How it works
The h-parameter (hybrid parameter) model represents the transistor as a two-port network. For the CE configuration, the input port equation is: v1 = hie × i1 + hre × v2, and the output port equation is: i2 = hfe × i1 + hoe × v2. Here hie is input impedance (typically 1–2 kΩ), hfe is forward current gain (β, typically 100–300), hre is the reverse voltage feedback ratio (very small, ~10⁻⁴, usually neglected), and hoe is output conductance (typically 25 μS). Voltage gain AV = −hfe × ZL / (hie + (1 − hre×hfe)×ZL); for practical analysis hre is set to zero, simplifying to AV = −hfe × ZL / hie. Input impedance Zi = hie when source is ideal.
Key points to remember
The four h-parameters for CE configuration: hie (input impedance, ~1 kΩ), hfe (forward current gain = β), hre (reverse voltage ratio, ~10⁻⁴, neglected in most calculations), hoe (output admittance, ~25 μS; its reciprocal 1/hoe ≈ 40 kΩ appears in parallel with RL). Simplified voltage gain: AV = −hfe × RL′/hie where RL′ = RC ∥ (1/hoe). Current gain: AI = −hfe/(1 + hoe × RL). These parameters change with operating point — hfe at IC = 2 mA differs from hfe at IC = 10 mA, so the Q-point must always be specified before h-parameters are applied. Conversion between CB, CE, and CC configurations is a common exam problem.
Exam tip
The examiner always asks you to draw the h-parameter equivalent circuit of a CE amplifier and derive expressions for voltage gain and input impedance — label all four h-parameters on the diagram even if hre is later neglected.