Transistor Control

Switch an LED on and off with a small base current.

Lesson progress

1/4
Saturation · ON
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Ic is limited by Rc and the LED, not by β. The transistor acts like a closed switch.

Input Vin
3.3 V
Base current Ib
257 µA
Collector current Ic
20.7 mA
max 20.9 mA
Emitter current Ie
20.9 mA
Ib + Ic
Base–emitter Vbe
734 mV
Collector–emitter Vce
284 mV
of 9.0 V
Effective gain Ic/Ib
× 80
Transistor heat P
6.04 mW

Live scope

Move the Vin slider, or try Pulse mode
5 V0━ Vin23 mA0━ Ic−6snow

Transfer curve · Ic vs Vin

Recalculated when components change
ActiveSaturation0 A12 mA23 mA0V1V2V3V4V5VVin →
The white dot shows where the circuit is now. Hover the curve to read values.

A small current controls a large one

A microcontroller pin can safely supply only a few milliamps, but motors, relays and bright LEDs need much more. A bipolar junction transistor (BJT) solves this. A small current into its base lets a current about β times larger flow from collector to emitter.

In this lab the input voltage drives the base through Rb. The transistor sits on the low side of an LED and its current-limiting resistor Rc. Raise Vin and watch the transistor move through its three operating regions.

What you'll learn

  • Why the base needs about 0.6–0.7 V before anything happens
  • How current gain β links Ib to Ic
  • The difference between amplifying (active) and switching (saturation)
  • How to choose Rb and Rc for a reliable switch

The three regions

Cutoff · OFF

Vbe is too low for base current to flow, so the transistor acts like an open switch.

Active · partly on

Ic = β·Ib. The transistor amplifies the base current, but it is not fully on.

Saturation · ONnow

Ic is limited by Rc and the LED, not by β. The transistor acts like a closed switch.