Open collector
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Open-collector is one of the many different electrical input/output standards in digital designs today.
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[edit] Definition
Open-collector refers to the collector terminal of a BJT transistor. (Analogous to the open drain in MOSFET devices.) The collector is "open" in that it is not connected to a positive voltage inside the logic chip, and requires an external pull-up resistor. In the logic 1 state, the collector has high impedance, prohibiting current flow. There is no voltage drop over the pull-up resistor, so (if there is no external load) the output is at the same level as the source on the other end of the resistor. In the logic 0 state, the collector sinks current, which is pulled from the voltage source through the pull-up resistor. A voltage differential develops, and the output line is held at the low voltage.
[edit] 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" gate: that is, it 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.
Open collector BJTs (which are usually NPN) exhibits faster fall time and greater current handling capabilities than FET, but have other problems. One of them is that they consume a lot of power.
[edit] Possible problems
As mentioned above, open-collector devices can handle more current, but they also have higher current minimums for correct operation. Even in the "off" state, open-collectors have some few nanoamps of leakage current (the exact amount varies with temperature.)
[edit] References
- Open Collector Outputs.
- Chapter 4: Circuits. Honeywell Solid State Technical Documentation.
- Horowitz, Paul, Winfield Hill (1989). The Art of Electronics, Second Edition, Cambridge University Press.
