Small-signal models

hybrid-pi model

h- parameter model

The h parameter model of an NPN BJT

Another model generally used to analyze BJT circuits is the "h-parameter" model, closely related to y-parameter two-port and hybrid-pi model, but using input current and output voltage as independent variables, instead of input and output voltages. This two port network is particularly appropriate to BJTs as it lends itself easily to analysis of circuit behavior, and can be used to develop further accurate models. As shown, the term "x" in model represents a different  BJT  lead depending on topology used. For common emitter mode several symbols take on specific values as:

• x = 'e' because it is a common emitter topology
• Terminal 1 = Base
• Terminal 2 = Collector
• Terminal 3 = Emitter
• i_i = 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 h-parameters can be given by:

• h_ix = h_ie - The input impedance of transistor (corresponding to the emitter resistance r_e).

• h_rx = h_re - Represents dependence of transistor's I_B-V_BE curve on value of V_CE.

It is very small usually and is often neglected (assumed to be zero).

• h_fx = h_fe - The current-gain of transistor. This parameter is 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 the impedance.

As shown, h-parameters have lower case subscripts and hence signify AC conditions or analyses. For DC conditions they are specified in upper-case. For CE topology, the approximate h-parameter model is commonly used which simplifies the circuit analysis further. For this the hoe  and hre  parameters are neglected (which means 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 which are important at the high frequencies should be added.

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