---
title: "BJT as a switch"
subject: "Fundamental of Electrical and Electronics Engineering"
chapter: 5
key: "semester_1/fundamental_of_electrical_and_electronics_engineering:bjt-as-switch"
source: https://www.bctengineer.com/q/semester_1/fundamental_of_electrical_and_electronics_engineering/bjt-as-switch
site: BCT Engineering
---

# BJT as a switch

Fundamental of Electrical and Electronics Engineering · Chapter 5: Transistor

- Asked in: 2081 Chaitra (new course), 3 marks
- Video: [Electronic Basics #22: Transistor (BJT) as a Switch](https://www.youtube.com/watch?v=WRm2oUw4owE) — GreatScott!
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/fundamental_of_electrical_and_electronics_engineering/bjt-as-switch
- Whole chapter: https://www.bctengineer.com/md/semester_1/fundamental_of_electrical_and_electronics_engineering/chapter-5

## Question

Draw a circuit diagram of BJT switch and explain its operation.

## Short answer

In cut‑off (V_BE < 0.7 V), I_C = 0, V_out = V_CC. In saturation, I_B ≥ I_C(sat)/β, V_CE ≈ 0.2 V, V_out ≈ 0. The base resistor sets I_B; the circuit acts as an inverter with low power dissipation.

## Answer

A BJT works as an electronic switch when it is driven between **cutoff** (switch OFF) and **saturation** (switch ON), never staying in the active region.

#### Circuit

```text
              +VCC
               |
              RC (load, e.g. lamp/relay)
               |
               +------ Vout
               |C
 Vin --[RB]--|/   npn
             |\
               |E
              GND
```

#### Operation

**Switch OFF (cutoff):** Vin = 0 V (logic 0). V_BE < 0.7 V, so I_B = 0 and I_C ≈ 0. No current flows through RC, so Vout = VCC. Collector–emitter acts like an **open switch**.

**Switch ON (saturation):** Vin = high (e.g. 5 V). Base current I_B = (Vin − 0.7)/RB is made large enough that the transistor saturates:

```text
I_C(sat) = (VCC − V_CE(sat))/RC ≈ VCC/RC
Condition: I_B ≥ I_C(sat)/β  (use 2-5 times for safety)
```

Then V_CE = V_CE(sat) ≈ 0.2 V, so Vout ≈ 0. Collector–emitter acts like a **closed switch** and the load gets nearly full VCC.

**Example:** VCC = 5 V, RC = 1 kΩ, β = 100: I_C(sat) ≈ 5 mA, I_B(min) = 50 µA. With Vin = 5 V, RB = 10 kΩ gives I_B = 0.43 mA, which ensures saturation.

The circuit is also a logic **inverter** (high input → low output). It has very small power loss in both states, since either I_C ≈ 0 or V_CE ≈ 0. Uses: driving relays, LEDs, lamps and digital logic.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A BJT switch operates between:
   A. Active and cutoff
   B. Active and saturation
   C. Breakdown and cutoff
   D. Cutoff (OFF) and saturation (ON)
   Answer: D. Staying out of the active region keeps power loss small. (Key point: Switching states)

2. When Vin = 0 V, what is Vout of the npn switch with RC load to VCC?
   A. VCC, since I_C about 0
   B. 0 V
   C. VCC/2
   D. V_CE(sat)
   Answer: A. With I_B = 0, I_C = 0, no drop across RC, so Vout = VCC. (Key point: Switch OFF)

3. Which is needed for saturation?
   A. I_B = 0
   B. I_B >= I_C(sat)/beta (usually 2-5 times more)
   C. I_B much less than I_C(sat)/beta
   D. V_BE = 0
   Answer: B. For VCC = 5 V, RC = 1k, beta = 100, I_C(sat) = 5 mA, so I_B(min) = 50 uA. (Key point: Saturation condition)

## Related in Fundamental of Electrical and Electronics Engineering

- [Common emitter BJT characteristics](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Ace-input-output-characteristics) — Fundamental of Electrical and Electronics Engineering, chapter 5
- [Emitter-bias BJT Q point and load line](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Aemitter-bias-q-point) — Fundamental of Electrical and Electronics Engineering, chapter 5

## Related in other subjects

- [BJT as a switch in power circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Abjt-as-power-switch) — Power Electronics, chapter 1, asked 2×
- [BJT as a switch](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Abjt-as-switch) — Electronics Devices and Circuit, chapter 2, asked 10×
- [BJT as switch; MOSFET versus BJT](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Abjt-as-switch-mosfet-vs-bjt) — Basic Electronics Engineering, chapter 3
- [CMOS switch operation](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acmos-switch) — Basic Electronics Engineering, chapter 3