Fan-in is the number of inputs of an electronic logic gate. For instance the 'fan-in' for the AND gate shown below is 3. Logic gates with a large fan-in tend to be slower than those with a small fan-in, because the complexity of the input circuitry increases the input capacitance of the device.
Fan-out specifies the number of standard loads that the output of a typical gate can drive without impairing its normal operation. A standard load is usually defined as the amount of current needed by an input of another similar gate of the same family.
The fan-out is calculated from the amount of current available in the output of a gate and the amount of current needed in each input of a gate.
Consider the connections shown in Fig. 10-3. The output of one gate is connected to one or more inputs of other gates. The output of the gate is in the high voltage level in Fig. 1O-3(a). It provides a current source IOH to all the gate inputs connected to it. Each gate input requires a current 1m for proper operation. Similarly, the output of the gate is in the low voltage level in Fig. 1O-3(b). It provides a current sink IOL for all the gate inputs connected to it.
Each gate input supplies a current I’ll .. The fan-out of the gate is calculated from the ratio IoH/lm or lad IlL, whichever is smaller. For example, the standard TTL gates have the following values for the currents:
The two ratios give the same number in this case:
Therefore, the fan-out of standard TTL is 10. This means that the output of a TTL gate can be connected to no more than ten inputs of other gates in the same logic family.
Otherwise, the gate may not be able to drive or sink the amount of current needed from the inputs that are connected to it.