The mental picture
The base acts as a control input for a larger load current. Load energy comes from the supply, not from a miraculous multiplication of base-current energy. Depending on its operating point, the transistor acts as an analogue control element or approaches a closed switch.
The principle
The NPN transistor considered here has base, collector and emitter terminals. Pin order depends on the component. In the active region, IC ≈ β·IB. In switching operation this approximation must not extend beyond the current set by load and supply.
Try it yourself
The bipolar transistor
Increase the control voltage. Collector current initially rises, then reaches the load limit. More base current then produces little additional load current in the model.
Model limits: Highly simplified NPN: β = 100, VBE = 0.7 V, VCEsat = 0.2 V. Not the curves of an actual device.
A worked example
With a 3.3 V control signal, an assumed 0.7 V base-emitter drop and 10 kΩ base resistor, base current is about 0.26 mA. β = 100 suggests 26 mA, but a 500 Ω load at 5 V limits current to about 9.6 mA.
A closer look · equations and assumptions
β varies substantially between devices and with current, temperature and collector voltage. Robust switch design uses saturation conditions to size base drive. Analogue amplifiers use bias stabilisation and feedback instead of relying on one precise β value.
A common pitfall. The base needs current limiting. A relay load also requires suitable handling of the turn-off voltage.
Workshop takeaway
A clean amplifier and an on/off switch need different checks. Start with load current, then consider drive and heat.
Sources & transparency
Original illustrations and educational calculations. No measurements of a specific product. Edited 11 October 2026.
Work it out · answers unfold
Write down your prediction first, then your calculation. Open the answer only when you are ready to compare.
08 · An NPN operates in its active region: β = 100, base current 0.1 mA. What collector current does the model predict?
Show worked answer
- Collector current = β · base current.
- 100 · 0.1 mA = 10 mA.
- Valid only while the external circuit permits it.
09 · NPN switch: 100 Ω load on 5 V, saturated VCE = 0.2 V. Find load current.
Show worked answer
- Load voltage: 4.8 V.
- Divide by 100 Ω.
- Collector current = 48 mA.
10 · For task 9 choose forced current gain 10. What base current should be used?
Show worked answer
- Divide load current by forced gain.
- Base current = 48/10 = 4.8 mA.
- The driver must be able to supply it.
11 · 3.3 V drive, assumed VBE = 0.8 V and base current 4.8 mA. Find base resistor.
Show worked answer
- Base resistor voltage is 2.5 V.
- R = 2.5/0.0048.
- Base resistor = 521 Ω; check actual driver and datasheet.
12 · Find collector-emitter dissipation of the on-state switch in task 9.
Show worked answer
- P = saturated VCE · collector current.
- 0.2 V · 0.048 A.
- P = 9.6 mW, excluding base and switching losses.
