Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Transistors
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Transistors
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Subject: Basic Electronics
Class: Senior Secondary 2
Term: 3rd Term
Week: 1
Theme: Semiconductor Devices
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Explain the concept of a transistor Explain biasing of a transistor. Explain bipolar transistor circuits Explain types of transistors and symbols. Explain applications of transistors
A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is one of the fundamental building blocks of modern electronic devices. The term "transistor" is a portmanteau of "transfer" and "resistor," referring to its ability to transfer current across a resistor.
Basic Function: A transistor typically has three terminals. A small current or voltage applied to one terminal (the control terminal) can control a much larger current flowing between the other two terminals.
As a Switch: When used as a switch, a small signal can turn a larger current ON or OFF, much like a faucet controls water flow. This is fundamental to digital logic circuits.
As an Amplifier: When used as an amplifier, a small input signal (e.g., from a microphone) can be increased in strength (amplified) to produce a larger output signal (e.g., to drive a loudspeaker). BJTs are current-controlled devices.
They have three terminals: Emitter (E): Heavily doped, emits charge carriers into the base.
Base (B): Lightly doped, very thin, controls the flow of charge carriers.
Collector (C): Moderately doped, collects charge carriers from the base. BJTs are formed by sandwiching a layer of one type of semiconductor material between two layers of the other type.
This gives rise to two main types: NPN Transistor: Consists of a thin P-type semiconductor sandwiched between two N-type semiconductors (N-P-N). The arrow in its symbol points out from the base.
Symbol: ``` C (Collector) | / \ / \ / \ |-------B (Base) \ / \ / \ / V | E (Emitter) | E (Emitter) <-- Arrow points IN ```
Note: The arrow on the emitter indicates the direction of conventional current flow when the transistor is in operation. For NPN, current flows from base to emitter; for PNP, current flows from emitter to base. There are two main families of transistors: Bipolar Junction Transistors (BJTs) and Field-Effect Transistors (FETs). For Senior Secondary 2, the focus is primarily on Bipolar Junction Transistors (BJTs) due to their fundamental nature in introductory electronics.
Fixed Bias (Base Resistor Bias): Circuit: A single resistor (Rb) connects the base to the supply voltage (Vcc). The collector resistor (Rc) connects the collector to Vcc, and the emitter is grounded.
Explanation: Provides a fixed base current (Ib) determined by Vcc and Rb. Simple to implement.
Disadvantage: Highly unstable with temperature variations. The Q-point shifts significantly with changes in Beta (β), the current gain, which varies with temperature. Not suitable for amplification where stability is critical.
Collector-to-Base Bias (Feedback Bias): Circuit: The base resistor (Rb) is connected between the collector terminal and the base terminal.
Explanation: Introduces negative feedback. If collector current (Ic) increases, the voltage drop across Rc increases, causing Vc to decrease. This, in turn, reduces Vb and thus Ib, counteracting the initial increase in Ic.
Advantage: Provides better thermal stability than fixed bias.
Disadvantage: The input resistance of the amplifier is reduced due to the feedback, which can affect signal performance. Voltage Divider Bias (Self-Bias / Emitter Bias): Circuit: Uses two resistors (R1 and R2) in a voltage divider network to set the base voltage. An emitter resistor (Re) is included to provide significant stability.
Explanation: R1 and R2 set a stable voltage at the base (Vb). The emitter resistor Re helps stabilize the circuit against variations in β and temperature. If Ic tries to increase, Ie (which is approximately Ic) increases, causing the voltage drop across Re (Ve = Ie Re) to increase. Since Vb is relatively fixed by the voltage divider, Vbe (Vb - Ve) decreases, reducing Ib, and thus controlling Ic.
Advantage: Provides excellent thermal stability and a very stable Q-point. This is the most widely used biasing technique for linear amplifier circuits.
Mobile Phone and Device Repair: Transistors are miniature switches and amplifiers within smartphones, tablets, and feature phones commonly used across Nigeria. Local electronics repair technicians (e.g., those in Computer Village, Lagos, or GSM Village, Abuja) frequently diagnose and replace faulty transistors in power management circuits, audio amplifiers, and radio frequency stages. Understanding transistors helps these technicians troubleshoot and repair devices, supporting the local economy.
Solar Power Systems: With unreliable grid power, solar power systems are increasingly popular in Nigerian homes and businesses. Transistors are critical components in solar charge controllers (regulating battery charging from solar panels) and inverters (converting DC battery power to AC for household appliances). Knowledge of transistor operation allows individuals to understand how these systems function, troubleshoot basic faults, and even build simple solar-powered projects. Local Audio Systems (Public Address & Entertainment): Transistors form the core of amplifier circuits in public address (PA) systems used in churches, mosques, schools, and community events across Nigeria. They are also crucial in home stereo systems and car audio setups. Learning about transistors provides insight into how sound signals are boosted, enabling learners to appreciate the technology behind local entertainment and communication.