Transistors — KCSE Physics

KCSE Physics · 112 practice questions · 7 syllabus objectives

38 easy36 medium38 hard

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Explain the concept of current gain (β = Ic/Ib) and calculate collector current from base current and β

Identify the three terminals (base, collector, emitter) of an NPN and PNP transistor and describe their functions

Describe how a transistor is used as a switch and as an amplifier in simple circuits

Describe transistor as amplifier; identify logic gates and draw truth tables

Distinguish between npn and pnp transistors; draw common-emitter circuits with current directions

Apply IE = IC + IB; calculate base current, collector current, current gain α and β

Transistors

Sample Questions

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1
easySHORT ANSWER4 marks

List two types of logic gates and state their primary functions. (4 marks)

View Marking Scheme
Part (a) — 2 marks
AND gate (1 mk)
OR gate (1 mk)
Part (b) — 2 marks
Outputs high only if all inputs are high (for AND gate) (1 mk)
Outputs high if at least one input is high (for OR gate) (1 mk)
2
easySHORT ANSWER3 marks

A certain transistor has a base current (IB) of 10 mA and a collector current (IC) of 100 mA. Calculate the emitter current (IE) and the current gains α and β. (3 marks)

View Marking Scheme
Part (a) — 1 mark
IE = IC + IB = 100 mA + 10 mA = 110 mA (1 mk)
Part (b) — 1 mark
β = IC / IB = 100 mA / 10 mA = 10 (1 mk)
Part (c) — 1 mark
α = IC / IE = 100 mA / 110 mA = 0.909 (1 mk)
3
easySHORT ANSWER2 marks

Name two key differences between npn and pnp transistors in terms of charge carriers. (2 marks)

View Marking Scheme
Part (a) — 2 marks
npn transistors use electrons as majority charge carriers (1 mk)
pnp transistors use holes as majority charge carriers (1 mk)
4

In a common-emitter transistor configuration, a transistor has a base current (Ib) of 2 mA and a current gain (β) of 50. Calculate the collector current (Ic). (3 marks)

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