the base as shown in the below picture.

           Fig(c)

                                                                                    Fig(d)

         

                                                                                          

Equations of current:

This thing also produce the equation of current,

I_E = I_B + I_C

Where I_E is the Emitter current, I_B is the base current and, I_C is collector current.

I_B = I_E - I_C

I_C = I_E - I_B