Transistor Modes

Transistor biasing is the process of setting the operating voltage across the transistor terminals. BJT (Bipolar junction transistor) has two junctions, one is base-emitter junction and another one is base-collector junction. Depending on the forward and backward biasing of this junction, there are three modes of the transistor. The transistor base to emitter junction depends upon its threshold voltage. When base to emitter voltage level drops below this threshold voltage, the transistor goes into its Cutoff State. When base to emitter voltage level is above this threshold voltage then the transistor is either in its Saturation State or Active State. Theoretically, the value of threshold voltage of the diode is 0.7V but practically, it is 0.65V.

Components Required :

 BC547 NPN transistor

 Potentiometer of 1k

 1k resistance

 Some jumper wires

 Breadboard

 Power supply (+5V)

 Multimeter

Fig. 1: Circuit Diagram of Transistor in Cut Off Mode

Cutoff Mode :

In this mode, both, base to emitter and base to collector junctions are in reverse bias. When there is a very low voltage at the base of the transistor then the base to emitter voltage is below its threshold voltage. Due to this, the transistor is in its OFF state and acts like an open switch. As the emitter is connected to the ground, its current is zero but there is a small flow of current at the collector of the transistor due to the thermal vibration of electrons. This small current is known as reverse saturation current (Ico). As the base voltage is zero, so there should be zero current but still, there is a flow of small negative current at base of the transistor and opposite of the reverse saturation current (-Ico). Thus due to negative current at base of the transistor, the base to emitter and base to collector junctions are reversed bias. Hence no current will flow from collector to emitter of the transistor and we get low at the output which turns off the LED.

The Cutoff Mode of the transistor is used in switching operation for Switch OFF application.

Fig. 2: Circuit Diagram of Transistor in Saturation Mode

Saturation State :

In this mode, base to emitter and base to collector junction both are in forward bias. As we increase the value of the voltage at the base of the transistor, this will make the base to emitter voltage greater than the threshold voltage. In this case, the transistor is now in its fully ON state and acts like a shot circuit or close switch. As the base current is very high to derive a large amount of collector current, the voltage drop across the resistance R3 is big enough. The base to emitter junction is in the biased state as the emitter is connected with ground and base is with positive power supply. Hence the collector to emitter path is acting like a short circuit due to which the current in emitter and collector of the transistor is nearly equal. Now current gets a shorted path from collector to emitter and it travels through transistor collector to base and goes to emitter which lights up the LED.

The Saturation Mode of the transistor is used in switching operation for Switch ON application.

Fig. 3: Circuit Diagram of Transistor in Active Mode

Active Mode :

In this mode the base to emitter junction of the transistor is in forward bias while base to collector junction is in reverse bias. In this mode, transistor is in middle of the cutoff and saturation state. The voltage at the base of the transistor is less than the collector, so the base to collector junction is in reverse bias. On the other hand, the emitter is at the ground so its voltage is less than base which makes a base to collector junction in reverse bias. When we apply a voltage in between cutoff and saturation mode at the base of transistor then transistor reaches into its active state. In this state the electrons which are flowing to the base of the transistor is attracted more towards the collector due to the positive supply at the collector (due to reverse bias). Thus there is a large amount of current flow from collector to the base which is greater than the current at emitter to base. Therefore by applying a small voltage at base we get a high value of current at the collector. This is called the amplification of input signal. Due to this property of transistor in its active state, it is used in Amplification.