• The key to transistor
small-signal analysis is the use of the equivalent circuits
(models). A MODEL IS A
COMBINATION OF CIRCUIT ELEMENTS
LIKE VOLTAGE OR
CURRENT SOURCES, RESISTORS, CAPACITORS
etc, that best approximates the
behavior of a device under specific operating
conditions. Once the model (ac
equivalent circuit) is determined, the schematic
symbol for the device can be
replaced by the equivalent circuit and the basic
methods of circuit analysis
applied to determine the desired quantities of the
network.
• Hybrid equivalent network –
employed initially. Drawback – It is defined for a set
of operating conditions that
might not match the actual operating conditions.
• re model: desirable, but does
not include feedback term
• model: model of choice.p• Hybrid
AC equivalent of a
network
• AC equivalent of a
network is obtained by:
• Setting all dc sources
to zero and replacing them by a short – circuit equivalent
• Replacing all
capacitors by short – circuit equivalent
• Removing all elements
bypassed by the short – circuit equivalents
• Redrawing the network
in a more convenient and logical form.
re model
• In re model, the transistor
action has been replaced by a single diode between
emitter and base terminals and a
controlled current source between base and
collector terminals.
• This is rather a simple equivalent circuit for a
device
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