Current Sinking and Sourcing in TTL Circuits

Current sinking and sourcing is a term used in the field of digital electronics to refer to the direction of direct current (DC) flow between Transistor-Transistor Logic (TTL) gates. It refers to the flow of conventional current from the positive, which is the sourcing end, to the ground, which is the sinking end.

These concepts and terms have expanded to cover the wider electronics field to include the flow of current through loads such as LEDs. The flow of direct current usually involves one part of a circuit sourcing current and another part sinking that current to complete the circuit. In TTL logic gates, the sinking and sourcing action occurs simultaneously when one gate feeds a signal to another gate.

In TTL logic gates, there must be a voltage source and a ground reference so that current may flow to complete a circuit. The input terminal of a logic gate can either sink or source DC current, and the output terminal can sink or source DC current as well.

Usually a sourcing output connects to a sinking input. This is the easiest action for beginners and students to visualise and understand. However, it is also possible to have a sinking output and a sourcing input.

Source Current

Source current refers to the amount of current a pin can supply to drive an externally connected load such as an LED. Sourcing occurs through a PNP transistor, which provides a path to the positive potential.

Sink Current

Sink current refers to the maximum amount of current that a pin can absorb through a load connected to an external supply. Sinking occurs through an NPN transistor, which provides a path to zero potential or ground.

TTL Device Example

A TTL device can sink or source current, and the 7400 series of logic chips can sink up to 16 mA, and source up to -400 µA. The negative sign indicates conventional flow of current, which is out of the device. Since the output currents are ten times larger than input currents, a single TTL output can drive up to ten TTL inputs.

Chart

History

Another name for TTL was current sinking logic, because of its ability to sink considerable amounts of current. The chart above shows that Schottky TTL can provide a maximum drive current of 20 mA.

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