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Transistors — KCSE Physics

KCSE Physics · 112 practice questions · 7 syllabus objectives · 7 revision lessons

38 easy36 medium38 hard

Last updated · Aligned to the KNEC KCSE syllabus

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

Revision Notes

Concise lesson notes for Transistors, written to the KCSE Physics marking standard. Read the first lesson free below.

Understanding Current Gain in Transistors

Current gain, denoted as β (beta), is a crucial parameter in transistors that indicates the amplification capability of the device. It is defined by the formula:
β = Ic / Ib
Where:

  • Ic is the collector current
  • Ib is the base current

A transistor amplifies the input current (Ib) to produce a larger output current (Ic). For example, if a transistor has a current gain of β = 100 and the base current (Ib) is 0.02 A, we can calculate the collector current (Ic) as follows:

Ic = β × Ib
Ic = 100 × 0.02 A = 2 A

This means that a small base current can control a much larger collector current, showcasing the transistor's ability to amplify signals. Understanding this concept is essential for analyzing transistor circuits effectively.

Key points to remember

  • Current gain (β) measures amplification in transistors.
  • Formula: β = Ic / Ib.
  • Ic is the collector current; Ib is the base current.
  • Higher β indicates greater amplification ability.
  • Transistors are essential in electronic circuits for signal processing.

Worked example

Calculate the collector current if Ib = 0.01 A and β = 50.
Ic = β × Ib = 50 × 0.01 A = 0.5 A.

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More lessons in this topic

Lesson 2: Understanding Transistor Terminals

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

Transistors are crucial components in electronics, acting as switches or amplifiers. There are two main types of transistors: NPN and PNP. Each type has three terminals: base, collector, and emitter.

  • Base: This terminal controls the transistor's operation. A small current flowing into the base allows a larger current to flow between the collector and emitter.
  • Collector: This terminal collects the current from the power supply. In an NPN transistor, it is connected to a positive voltage; in a PNP transistor, it connects to a negative voltage.
  • Emitter: This terminal emits current out of the transistor. In an NPN transistor, it is connected to the ground or a lower voltage; in a PNP transistor, it connects to a higher voltage.

Understanding these terminals helps in comprehending how transistors amplify signals or switch currents in circuits.

  • NPN and PNP transistors have three terminals: base, collector, emitter.
  • Base controls the transistor's operation with a small current.
  • Collector gathers current from the power supply.
  • Emitter releases current, connecting to lower or higher voltage.
  • NPN uses positive voltage at collector, PNP uses negative.

Identify the terminals of an NPN transistor and describe their functions.

  • The terminals are base, collector, and emitter.
  • Base controls the current flow; collector receives current; emitter outputs current.
Lesson 3: Transistors as Switches and Amplifiers

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

A transistor is a semiconductor device that can function as both a switch and an amplifier in electronic circuits. As a switch, a transistor can control the flow of current. When a small current is applied to the base terminal, it allows a larger current to flow from the collector to the emitter. This action can turn devices on or off.

As an amplifier, a transistor increases the strength of a signal. A small input voltage at the base results in a larger output voltage between the collector and emitter. This property is crucial in audio devices and communication systems.

In practical applications, transistors are often used in circuits to control lights, motors, or audio signals. Understanding these functions is essential for designing effective electronic devices.

  • Transistor acts as a switch by controlling current flow.
  • Small base current allows larger collector-emitter current.
  • Transistor amplifies signals, enhancing weak inputs.
  • Used in circuits for lights, motors, and audio.
  • Key component in modern electronic devices.

Explain how a transistor functions as a switch.

  • A small current at the base terminal turns the transistor on.
  • This allows a larger current to flow from collector to emitter, activating the circuit.
Lesson 4: Transistors as Amplifiers and Logic Gates

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

A transistor is a semiconductor device that can amplify electrical signals. It has three main regions: the emitter, base, and collector. When a small input current flows into the base, it allows a larger current to flow from the collector to the emitter, thus amplifying the signal.

Logic gates are circuits that perform basic logical functions on one or more binary inputs to produce a single output. Common logic gates include:

  • AND gate: Output is high only if all inputs are high.
  • OR gate: Output is high if at least one input is high.
  • NOT gate: Inverts the input signal.

To illustrate, here’s how to draw a truth table for an AND gate:

| Input A | Input B | Output (A AND B) | |---------|---------|------------------| | 0 | 0 | 0 | | 0 | 1 | 0 | | 1 | 0 | 0 | | 1 | 1 | 1 |

  • Transistors amplify signals using small input currents.
  • Logic gates perform logical operations on binary inputs.
  • AND gate outputs high only when all inputs are high.
  • OR gate outputs high if at least one input is high.
  • Truth tables summarize logic gate outputs for all input combinations.

Question: Draw the truth table for an OR gate. Answer: | Input A | Input B | Output (A OR B) | |---------|---------|------------------| | 0 | 0 | 0 | | 0 | 1 | 1 | | 1 | 0 | 1 | | 1 | 1 | 1 |

Sample Questions

Read 3 questions and answers free. Sign up to access all 112 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER4 marks

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

Answer & 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)

Answer & 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)

Answer & 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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Frequently asked questions

What does the KCSE Physics topic "Transistors" cover?

Transistors covers 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, and more, all aligned to the official KNEC KCSE Physics syllabus.

How many practice questions are available for Transistors?

HighMarks has 112 Transistors practice questions for KCSE Physics, each with a full marking scheme. The first 3 are free; sign up to access the rest, plus all KCSE mock exams and past papers.

Are these aligned with the KNEC KCSE syllabus?

Yes. Every objective on this page is taken directly from the official KNEC KCSE Physics syllabus. Practice questions match the KCSE exam format and are graded against the standard KNEC marking scheme.

How should I revise Transistors for the KCSE exam?

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