Fan-out (or fan out or fan-out ) is a measure of the ability of a logic gate output transition (English logic gate) to control inputs of other logic devices of the same type, that is, the required electrical voltages and currents for the fault- free operation. The load factors of electronic components must be between fan-out and fan-in (or fan in or fanin ) can be distinguished.

The output of a logic device may vary according to type an H level ( electric current flows from the component out ) or an L level ( current flows in the device inside ) actively drive. For the driving capability at the output fan-out is a standardized parameter that refers to the load of a standard component of each logic family.

To an input of a logic device, an L level and an H level may be applied. At a high level, current flows into the device, while the L level, current flows from the device out to a. For this behavior to the input of a logic device fan-in is a standardized parameter.

The fan-out and fan-in can be specified for logic gates and flip-flops.


Load behavior

Connected to the output of a single device only a maximum number of additional component inputs can be connected so that the levels for high and low can be met (For TTL devices, these are, for example, 0 V to 0.8 V for " low", and 2.4 V to 5 V for "High "). This number of connectable component inputs is called fan-out of a component.

Most TTL gates can control up to 10 digital gates (or devices with the same load behavior ) at their outputs. Therefore, a typical TTL gate a fan-out of 10, these values ​​are dependent on the logic families and the individual component types of a logic family.

When exceeding the maximum fan -out, the very low output impedance associated with the pull- down resistor can no longer reach the high state of the logic signals. This allows subsequent devices ( gates), do not interpret the incoming signals properly (from the output). This can be avoided by interposing an additional driver component.


The fan-out is calculated from the ratio of the output current and the input current. So the rounded value of the quotient of dividing the results by fan-out:

Exceeding the fan - outs

We consider the case when the maximum allowable fan-out of a device is exceeded. Each digital device has a maximum current drive capability at the output of an L level ( the output is switched to ground) and for an H level ( the output is switched to the supply voltage ). If the maximum current is exceeded at the output of the sending device, it can come to power overload and in the worst case to the destruction of the device.

If it does not come in damage to the device, but has an overload but repercussions on the switching behavior of the device. When an overload of a component can lead to changes in the logic level. For example, consider the input range of an L- level of a component. When the device transmits an L signal, is, for example, at the output to a voltage range between 0 V and 0.8 V. When overloaded, can bear an output level that is greater than the maximum value of 0.8V. On subsequent devices, there is a risk that this voltage level is excessive in the non- defined input voltage range. The same case can occur at the H level. For example, a voltage range of 2.4 V to 5.0 V is provided to the H level. When overloaded, the actual output voltage value may be below the 2.4 V, which also may be the recipient component in undefined input voltage range and thus may lead to a malfunction.

As a further consequence, it can come at a very slow increase in voltage, which manifests itself in a bad slope. Inadequate slope of the transmitted signal at the receiver can then lead to problems, since the switching process is not defined input voltage range of the following component is crossed too slow.

Oversized fan-out

Consider the second case where the fan-out is many times more than the required fan-out. Very strong driver components with a high fan-out usually have a high current driving capability. This has on the signal level special advantages, because these components of the actual output voltage value is at the L level usually at the lower definition limit of 0.0 V and at the H level usually on the upper definition limit to the amount of the supply voltage (analogous to the above example, usually slightly below 5.0 V). For the absolute voltage level does this affect positive.

It comes in this application usually also very fast switching processes, which is reflected in a very high slope. Because of this very fast transients can occur for emission of noise (electromagnetic compatibility). Furthermore, it may lead to transients in the output signals. Due to construction technique of the circuit ( usually in the form of printed circuit boards ) these parasitic factors ( capacitors and inductors ) come with fast switchover amplified the effect and cause interference.


The input of a logic device flows out as described above at a high level in flow and at a low level current. Depending on the design of a device, these currents may be different. In order to standardize these flows, there is the fan-in. This indicates how big is the actual power of a specific component with respect to a standardized comparison component of the same family.

In a standardized component of logic family is the fan-in 1 is for an IC family (such as transistor-transistor logic ) of the fan-in for all component types are equal, can be any device with a fan-in ( input load factor) of 1 are accepted.

Components of the considered logic family which type of construction, for example, have the double input currents, have a fan-in of 2 (based on the standardized component of logic family). When calculating the load response at the output of the upstream device driver this double current load must be considered.

Components with a high fan-in have higher input currents result. This leads to slower switching operations and to increase the burden of the sending device. Furthermore, this voltage value is reduced to an H level, while the voltage value for an L level is raised. In the extreme case, the voltage level may be in the non-defined input voltage range.