Skip to main content

Active and passive components, transconductance

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).

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

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.

ActivePassive
Needs an external supply to workWorks without an extra supply
Can amplify the signalCannot amplify
Can deliver power gainPower gain ≤ 1
e.g. transistor, op-ampe.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.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Basic Electronics Engineering chapter 1 questions →