How it works
Transconductance gm = IC / VT, where VT ≈ 26 mV at room temperature. At IC = 1 mA, gm = 1/26 Ω⁻¹ ≈ 38.5 mA/V. Input resistance rπ = β / gm; for β = 100, rπ = 2.6 kΩ. Output resistance ro = VA / IC, where VA is the Early voltage (typically 50–100 V for BC547). For the common-emitter configuration with RC = 3.9 kΩ, voltage gain Av = −gm·(RC ∥ ro) ≈ −gm·RC when ro >> RC. The h-parameter model is an alternative: hie = rπ, hfe = β, hre is negligible for most calculations, hoe = 1/ro. Input impedance of CE stage is Rin = R1 ∥ R2 ∥ rπ.
Key points to remember
At IC = 1 mA, gm ≈ 38.5 mA/V — this numerical value appears so often in exam problems that you should compute it instantly from gm = IC/26mV. The hybrid-π model is preferred for high-frequency analysis because it explicitly shows the base-collector capacitance Cμ and base-emitter capacitance Cπ. h-parameters are preferred for low-frequency two-port analysis when the question says "using h-parameter equivalent circuit." The CE amplifier gives the highest voltage gain among the three configurations; CC (emitter follower) gives high input impedance and near-unity voltage gain; CB gives the widest bandwidth. Early voltage VA shifts the output characteristics upward when included.
Exam tip
The examiner always asks you to draw the hybrid-π equivalent circuit of a CE amplifier with specific bias values and then derive expressions for Av, Rin, and Rout — label gm, rπ, and ro on the diagram before writing any equation.