Bipolar Junction Transistor (BJT) switch circuits use a low power/current signal to control whether a higher power load is turned on or off.
NPN component Diagram
Basic NPN BJT switch circuit schematic
Some general topics related to BJT switch circuits.
- Cutoff – Less than about 0.6V to the base will ensure that the transistor is fully off.
- Active region (avoid with switch circuits) – Some base-emitter current flows, but not enough for transistor collector-emitter to conduct fully. Conducting fully in this case means as much as the load will allow.
- Saturated – Base-emitter current is sufficient enough that the collector conducts well enough that the load limits current.
- Low side switching – Note that in realationship to the load, the NPN BJT is on the ground 0V reference side of the circuit. This is considered the lower voltage side of the circuit than Vcc. So, the transistor is switching on the low side of the circuit. For high side switching, a PNP transistor can be used (will be covered on a separate page).
- Separate power supplies can be used for the base and the collector current.
- Gain/Beta (ß) – The multiple amount of current (compared to base current) that a given transistor’s collector will pass. Varies between transistor part numbers and operating conditions. Consult the manufacturer’s datasheet for the particular one you are using.
Demonstration circuit using 2N3904 NPN BJT and resistor protected LED
- PNP BJT switch – Bipolar Junction Transistor – 2N3906
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