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Component 1: Semiconductor Components
Quick questions on Transistors as switches: BJT, MOSFET, saturation and base resistor - WJEC A-Level Electronics
4short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is sizing the base resistor (BJT)?Show answer
To saturate the transistor you must supply at least the base current . The base resistor drops the difference between the driving voltage and the base-emitter voltage. In practice a smaller resistor is chosen so that more base current flows than the minimum, driving the transistor hard into saturation for reliable switching.
What are switching real loads?Show answer
Output loads (relays, motors, lamps) are placed in the collector (or drain) circuit. Inductive loads must have a flyback diode across them to absorb the back-EMF when the transistor switches off, or the voltage spike will destroy the transistor.
What is q1?Show answer
A transistor switch has and must pass a collector current of . Find the minimum base current to saturate it. [2 marks]
What is q2?Show answer
Explain why a transistor used as a switch dissipates little power in both of its states. [2 marks]
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