Open Collector Meaning

[Torie Crivello]

These open outputs configurations are often used for digital applications when the transistor acts as a switch, to allow for logic-level conversion, wired-logic connections, and line sharing. External pull-up/down resistors are typically required to set the output during the Hi-Z state to a specific voltage. Analog applications include analog weighting, summing, limiting, and digital-to-analog converters.

The NPN BJT (n-type bipolar junction transistor) and nMOS (n-type metal oxide semiconductor field effect transistor) have greater conductance than their PNP and pMOS relatives, so may be more commonly used for these outputs. Open outputs using PNP and pMOS transistors will use the opposite internal voltage rail used by NPN and nMOS transistors.

For NPN open collector outputs, the emitter of the NPN transistor is internally connected to ground,[1] so the NPN open collector internally forms either a short-circuit (technically low impedance or "low-Z") connection to the low voltage (which could be ground) when the transistor is switched on, or an open-circuit (technically high impedance or "hi-Z") when the transistor is off. The output is usually connected to an external pull-up resistor, which pulls the output voltage to the resistor's supply voltage when the transistor is off.

For PNP open collector outputs, the emitter of the PNP transistor is internally connected to the positive voltage rail, so the collector outputs a high voltage when the transistor is on or is hi-Z when off. This is sometimes called "open collector, drives high".

An nMOS open drain output connects to ground when a high voltage is applied to the MOSFET's gate, or presents a high impedance when a low voltage is applied to the gate. The voltage in this high impedance state would be floating (undefined) because the MOSFET is not conducting, which is why nMOS open drain outputs require a pull-up resistor connected to a positive voltage rail for producing a high output voltage.

Microelectronic devices using nMOS open drain output may provide a 'weak' (high-resistance, often on the order of 100 kΩ) internal pull-up resistor to connect the terminal in question to the positive power supply of the device so their output voltage doesn't float. Such weak pullups reduce power consumption due to their lower V 2 / R \displaystyle V^2/R ohmic heating and possibly avoid the need for an external pull-up. External pullups may be 'stronger' (lower resistance, perhaps 3 kΩ) to reduce signal rise times (like with IC) or to minimize noise (like on system RESET inputs).

Because the pull-up resistor is external and does not need to be connected to the chip supply voltage, a lower or higher voltage than the chip supply voltage can be used instead (provided it does not exceed the absolute maximum rating of the chip's output). Open outputs are therefore sometimes used to interface different families of devices that have different operating voltage levels. The open collector transistor can be rated to withstand a higher voltage than the chip supply voltage. This technique is commonly used by logic circuits operating at 5 V or lower to drive higher voltage devices such as electric motors, LEDs in series,[8] 12 V relays, 50 V vacuum fluorescent displays, or Nixie tubes requiring more than 100 V.

Another advantage is that more than one open collector output can be connected to a single line. If all open collector outputs attached to a line are off (i.e. in the high-impedance state), the pull-up resistor will be the only device setting the line's voltage, and will pull the line voltage high. But if one or more open collector outputs attached to the line are on (i.e. conducting to ground), since any one of them are strong enough to overcome the pull-up resistor's limited ability to hold the voltage high, the line voltage will instead be pulled low. This wired logic connection has several uses.

One problem such open-collector and similar devices with a pull-up resistor is the resistor consumes power constantly while the output is low. Higher operating speeds require lower resistor values for faster pull-up, which consume even more power.

Pseudo open drain (POD) drivers have a strong pull-down strength but a weaker pull-up strength. The purpose is to reduce the overall power demand compared to using both a strong pull-up and a strong pull-down.[10] A pure open drain driver, by comparison, has no pull-up strength except for leakage current: all the pull-up action is on the external termination resistor. This is why the term "pseudo" has to be used here: there is some pull-up on the driver side when output is at high state, the remaining pull-up strength is provided by parallel-terminating the receiver at the far end to the HIGH voltage, often using a switchable, on-die terminator instead of a separate resistor.

