Transistor
Basic definition: A transistor is a small electronic component made from semiconductor materials. It can act as a switch to turn electricity on or off or as an amplifier to make electrical signals stronger. Transistors are found in almost every electronic device, from computers to radios.
Current control: A transistor works by using a small electric current or voltage at one terminal to control a larger current between two other terminals. This is how it helps regulate and control electricity in circuits.
Structure
Terminals: A transistor has three main parts, called terminals: the emitter (E), the base (B), and the collector (C). Each terminal plays a special role in making the transistor work.
Emitter function: The emitter is like a source that pushes out tiny charged particles called electrons (or holes). These particles are needed to create the flow of current in the transistor.
Base function: The base is the control center of the transistor. It decides how many charge carriers from the emitter are allowed to move toward the collector. It is very thin and only allows a small current.
Collector function: The collector receives or “collects” the charge carriers that come from the emitter through the base. It is the output side of the transistor where most of the current flows out.
NPN type: In an NPN transistor, there are two layers of n-type material with a layer of p-type material in between. Current flows from the collector to the emitter when a small current flows into the base.
PNP type: In a PNP transistor, there are two layers of p-type material with a layer of n-type in the middle. Current flows from the emitter to the collector when the base is at a lower voltage.
Terminal connections: The three terminals are connected to metal wires or leads that stick out from the transistor body. These leads connect the transistor to the rest of the electronic circuit.
Transistor Circuits
Basic setup: A simple circuit using a transistor includes a power supply (voltage source), a resistor to limit current, and connections to the base, collector, and emitter. This setup allows the transistor to control current in the circuit.
Base current role: A small amount of current entering the base (IB) acts like a signal to turn on a much larger current between the collector and emitter. This is why transistors are very useful.
Current gain (β): The transistor can multiply the small base current. The number that shows how much bigger the output current (IC) is compared to the base current (IB) is called beta (β). It shows how good the transistor is at amplifying.
Transistor as a Switch
Switching role: A transistor can act like a switch by turning the current on or off. This makes it useful for controlling parts of a circuit without needing a moving switch.
Closed switch state: When the current going into the base is strong enough, the transistor turns “on” and lets current pass from the collector to the emitter. This is like closing a switch.
Open switch state: If the base current is too small or not present, the transistor will not allow current to flow between collector and emitter. This is like opening a switch.
Minimum voltage (VBE): To make the transistor turn on, the voltage between the base and emitter must reach a certain level. This voltage is called VBE. For silicon transistors, it is about 0.7 volts, and for germanium ones, it’s about 0.3 volts.
Relay function: Just like a relay, a transistor can use a small current to control a much larger one. This makes it helpful in circuits where you need to turn on a big device using a small signal.
Transistor as an Amplifier
Amplification use: Besides switching, a transistor can make weak electrical signals stronger. This is called amplification, and it’s important in radios, microphones, and sound systems.
Signal increase: When the base current changes just a little, it can cause a big change in the collector current. That means the transistor can make small signals much bigger.
Output magnitude: After amplification, the output signal may have more voltage, more current, or more power than what went in. This helps in boosting weak signals.
Amplification factor: The amplification factor shows how much the output current changes when the input (base current) changes. A higher value means the transistor is a better amplifier.
Transistor Applications
General use: Transistors are everywhere! They are used in electronic circuits as switches to turn things on and off, and as amplifiers to make signals louder or stronger.
Automatic Switches
Environmental response: Some circuits use transistors to respond automatically to things like light or temperature. These are called automatic switches.
Light-Controlled Switch
LDR function: In a light-controlled switch, a special component called an LDR (light-dependent resistor) helps change the base current depending on how much light is around.
LDR resistance change: When more light shines on the LDR, its resistance goes down. This change affects the voltage at the base of the transistor.
Switching result: Because the base voltage changes, the transistor may turn on or off. This can be used to turn on a light automatically when it gets dark.
Photo-sensitive resistor: An LDR is also called a photo-sensitive resistor because it reacts to light. More light means lower resistance.
Temperature-Controlled Switch
Thermistor use: In temperature-sensitive circuits, a thermistor is used. A thermistor is a type of resistor that changes its resistance with temperature.
Temperature effect: When the temperature goes up, the resistance of the thermistor changes. This affects the voltage at the transistor’s base and turns it on or off.
Thermistor types: There are two main types of thermistors: PTC (positive temperature coefficient) gets more resistant when it gets hotter, and NTC (negative temperature coefficient) gets less resistant as it heats up.