on video What is a Transistor? How Does a Transistor Work? (BJT Transistor Tutorial)


In this lesson, I will try to explain you the transistor's working principle without touching upon the structure and types of the transistor. A transistor is a circuit element that acts as an amplifying or switching in a circuit.  It consists of joining three semiconductors of type-P and type-N.  Here I will describe the Bipolar Junction Transistor.

On the left, you see the shape of the BJT transistor,  and on the right, the symbol. The BJT Transistor has three pins.  One of them is Base,  the other one is Collector and  the last one is emitter. They are denoted by the abbreviations B, C and E. Their locations are not always like this, they can change.  In its symbol, Base, Collector and Emitter pins are like this. So how does the transistor work? With a low trigger current coming from the Base, a high current is controlled from Collector to Emitter or from Emitter to Collector.  We can compare the transistor to a faucet. The valve of the faucet is Base and the direction of water is between Collector and Emitter. In other words, while controlling the current in the transistor, we can control the flow of water in the faucet.   When we open the valve, we control the water.

BJT transistors are two types. The first one is the NPN transistor and the second one is the PNP transistor. There is not much difference between them.  NPN transistor controls a high current from  Collector to Emitter with a low trigger current from Base, while  PNP transistor controls a high current from  Emitter to Collector with a low trigger current from Base. In other words, between the trigger current and the collector-emitter current in the NPN transistor, the emitter-collector current is controlled in the PNP transistor.

Here you see two circuits using the NPN transistor. No voltage is applied to the base of the transistor in the circuit on the left. So there is no current in the Base. Since there is no current in the base, the transistor is not triggered. No current, flows from Collector to Emitter and the LED does not light up. In the circuit on the right, a 5V voltage is supplied to the Base.  In this circuit, a low current flows from Base to Emitter. The transistor is triggered by this current.  Then a high current flows through the Collector towards Emitter,  and the LED lights up.

Hereby, what we mean by the low current in the Base is a current at the level of microampere. (µA). By the high current between Collector and Emitter, it is meant a milliampere (mA) current.

Let's reinforce the principle of working over another circuit. You see two circuits here. A control circuit on the left ,namely, is an IR transmitter circuit. The right one is a receiver circuit. The circuit on the left has a power source, a button, a resistor and an Infrared LED. With this circuit, we will try to light up the lamp by controlling the circuit on the right.  When we press the button here,  a current flows through this circuit and the  Infrared LED emits an infrared light. With this infrared light, the photodiode is triggered and a short circuit occurs.  When triggered, a current flows through this circuit. In other words, in the transistor circuit, a low trigger current flows from Base to Emitter. The transistor is triggered by this current.    Then, a current transition from Collector to Emitter is provided.  With this current, the lamp lights up. Hereby, we have  checked a microampere (µA) current between the Base and Emitter,   and a milliampere (mA) current between the Collector and Emitter. With this current, we have ensured that the lamp lighted up. This is how the BJT Transistor functions as a switch. I hope it was useful for you and you enjoyed it.

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