---
title: "Active and passive components, transconductance"
subject: "Basic Electronics Engineering"
chapter: 1
key: "semester_2/basic_electronics_engineering:active-passive-component"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/active-passive-component
site: BCT Engineering
---

# Active and passive components, transconductance

Basic Electronics Engineering · Chapter 1: Basic Circuits Concepts

- Asked in: 2074 Bhadra, 2+4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/active-passive-component
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-1

## Question

What is Active and Passive Component? Define transconductance and voltage gain with reference to BJT.

## Short answer

Active components (e.g., transistor, op-amp) supply energy or amplify using an external source. Passive components (e.g., R, L, C) only dissipate or store energy. Transconductance gm = ΔIC/ΔVBE = IC/VT (S). Voltage gain Av = vo/vin = -gm(RC||RL).

## Answer

#### Active component
A component that can supply energy to a circuit, amplify a signal, or control the flow of current by using an external power source. Examples: transistors (BJT, FET), diodes in some uses, op-amps, ICs, batteries.

#### Passive component
A component that cannot generate energy and cannot amplify; it only absorbs, stores or dissipates energy. Examples: resistors, capacitors, inductors, transformers.

| Active | Passive |
|---|---|
| Needs an external supply to work | Works without an extra supply |
| Can amplify the signal | Cannot amplify |
| Can deliver power gain | Power gain ≤ 1 |
| e.g. transistor, op-amp | e.g. R, L, C |

#### Transconductance (gm)
The ratio of the change in output (collector) current to the change in input (base-emitter) voltage, with VCE constant:

gm = ΔIC/ΔVBE = IC/VT

where VT = kT/q ≈ 25 mV at room temperature. For IC = 1 mA, gm = 1 mA/25 mV = 40 mS. Unit: siemens (A/V).

#### Voltage gain
The ratio of output signal voltage to input signal voltage:

Av = vo/vin

For a common-emitter BJT amplifier with collector load RC (and load RL), the small-signal voltage gain is

Av = −gm (RC ∥ RL)

The negative sign shows a 180° phase shift. For example, with gm = 40 mS and RC ∥ RL = 2 kΩ, Av = −40 × 10⁻³ × 2000 = −80.

## Check yourself

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

1. Which statement correctly separates active from passive components?
   A. Active components can amplify or supply energy using an external supply; passive components only absorb, store or dissipate energy
   B. Passive components can amplify
   C. Active components work without any supply
   D. Resistors are active
   Answer: A. Active devices (transistor, op-amp) need an external supply and can give power gain; R, L, C cannot. (Key point: Active vs passive)

2. What is transconductance gm of a BJT?
   A. ΔVBE/ΔIC
   B. ΔIC/ΔVBE = IC/VT
   C. ΔIB/ΔIC
   D. VCE/IC
   Answer: B. gm = ΔIC/ΔVBE = IC/VT, measured in siemens. (Key point: Transconductance)

3. For IC = 1 mA and VT = 25 mV, what is gm?
   A. 40 mS
   B. 25 mS
   C. 0.04 mS
   D. 4 S
   Answer: A. gm = 1 mA/25 mV = 40 mS. (Key point: Value of gm)

4. The small-signal voltage gain of a CE amplifier is
   A. +gm (RC ∥ RL)
   B. −gm (RC ∥ RL), with 180° phase shift
   C. gm/(RC ∥ RL)
   D. −gm RC only at dc
   Answer: B. Av = −gm(RC ∥ RL); with gm = 40 mS and 2 kΩ it is −80. (Key point: CE voltage gain)

## Related in Basic Electronics Engineering

- [Transconductance and voltage gain of BJT](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Avoltage-gain-transconductance-bjt) — Basic Electronics Engineering, chapter 3, asked 2×
- [Active/passive components and color code](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aactive-passive-color-code) — Basic Electronics Engineering, chapter 1

## Related in other subjects

- [Insertion gain/loss, design steps, active vs passive](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Ainsertion-loss-steps-active-passive) — Filter Design, chapter 1
- [Mirror error and active-load differential gain](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Amirror-error-active-load-gain) — Advanced Electronics, chapter 1
- [Transconductance gm of BJT](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Abjt-transconductance-gm) — Electronics Devices and Circuit, chapter 2, asked 2×
- [BJT bipolar vs FET unipolar; MOSFET transconductance](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Abjt-bipolar-fet-unipolar-gm) — Electronics Devices and Circuit, chapter 3