2015-08-21
327
In the case of the y parameters, it can be seen that for I to equal yV, y must
represent admittance and have the units of siemens. At low frequencies the
y parameters can be considered to be purely conductive. We shall make this
assumption, so that ' y ' could be replaced by ' g '.
In the case of the h parameters, the situation is a little more complex. We have
to consider each term on its merits. Take the first equation of the pair; as the
left-hand side is in volts, each term on the right-hand side must also represent
voltage and have the units of volts. Thus, in the term h 11 I 1, h 11 must represent
impedance and have the unit of ohms. At low frequencies h 11 will be resistive.
In the term h 12 V 2, V 2 already represents volts and therefore h 12 must be
dimensionless. In fact, it represents a reverse voltage gain from output to
input, i.e. it gives feedback.
In the second equation, each term must represent current and have the units of
amperes. Consider the first term h 21 I 1. I 1 is already current and therefore h 21
is dimensionless. It, in fact, represents forward current gain. Finally, in the
term h 22 V 2, h 22 must represent admittance and have the units of siemens. At
low frequencies h 22 can be regarded as purely conductive.
The ' h ' stands for hybrid, reflecting the fact that the right-hand side of the h
equations are a mixture of voltages and currents, giving parameters that are a
mixture of impedance, admittance and dimensionless gains. Contrast this state
of affairs with the parameters of the z and y models, whose parameters have
units of ohms and siemens respectively.
