h-PARAMETER MODEL

Generalized h-parameter model of an NPN BJT.replace x with e, b or c for CE, CB and CC topologies respectively.

Another model commonly used to analyze BJT circuits is the "h-parameter" model, closely related to the hybrid-pi model and the y-parameter two-port, but using input current and output voltage as independent variables, rather than input and output voltages. This two-port network is particularly suited to BJTs as it lends itself easily to the analysis of circuit behaviour, and may be used to develop further accurate models. As shown, 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


• iin = Base current (ib)


• io = Collector current (ic)


• Vin = Base-to-emitter voltage (VBE)


• Vo = Collector-to-emitter voltage (VCE)

and the h-parameters are given by –

• hix = hie – The input impedance of the transistor (corresponding to the emitter resistance re).


• hrx = hre – Represents the dependence of the transistor's IB–VBE curve on the value of VCE. It is usually very small and is often neglected (assumed to be zero).


• hfx = hfe – The current-gain of the transistor. This parameter is often specified as hFE or the DC current-gain (βDC) in datasheets.


• hox = hoe – The output impedance of transistor. This term is usually specified as an admittance and has to be inverted to convert it to an impedance.

As shown, the h-parameters have lower-case subscripts and hence signify AC conditions or analyses. For DC conditions they are specified in upper-case. For the CE topology, an approximate h-parameter model is commonly used which further simplifies the circuit analysis. For this the hoe and hre parameters are neglected (that is, they are set to infinity and zero, respectively). It should also be noted that the h-parameter model as shown is suited to low-frequency, small-signal analysis. For high-frequency analyses the inter-electrode capacitances that are important at high frequencies must be added.