- 2070 Magh · 4+2 marks
BJT as switch; MOSFET versus BJT
Short answer
A BJT acts as a switch by operating in cut-off (OFF, VCE ≈ VCC) when base current is zero, and in saturation (ON, VCE(sat) ≈ 0.2 V) with sufficient base drive. MOSFET is voltage-controlled, unipolar, high input impedance, and fast switching; BJT is current-controlled, bipolar, slower, and prone to thermal runaway.
Explain how BJT can be used as a switch. What are the differences between MOSFET and BJT?
BJT as a switch
A transistor used as a switch operates only in two regions: cut-off (switch OFF) and saturation (switch ON), and not in the active region.
+VCC
|
[RC] (load)
|
+----o Vout
|C
Vin o--[RB]--B-| NPN
|E
|
GND
- OFF state (cut-off): Vin = 0 (or below 0.7 V). The base-emitter junction is not forward biased, so IB = 0, IC ≈ 0 and VCE ≈ VCC. The transistor acts like an open switch.
- ON state (saturation): Vin is high (for example 5 V). IB = (Vin − 0.7)/RB is made large enough that IB > IC(sat)/β, where IC(sat) = (VCC − VCE(sat))/RC. Then VCE = VCE(sat) ≈ 0.2 V and the transistor acts like a closed switch.
The output is the inverse of the input, so the circuit also works as a NOT gate (inverter). It is used in digital circuits, LED and relay drivers, and logic families (TTL, RTL).
Example: VCC = 5 V, RC = 1 kΩ, β = 100. IC(sat) = 4.8 mA, so IB(min) = 48 µA. For Vin = 5 V, RB ≤ (5 − 0.7)/48 µA ≈ 90 kΩ; choose RB = 47 kΩ for firm saturation.
MOSFET versus BJT
| Point | MOSFET | BJT |
|---|---|---|
| Carriers | unipolar (one carrier type) | bipolar (electrons and holes) |
| Control | voltage controlled (VGS) | current controlled (IB) |
| Input impedance | very high (about 10¹² Ω) | low (kΩ) |
| Gate/base current | almost zero | not zero |
| Size and packing | small, easy to integrate | larger |
| Power consumption | very low (CMOS) | higher |
| Switching speed | fast, good for logic | slower (storage time) |
| Noise and thermal stability | low noise, no thermal runaway | thermal runaway possible |
| Static damage | gate oxide can be damaged by static | more robust |
✓ Answer checked by Claude Opus 5.5 (high effort)
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