E4: Diodes & Transistors
Semiconductor diodes, LEDs, bipolar transistors as switches and amplifiers
Diodes & Transistors
Semiconductor diodes, LEDs, bipolar transistors as switches and amplifiers
Key Fact: A semiconductor diode allows current to flow in one direction only; it has a forward voltage drop of about 0.7 V for silicon.
Key Fact: An LED (light-emitting diode) emits light when forward biased; it requires a series resistor to limit current.
Key Fact: A bipolar junction transistor (BJT) has three terminals: base, collector and emitter.
Key Fact: An NPN transistor switches on when the base-emitter voltage exceeds ~0.7 V, allowing collector current to flow.
Key Fact: In switching mode, a transistor is driven between cut-off (off) and saturation (fully on).
Key Fact: Current gain h_FE = I_C/I_B; a small base current controls a much larger collector current.
Key Fact: As an amplifier, a transistor operates in its active region, producing a magnified copy of the input signal.
Key Fact: A common-emitter amplifier inverts the signal and provides voltage, current and power gain.
Key Fact: A base resistor limits base current to protect the transistor; R_B = (V_in − 0.7)/I_B.
Key Fact: A PNP transistor operates with opposite polarity: current flows when the base is pulled low.
Key Fact: Transistor saturation occurs when I_C reaches its maximum, limited by the supply and load resistance.
Key Fact: Protection diodes are placed across inductive loads (relays, motors) to prevent back-EMF damage.
📋 Key Vocabulary and Concepts
For Diodes & Transistors, you must know:
- Forward bias: Applying voltage across a diode in the direction that allows current to flow.
- Reverse bias: Applying voltage across a diode in the direction that blocks current flow.
- Current gain (h_FE): The ratio of collector current to base current in a bipolar transistor.
- Saturation: The transistor state where collector current is at its maximum and V_CE is near zero.
- Cut-off: The transistor state where base current is zero and no collector current flows.
- Active region: The transistor operating region between cut-off and saturation, used for amplification.
❓ Practice Questions
Q: What is the typical forward voltage drop of a silicon diode?
Q: A transistor has h_FE = 100 and I_B = 50 μA. What is I_C?
Q: Why does an LED need a series resistor?
Q: What are the three terminals of a BJT?
Q: Describe the difference between using a transistor as a switch and as an amplifier.
✅ Answers
- Approximately 0.7 V.
- I_C = h_FE × I_B = 100 × 50 × 10⁻⁶ = 5 mA.
- To limit the forward current to a safe value; without it, excessive current would destroy the LED.
- Base, collector and emitter.
- As a switch, the transistor operates between cut-off and saturation; as an amplifier, it operates in the active region where small base current changes produce proportionally larger collector current changes.
🎯 Exam Tips
- Always include a series resistor when calculating LED circuits.
- State the value of V_BE (0.7 V for silicon) before using it in calculations.
- For switching circuits, verify the transistor is in saturation by checking I_C < h_FE × I_B.
- Label base, collector and emitter clearly on any circuit diagram.
- Remember that a common-emitter amplifier inverts the output signal.
📝 Exam Technique
GCSE Electronics Exam Tips — Diodes & Transistors:
1. For Diodes & Transistors questions, use correct electronic symbols and terminology
2. Always show your working in calculations, including units at each step
3. When analysing circuits, state which law or rule you are applying first
4. For evaluation questions on Diodes & Transistors, compare component choices and consider cost, reliability and tolerance
5. Draw circuit diagrams neatly with conventional symbols
1. For Diodes & Transistors questions, use correct electronic symbols and terminology
2. Always show your working in calculations, including units at each step
3. When analysing circuits, state which law or rule you are applying first
4. For evaluation questions on Diodes & Transistors, compare component choices and consider cost, reliability and tolerance
5. Draw circuit diagrams neatly with conventional symbols
⚠️ Common Errors
✗ Omitting the series resistor for an LED in a circuit diagram. ✓ An LED must always have a series resistor to limit current to a safe value.
✗ Confusing NPN and PNP transistor biasing. ✓ NPN turns on when base is higher than emitter by ~0.7 V; PNP turns on when base is lower than emitter.
✗ Assuming a transistor in saturation has a large V_CE. ✓ In saturation, V_CE(sat) is very small, typically 0.2 V or less.
✗ Forgetting to check that the base resistor provides enough current for saturation. ✓ Calculate I_B = (V_in − 0.7)/R_B and verify I_C = h_FE × I_B exceeds the load current needed.
✍️ Model Answer
Full-Mark Response
Explain how a bipolar junction transistor can be used both as an electronic switch and as an amplifier.
A bipolar junction transistor (BJT) has three terminals: base, collector and emitter. A small base current I_B controls a larger collector current I_C, where I_C = h_FE × I_B. As a switch, the transistor is driven between two states: cut-off (no base current, no collector current, switch open) and saturation (base current sufficient to allow maximum collector current, switch closed). A logic-level input at the base turns the load on or off. As an amplifier, the transistor is biased into the active region between cut-off and saturation. A small varying input signal at the base produces proportional variations in collector current, resulting in a larger output signal at the collector. In a common-emitter configuration, the amplifier provides voltage, current and power gain but inverts the signal. The bias components set the operating point so the output can swing symmetrically without clipping.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of diodes & transistors, including electronic components, circuit theory and systems concepts relevant to WJEC Eduqas C690QS.
AO2 (Application): Apply knowledge and understanding of diodes & transistors to analyse, design and construct electronic circuits and systems.
AO3 (Evaluation): Evaluate electronic circuits and systems, making reasoned judgements about design choices, performance and practical considerations, constructing supported arguments.
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