DDR4 memory uses POD12 drivers but with the same driver strength (34 Ω/48 Ω) for pull-down (RonPd) and pull-up (RonPu). The term POD in DDR4 referring only for termination type that is only parallel pull-up without the pull-down termination at the far end.[clarification needed] The reference point (VREF) for the input is not half-supply as was in DDR3 and may be higher. A comparison[15] of both DDR3 and DDR4 termination schemes in terms of skew, eye aperture and power consumption was published in late 2011.[relevant?]

We know from our previous tutorials that a bipolar junction transistor, whether it is an NPN type or a PNP type, is a 3-terminal device. These three terminals are identified as being the Emitter, the Base, and the Collector.

Since the Bipolar Junction Transistor (BJT) is a 3-terminal device, it can be configured and operated in one of three different switching modes. These being Common Base (CB), Common Emitter (CE), and Common Collector (CC).

This allows the transistors collector current to be controlled between zero (cut-off) and some maximum value (saturation). This is the standard arrangement for the common emitter configuration, either biased to operate as a class-A amplifier or as a logical ON/OFF switch.

The problem here is that both the transistor and its collector load resistance are linked together to one common supply voltage. The collector resistor, RC is used here to allow the collectors voltage, VC to change value in response to an input signal applied to the transistors base terminal, thus allowing the transistor to produce an amplified output signal. As without RC the voltage on the collector terminal would always be equal to supply voltage.

As mentioned earlier, a bipolar junction transistor can be operated between it cut-off and saturation regions when VBE is much less than 0.7 volts (zero base current), or when it is much greater than 0.7 volts (maximum base current) respectively.

One way to overcome this inversion of the transistors switching state is to remove the collector resistor, RC completely and have the transistors collector terminal available to be connected to some external load. This type of set-up produces what is commonly called an open collector output configuration.

That is with no base bias voltage applied, the transistor will be fully-OFF, and when a suitable base bias voltage is applied, the transistor will be fully-ON. So when the transistor is operated between its cut-off (OFF) and saturation regions (ON), it does not operate as an amplifying device as it would do if controlled in its active region.

The switching of the transistor between cut-off and saturation allows for the open collector outputs the capability of driving external connected loads which require higher voltages and/or currents than allowed by the previous common emitter configuration. The only limit is the maximum allowable voltage and/or current values of the actual switching transistor.

Then the advantage of open collector outputs is that any output switching voltage can be obtained simply by pulling up the collector terminal to the single positive supply as before, or by powering the load from a separate supply rail. For example, you might want to drive a low-current lamp or relay that requires a +12 volt supply from the output of a +5 volt logic gate or Arduino, Raspberry-Pi output pin.

However, the disadvantage is that when using open collector outputs to switch digital signals, gates, or inputs of electronic circuits, an externally connected pull-up resistor is generally required as the collector terminal of the transistor has no output drive capacity. This is because for an NPN transistor, it can only pull the output LOW to ground (0V) when energised, it cannot return or push it back HIGH again when it is in the OFF state.

When the transistors positive base drive is removed (OFF), the NPN transistor stops conducting and the load, which could be a relay coil, solenoid, small dc motor, lamp, etc. is de-energised and also turns OFF. Then the output transistor can be used to control an externally connected load as the current-sink switching action of the NPN transistors open-collector acts as either an open circuit (OFF) or a short circuit (ON).

The advantage here is that the collector load does not need to be connected to the same voltage potential as the transistors driving circuit, as it could use a lower or higher voltage potential, for example 12 volts, or 30 volts DC.

Also the same simple digital or analogue circuit can be used to switch many different loads by simply changing the output transistor. For example, 6 VDC at 10mA (2N3904 transistor), or 40 VDC at 3 amperes (2N3506 transistor), or even use an open collector Darlington transistor.

Thus for an NPN transistor with a DC current gain of 50, a base current of 2.4mA is required, ignoring the collector-emitter saturation voltage, (VCE(sat)) of about 0.2 volts. Recall that a transistors DC current gain is its specification of how much base current is required to produce the resulting collector current.

Then we can see that an NPN-type or a PNP-type open collector output configuration can only actively pull its output LOW to ground, or HIGH to a supply rail (depending on transistor type) when ON, but its collector terminal must be pulled up or down passively by the use of a pull-up or pull-down resistor connected to its output terminal if the connected load is not able to do this. The type of output transistor used, and therefore its switching action, produces either a current sink or a current source condition.

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