The fan-out is a measurement of the capacity of a logical Porte to be been used as source with other logical doors (of the same technology). Indeed in the majority of the electronic circuits, the logical doors are connected together to form a complex circuit. However the Technologie used for the realization of a logical door inevitably limits the number of entries of other logical doors that one can connect on the exit of this door, while keeping the levels of tension in the acceptable beaches, in order to guarantee a correct operation.
Traditionally, one defines the fan-out as: where is the current one which can provide or absorb the door, the current absorptive or provided by an entry.
All technologies do not have same the limits. For example old technology TTL is characterized by a fan-out from 2 to 10.
To calculate the fan-out of a circuit in technology CMOS is more delicate, because the running necessary to an entry is almost null. One thus finds figures raised for the fan-out, going until a few hundreds. It is necessary however to take account of another factor: indeed, each entry has a certain capacity. During the changes of level, the capacities of the various entries must be charged or discharged by the output current from the door which controls them. When one connects too many entries to the same exit, one thus slows down the operation of the system. To avoid that, it is recommended not to connect to an exit more than one ten entries.
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