transistor as a switch

transistor as a switch

1. What Switch Means for BJT

BJT as switch = operates in two extreme states: cutoff (OFF) and saturation (ON), not active, to drive load (relay, LED, motor driver, logic) with small base current controlling large collector current. This is the digital / switching use vs linear amplifier (active).

2. Two States

State	Bias (both junctions)	$V_{CE}$	$I_C$	Behaviour
Cutoff (OFF)	EB reverse (or $V_{BE}<0.5V$), CB reverse	≈Vcc (no drop)	≈0 ($I_{CEO}$ leakage)	Open switch (high resistance)
Saturation (ON)	EB forward, CB forward (both forward, $V_{BE}≈0.7–0.8V$, $V_{BC}$ forward ~0.5V)	$V_{CE,sat}≈0.2V$ (small, not zero)	$I_{C,sat}= (Vcc - V_{CE,sat})/R_C$ (limited by external $R_C$, not $β$)	Closed switch (low resistance)

Active is between, but for switch we avoid it (except transition).

3. Base Drive Requirements

• To saturate (ON): need $I_B ≥ I_{C,sat}/β$ (overdrive, e.g., $I_B=1.5·I_{C,sat}/β$ to ensure saturation despite $β$ variation).
• To cutoff (OFF): need $V_{BE}≤0$ (or $I_B=0$), often $R_{BE}$ pulldown.

Switching speed: limited by charge storage in saturation (base stored charge must be removed to turn off) ⇒ slower turn-off than cutoff linear.

4. Circuit Example (NPN low-side switch)

Vcc (12V) → Rc (load: LED with R) → Collector → Emitter → Ground
Base → Rb (e.g., 10kΩ) → Control Vin (0V OFF, 5V ON)
When Vin=0: I_B=0 ⇒ cutoff ⇒ V_C≈Vcc (OFF)
When Vin=5V: I_B=(5-0.7)/10k=0.43mA, if β=100, I_Csat limited to (12-0.2)/1k=11.8mA, needed I_B=0.118mA ⇒ 0.43>0.118 ⇒ saturated ON ⇒ V_CE≈0.2V (low)

5. Uses

• Digital logic (RTL, TTL), relay driver, LED driver, motor driver, inverter.
• Not for linear amplification — for on/off control.

6. Exam Points

• Contrast cutoff ($V_{CE}≈Vcc$, $I_C≈0$) vs saturation ($V_{CE,sat}≈0.2V$, $I_C=I_{C,sat}$) with bias — 5-7 marks, very frequent (almost certain to ask switch states).
• Show need $I_B ≥ I_{C,sat}/β$ to saturate — 5 marks, frequent.
• Draw switch circuit with $R_B$, $R_C$ — 5 marks, frequent.
• Mention $V_{CE,sat}$ small not zero — 3 marks.