OPEN COLLECTOR

Open-collector is one of the many different electrical input/output standards in digital designs today.

Contents

Applications of open-collector devices

See also

References

Applications of open-collector devices

One useful property is that the resistor need not be connected to the same voltage as the chip supply voltage (Vcc); a lower or higher voltage can be used instead. Open collector circuits are therefore sometimes used to interface two series of devices that have different operating logic levels (voltages.)
Another advantage is that more than one open-collector output can be attached to a single wire. If all outputs attached to the wire are in the high-impedance/logic 1 state, the pull-up resistor will hold the wire in a high voltage state. If at least one of the device outputs is in the ground/logic 0 state, it will sink current and bring the line voltage low.
By tying the output of several open collectors together, the common line becomes a "wired AND" (positive-true logic) or "wired OR" (negative-true logic) gate. A "wired AND" behaves like the boolean AND of the two (or more) input gates in that it will be logic 1 whenever both (all) are in the high impedance state, and 0 otherwise. A "wired OR" behaves like the boolean OR for negative-true logic, where the output is LOW if any one of the inputs is low.
Open collector BJTs (which are usually NPN) exhibit faster fall time and greater current handling capabilities than FET (Open drain), but have other problems. One notable problem is that they consume a lot of power. Open-collector devices have higher current minimums for correct operation, and even in the "off" state, open-collectors have a few nanoamps of leakage current (the exact amount varies with temperature.)

See also

★ Open drain

References

★ Open Collector Outputs

★ Chapter 4: Circuits

★ The Art of Electronics, , Paul, Horowitz, Cambridge University Press, 1989,