Editing Bipolar junction transistor (section)

==== h-parameter model ====
[[File:BJT h-parameters (generalised).svg|frame|Generalized h-parameter model of an NPN BJT.<br />''Replace '''x''' with '''e''', '''b''' or '''c''' for CE, CB and CC topologies respectively.'']]

Another model commonly used to analyze BJT circuits is the ''[[two-port network#Hybrid parameters (h-parameters)|h-parameter]]'' model, also known as the hybrid equivalent model, closely related to the [[hybrid-pi model]] and the [[admittance parameters|y-parameter]] [[two-port network#Y-parameters (admittance parameters)|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 behavior, 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:
* Terminal 1, base
* Terminal 2, collector
* Terminal 3 (common), emitter; giving ''x'' to be ''e''
* ''i''<sub>i</sub>, base current (''i''<sub>b</sub>)
* ''i''<sub>o</sub>, collector current (''i''<sub>c</sub>)
* ''V''<sub>in</sub>, base-to-emitter voltage (''V''<sub>BE</sub>)
* ''V''<sub>o</sub>, collector-to-emitter voltage (''V''<sub>CE</sub>)
and the h-parameters are given by:
* ''h''<sub>ix</sub> = ''h''<sub>ie</sub> for the common-emitter configuration, the '''i'''nput impedance of the transistor (corresponding to the base resistance ''r''<sub>pi</sub>).
* ''h''<sub>rx</sub> = ''h''<sub>re</sub>, a '''r'''everse transfer relationship, it represents the dependence of the transistor's (input) ''I''<sub>B</sub>&ndash;''V''<sub>BE</sub> curve on the value of (output) ''V''<sub>CE</sub>. It is usually very small and is often neglected (assumed to be zero) at DC.
* ''h''<sub>fx</sub> = ''h''<sub>fe</sub>, the "forward" current-gain of the transistor, sometimes written ''h<sub>21</sub>''. This parameter, with lower case "fe" to imply small signal (AC) gain, or more often with capital letters for "FE" (specified as ''h''<sub>FE</sub>) to mean the "large signal" or DC current-gain (''β''<sub>DC</sub> or often simply ''β''), is one of the main parameters in datasheets, and may be given for a typical collector current and voltage or plotted as a function of collector current. See below.
* ''h''<sub>ox</sub> = 1/''h''<sub>oe</sub>, the output impedance of transistor. The parameter ''h''<sub>oe</sub> usually corresponds to the output admittance of the bipolar transistor 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 ''h''<sub>oe</sub> and ''h''<sub>re</sub> parameters are neglected (that is, they are set to infinity and zero, respectively). 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.

===== Etymology of ''h''<sub>FE</sub> =====
The ''h'' refers to its being an h-parameter, a set of parameters named for their origin in a '''''h'''ybrid equivalent circuit'' model (see above). As with all h parameters, the choice of lower case or capitals for the letters that follow the "h" is significant; lower-case signifies "small signal" parameters, that is, the slope the particular relationship; upper-case letters imply "large signal" or [[Direct Current|DC]] values, the ratio of the voltages or currents. In the case of the very often used ''h''<sub>FE</sub>:
* ''F'' is from '''''F'''orward current amplification'' also called the current gain. 
* ''E'' refers to the transistor operating in a ''common '''E'''mitter'' (CE) configuration. 
So h<sub>FE</sub> (or hFE) refers to the (total; DC) collector current divided by the base current, and is dimensionless. It is a parameter that varies somewhat with collector current, but is often approximated as a constant; it is normally specified at a typical collector current and voltage, or graphed as a function of collector current.

Had capital letters not been used for used in the subscript, i.e. if it were written ''h<sub>fe</sub>'' the parameter indicate small signal ([[Alternating Current|AC]]) current gain, i.e. the slope of the Collector current versus Base current graph at a given point, which is often close to the hFE value unless the test frequency is high.