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Quick questions on The bipolar transistor explained: O-Level Electronics Electronic Components

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 are the three terminals?
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A bipolar junction transistor has three leads:
What is current control?
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The key behaviour is that a small base current IBI_B controls a much larger collector current ICI_C. The transistor does not create energy; it acts as a valve, letting the supply push a large current through the collector and emitter in response to the small base current. This is why a transistor can let a low-power circuit (a sensor, a logic gate) control a high-power load (a motor, a lamp).
What is current gain?
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The ratio of collector current to base current is the current gain, given the symbol β\beta (or hFEh_{FE}):
What is the transistor as a switch?
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Used as a switch, the transistor has two states:

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