Transistors
The tiny switch-and-amplifier inside nearly every electronic device.
The question
How can a tiny current, too small to light an LED, switch on something much bigger?
What you’ll discover: how a transistor works as an electronic switch and amplifier, and how to choose its base resistor.
In simple terms
BeginnerA switch you control with a trickle
A transistor is a switch with no moving parts. A small current into one leg controls a much bigger current through the other two. In the Automatic Night Light, a fraction of a milliamp from the light sensor switches on the LED’s 10 mA.
The common NPN transistor (like the 2N3904) has three legs:
- Base (B): the control. A small current in here turns it on.
- Collector (C): where the bigger current comes in.
- Emitter (E): where it all goes out, usually to −.
The transistor was invented at Bell Labs in 1947. Today a single computer chip contains billions of them, switching on and off billions of times a second.
Switch and amplifier
A transistor can work two ways:
- As a switch: give the base enough current and the transistor turns fully on, like a closed switch. Remove it, and it’s off. This is how computers work: billions of tiny switches.
- As an amplifier: in between, the collector current follows the base current, but much bigger. A small changing signal at the base becomes a large copy at the collector. That’s how a radio or guitar amplifier makes a faint signal loud.
Go deeper
IntermediateGain and the base resistor
A transistor’s current gain, called β (beta) or hFE, says how many times bigger the collector current can be than the base current. For a 2N3904 it’s typically somewhere around 100 to 300, and it varies from part to part, so designs never rely on an exact value.
Choosing a base resistor for a switch: say an LED needs 20 mA and the transistor’s gain is at least 100. The base needs at least 20 ÷ 100 = 0.2 mA. To make sure it switches fully on, designers give it several times more, say 1 mA. With a 4.5 V supply and about 0.7 V across the base-emitter junction: R = (4.5 − 0.7) ÷ 0.001 = 3,800 Ω. Use the standard 3.3 kΩ.
The base resistor also protects the transistor: never connect the base straight to + without one.
AdvancedNPN, PNP and the regions of operation
An NPN transistor turns on when its base is about 0.6 to 0.7 V above its emitter and current flows into the base. A PNP transistor (like the 2N3906) is the mirror image: its emitter goes to +, and it turns on when current flows out of the base. PNP transistors are handy for switching the + side of a load.
A transistor has three main regions: cut-off (off), active (amplifying, IC ≈ β × IB), and saturation (fully on, with only about 0.1 to 0.3 V left across collector and emitter). Switching circuits are designed to jump between cut-off and saturation, spending as little time as possible in the active region, where the transistor dissipates the most heat.
Know the limits: a 2N3904 is rated for a maximum collector current of 200 mA and should be kept well below that. Bigger loads, like motors, need bigger transistors, such as a power MOSFET.
Quick check
What are the three legs of an NPN transistor?
Base, collector and emitter.
If a transistor’s gain is 100 and the base current is 0.1 mA, what’s the most collector current it can control?
About 100 × 0.1 mA = 10 mA.
Why do we never connect the base straight to + without a resistor?
Too much base current would flow and could destroy the transistor.
Words to know
- Transistor
- A semiconductor switch and amplifier controlled by a small current.
- Base, collector, emitter
- The control leg, the current-in leg and the current-out leg of a bipolar transistor.
- Gain (β)
- How many times bigger the collector current can be than the base current.
- Saturation
- Fully on, like a closed switch.
Learn more
- Science World (Vancouver): Current Electricity: eight hands-on circuit activities
- Build: Automatic Night Light
- Build: Touch Switch
- Next: Switches and relays
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.