What it shows
A transistor turns a very small base current into a much larger collector current. The ratio Ic / Ib, called the current gain, can be found with a multimeter and two resistors.
The science
With the base fed through 1 MΩ, the base current is Ib = (9 V minus 0.7 V) / 1 MΩ = about 8 µA. The collector current Ic is found from the voltage across the 1 kΩ resistor: Ic = (9 V minus Vc) / 1 kΩ. The gain hFE = Ic / Ib is typically 100 to 400 for a BC547 and varies from one transistor to another. This works while Vc stays above about 1 V; below that the transistor is saturated and the formula does not hold.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Three BC547 transistors from the same pack
- A 1 MΩ resistor and a 1 kΩ resistor
- A 9 V battery with snap connector
- A multimeter
- A breadboard and jumper wires
- A notebook for a table
Safety
For this project: Battery power only. Check the transistor legs before connecting: with the flat face towards you they are collector, base, emitter from left to right.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fit the first BC547 and connect its emitter to the battery negative rail.
Connect the 1 kΩ resistor from the battery positive rail to the collector.
Connect the 1 MΩ resistor from the battery positive rail to the base.
Set the meter to DC volts and measure the voltage across the 1 MΩ resistor; divide by 1 MΩ to get Ib.
Measure the voltage from the collector to the negative rail, call it Vc.
Calculate Ic = (9 minus Vc) / 1000 amperes.
Divide Ic by Ib to get the gain; record it in your table.
Repeat for the other two transistors and compare their gains. Why are they different?
What you should see
Each transistor shows a gain in the hundreds, and different transistors of the same type give different values.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is current gain?
The ratio of collector current to base current.
2Why is a 1 MΩ base resistor used?
It keeps the base current tiny so the transistor stays in its amplifying range.
3What happens if the base resistor is too small?
The transistor saturates and the gain formula no longer applies.
4Why do identical-looking transistors have different gains?
Small differences in manufacturing give a wide range of hFE.
Take it further
Warm one transistor gently with your fingers and watch the collector voltage fall as the gain rises.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Hardware & repair
Using a Digital Multimeter
Measure voltage, current, resistance and continuity safely
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each