Transistor is a three terminal semiconductor device which is used in switching and to amplify the base signal but now they are mostly used in embedding integrated circuits. Most common category of Transistors is BJT – Bipolar Junction Transistor.
As these are semiconductor devices, they are of two types one is NPN and other is PNP. In NPN majority charge carriers are electrons while in PNP majority carriers are holes.
APPLICATION OF TRANSISTOR
- It is also in switching applications
- It is mainly used for signal amplification
BJT BC547 TECHNICAL SPECIFICATION
- Gain value of BJT BC547 is between 110 to 800.
- The maximum current able to flow through the Collector pin is 100 mA, We can not therefore connect loads which consume more than 100 mA using this transistor.
- To bias a transistor we must supply a base pin with current, The current (IB) is limited to 5 mA.
- Conclusion with statement 2 and 3 is, 0.5 mA base current IB drive 10 mA Collector current IC and 5 mA IB can drive 100 mA IC.
- If this transistor is fully biased then a maximum of 100 mA can flow through the collector and emitter. This stage is named Region of Saturation.
- When the base current is removed the transistor will turn off completely, This stage is known as the Cut off Region.
TRANSISTOR AS A SWITCH
Now we will see how the transistor works as a switch, here we are using an NPN transistor. When a positive signal (+) present on its base it will make a path for collector current to go through from the base towards the emitter. This is how it works. It just bypasses the signal present of the collector while having active base condition.
As soon as we press the button a signal (in form of voltage and current) will be received by the base of BJT which in turn allows electric current to pass from collector to emitter.
Current can flow through the circuit in this way (electrons flow in opposite direction of current):
Power (+5V) → resistor 280 W → LED → Collector-base-Emitter → ground
This simply means if we applied signal (voltage/current) across collector and emitter but not on the base, the transistor will not work. But a small base signal is enough for it to operate.
Pressing the button over here will light up the LED because now the circuit becomes complete from power source to ground.
TRANSISTOR AS AN AMPLIFIER
Generally people think amplifiers can amplify any signal up-to any level but that is not so true. Things to remember
- Transistors cannot amplify source voltage Vcc.
- Transistor can only amplify the voltage signal present at its base, at a maximum magnitude of applied Vcc voltage.
- It is also same as bypassing a signal with same waveform but with higher magnitude (voltage)
HISTORY OF TRANSISTORS & VACUUM TUBES
Before transistors we need to know about diodes, even about vacuum diodes, before the invention of solid state devices like diodes and transistors, the simplest vacuum tube is a diode which is bigger in size than today’s solid state devices.
Diode is the same as we discussed above, have anode i.e. positively charged and cathode terminals which are negatively charged. Works in one direction in forward bias condition.
Then another vacuum tube was invented which is known as “Triode” which is a three terminal device same as a transistor. Then transistors were invented in 1947, some people say transistors replace vacuum tubes, but it is not like that. That time vacuum tubes are used because the application in which they are using need more power like CRO needs about 200-300 V, but now researchers develop technology which works at lower voltage level that’s why we don’t need vacuum tubes everywhere but they are used in many applications where high voltage suits the application.
In the event of an atomic war, and a total radiation environment that would make radio communication almost impossible for ship to air, or air to air, or ground to air, transistors will not work properly in a high radiation environment. But vacuum tubes will work.
PULL UP RESISTOR AND PULL DOWN RESISTOR