---
title: "BJT as a switch"
subject: "Electronics Devices and Circuit"
chapter: 2
key: "semester_3/electronics_devices_and_circuit:bjt-as-switch"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/bjt-as-switch
site: BCT Engineering
---

# BJT as a switch

Electronics Devices and Circuit · Chapter 2: The Bipolar Junction Transistor

- Asked in: 2083 Baisakh, 4 marks; 2080 Bhadra, 4 marks; 2076 Ashwin, 4 marks; 2074 Ashwin, 6 marks; 2073 Shrawan, 4 marks; 2069 Chaitra, 4 marks; 2072 Kartik, 4 marks; 2071 Shrawan, 4 marks; 2071 Chaitra, 4 marks; 2070 Chaitra, 4 marks (asked 10 times)
- 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_3/electronics_devices_and_circuit/bjt-as-switch
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-2

## Question

Explain the operation of BJT as a switch.

## Short answer

In cut-off (OFF), base current is zero, IC ≈ 0, VCE = VCC (open switch). In saturation (ON), base current is large enough so IC(sat) ≈ VCC/RC, VCE(sat) ≈ 0.2 V (closed switch). It acts as an inverter.

## Answer

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
```text
        +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.
```text
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.

## Check yourself

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

1. In which regions does a BJT switch operate?
   A. Cut-off and saturation
   B. Cut-off and active
   C. Reverse active only
   D. Active and saturation
   Answer: A. OFF is cut-off, ON is saturation; the active region is avoided. (Key point: Regions used)

2. In the OFF state, what are IC and Vout?
   A. IC maximum, Vout about 0.2 V
   B. IC = beta IB, Vout = VCC/2
   C. IC about 0, Vout = 0
   D. IC about 0, Vout about VCC (high)
   Answer: D. With the BE junction not forward biased, IB = 0 and IC is only leakage, so Vout is high. (Key point: OFF state)

3. What is required for saturation?
   A. RB = 0
   B. IB less than IC(sat)/beta
   C. IB greater than IC(sat)/beta
   D. Vin = 0
   Answer: C. IB must exceed IC(sat)/beta, often 2 to 5 times IB(sat). (Key point: ON state)

4. What are VCE(sat) and the circuit function?
   A. About 0.7 V; an amplifier
   B. About 5 V; a buffer
   C. About 0 V; a rectifier
   D. About 0.2 V; an inverter
   Answer: D. Output is low (about 0.2 V) when input is high, so it inverts. (Key point: VCE(sat) and logic)

## Related in Electronics Devices and Circuit

- [Series regulator stability factor and overload protection](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Aseries-regulator-stability-overload) — Electronics Devices and Circuit, chapter 6

## Related in other subjects

- [BJT as a switch](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Abjt-as-switch) — Fundamental of Electrical and Electronics Engineering, chapter 5
- [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 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