Generalised h-parameter model of an NPN BJT. The term "x" in the model represents a different BJT lead depending on the topology used. For common-emitter mode the various symbols take on the specific values as:

• x = 'e' because it is a common-emitter topology
• Terminal 1 = Base
• Terminal 2 = Collector
• Terminal 3 = Emitter
• i_in = Base current (i_b)
• i_o = Collector current (i_c)
• V_in = Base-to-emitter voltage (V_BE)
• V_o = Collector-to-emitter voltage (V_CE)

and the h-parameters are given by –

• h_ix = h_ie – The input impedance of the transistor (corresponding to the emitter resistance r_e).
• h_rx = h_re – Represents the dependence of the transistor's I_B–V_BE curve on the value of V_CE. It is usually very small and is often neglected (assumed to be zero).
• h_fx = h_fe – The current-gain of the transistor. This parameter is often specified as h_FE or the DC current-gain (β_DC) in datasheets.
• h_ox = h_oe – The output impedance of transistor. This term is usually specified as an admittance and has to be inverted to convert it to an impedance.