BJT as a switch

A BJT used as a switch operates in only two regions: cut-off (switch OFF) and saturation (switch ON), not in the active region. The base current controls the collector-emitter path like a switch.

Circuit

        +VCC
          |
         RC
          |
          +----o Vout
 Vin--RB--|
          B  C
          |/
          |\
            E
            |
           GND

(NPN transistor, emitter grounded, load RC in the collector.)

Operation

OFF state (cut-off): Vin is low (≈ 0 V, below 0.7 V), so the base-emitter junction is not forward biased. IB = 0, IC ≈ 0 (only the leakage ICEO), VCE ≈ VCC, so Vout is high. The transistor behaves like an open switch.

ON state (saturation): Vin is high, so IB is large enough to forward bias both junctions.

IC(sat) = (VCC - VCE(sat))/RC ≈ VCC/RC
IB = (Vin - VBE)/RB  >  IB(sat) = IC(sat)/β

VCE(sat) ≈ 0.2 V, so Vout ≈ 0.2 V (low). The transistor behaves like a closed switch. In practice IB is made 2 to 5 times IB(sat) to make sure the transistor saturates.

Example

VCC = 5 V, RC = 1 kΩ, β = 100, Vin = 5 V: IC(sat) ≈ 4.8 mA, so IB(sat) = 48 µA; choosing IB = 0.1 mA (about twice IB(sat)) gives RB = (5 - 0.7)/0.1 mA = 43 kΩ.

Notes

• The output is the inverse of the input, so the circuit is an inverter (NOT gate), the basis of digital logic.
• Applications: driving relays, LEDs, motors (with a flyback diode) and logic circuits.
• Switching speed is limited by the turn-on delay and the storage time.