A transistor passes 100 mA of collector current with 0.5 mA of base current. Calculate hFE.

The gain hFE is 200. Gain = Ic ÷ Ib = 100 ÷ 0.5 = 200.

A 5 V logic output drives a transistor with an hFE of 100 and a 200 mA load. Calculate a suitable base resistor.

The base current needed is 200 mA ÷ 100 = 2 mA. Rb = (5 − 0.7) ÷ 0.002 = 2150 Ω, so the lower preferred value of 1.8 kΩ is chosen.

Why is a base resistor chosen slightly smaller than the calculated value?

A smaller resistor gives extra base current, ensuring the transistor is driven fully on, or saturated. A saturated transistor wastes little power and stays cool.

CCEA A-Level Technology and Design 8900

Material Selection and Wood

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Design for Manufacture, Quality and Safety

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Electronic Control SystemsOption A

Topic overview Control systems, feedback and electronics safety Resistors, Ohm's law and power ratings Capacitors and RC timing Potential dividers, sensors and comparators Switches, pull-up and pull-down resistors Output devices and LEDs Transistors, Darlington pairs and thyristors Logic gates, truth tables and flip-flops 555 monostable and astable timers

Microelectronic Control SystemsOption A

Topic overview Microcontroller systems, ADCs and DACs Strain gauges and bridge circuits Optical switches, encoded discs and Schmitt triggers Op-amp amplifiers and gain Zener diodes and voltage regulators Karnaugh maps and binary counters Seven-segment, LCD and dot matrix displays PIC flowcharts and programming DC motors, stepper motors and driver circuits

Mechanical and Pneumatic ComponentsOption B

Topic overview Open and closed loop mechanical and pneumatic control Safe working with compressed air and mechanisms Levers, linkages and moments Gear types and gear trains Pulleys, belts, chains and shaft fixings Cams, cranks and converting motion Compressed air supply and pneumatic valves Cylinders, shuttle valves, restrictors and reservoirs Pneumatic circuits for logic, timing and speed control

Mechanical and Pneumatic SystemsOption B

Topic overview Hydraulics, system limitations and plant safety Torque, work, power and air consumption Vacuum handling, proximity sensors and 5/3 valves Sequential and cascade pneumatic circuits Cam profiles and follower displacement diagrams Gear geometry, pulley blocks and ratchets Toggles, treadles, shafts and couplings Friction, brakes and clutches Bearings, lubrication and seals

Designing and Manufacturing ProductsOption C

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Electronic Control Systems

Transistors, Darlington pairs and thyristors

3 things to know
AS 1 Paper 2

Overview

Transistor switching and hFE, base resistor and Ic(max) calculations, Darlington pairs and their gain, protective diodes on relay coils, and thyristors in alarms.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

How does an npn transistor act as a switch?

Key term

What base–emitter voltage turns a silicon transistor on?

Key term

Define the current gain hFE of a transistor.

Questions from this subtopic

Each one with the model answer underneath.

1
How does an npn transistor act as a switch?
Model answer
A small current into the base allows a much larger current to flow from collector to emitter. The load sits between the positive supply and the collector.
2
What base–emitter voltage turns a silicon transistor on?
Model answer
A silicon transistor begins to conduct when Vbe reaches about 0.7 V. Almost no collector current flows below this voltage.
3
Define the current gain hFE of a transistor.
Model answer
Current gain hFE is the collector current divided by the base current, Ic ÷ Ib. The gain is a ratio of two currents, so it has no unit.
4
A transistor passes 100 mA of collector current with 0.5 mA of base current. Calculate hFE.
Model answer
The gain hFE is 200. Gain = Ic ÷ Ib = 100 ÷ 0.5 = 200.
5
A 5 V logic output drives a transistor with an hFE of 100 and a 200 mA load. Calculate a suitable base resistor.
Model answer
The base current needed is 200 mA ÷ 100 = 2 mA. Rb = (5 − 0.7) ÷ 0.002 = 2150 Ω, so the lower preferred value of 1.8 kΩ is chosen.
6
Why is a base resistor chosen slightly smaller than the calculated value?
Model answer
A smaller resistor gives extra base current, ensuring the transistor is driven fully on, or saturated. A saturated transistor wastes little power and stays cool.

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The output voltage is V.

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AThe total resistance is 2 kΩ.

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CThe series resistor adds another 1 kΩ.

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