Open Collector Outputs

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Open Collector Outputs are increasingly common in digital chip design, operational amplifiers and micro-controller (Arduino) type applications, for either interfacing with other circuits or for driving high-current loads such as indicator lamps and relays which maybe incompatible with the electrical characteristics of the control circuit. But what does “open-collector” mean, and how can we use it within our circuit designs.

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. We can use bipolar transistors to operate as either an Amplifier, that is the output signal has a greater amplitude than the input signal, or more commonly, as a solid state “ON/OFF” type electronic switch.

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), with the “Common Emitter” configuration being the most common transistor operation when used for amplification (active region) or switching (cut-off or saturation regions). So this is the transistor configuration we will look at here in this tutorial about open collector outputs.

Consider the standard common emitter amplifier configuration shown below.

Common Emitter Configuration

common emitter configuration

Here in this single stage common emitter configuraton, a resistance is connected between the collector terminal of the transistor and the positive supply rail, VCC. The input signal is applied between the transistors base and emitter junction, with the emitter’s terminal connected directly to ground. Hence the descriptive term “common emitter”, (CE).

The bias current, IB required to turn “ON” the transistor is fed directly into the base of the NPN transistor via base resistor RB with the output signal, which is 180o-phase inverted relative to the input signal, taken from between the collector and emitter terminals. 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. In this way the NPN bipolar transistor can be used as an electronic switch performing the operation of inversion, because when the transitor is “OFF”, its collector terminal, and thus VCE, is “HIGH” at VCC level, and when it is “ON”, (conducting) the output taken across VCE will be “LOW”, which is the opposite switching conditions if we want to control a relay, solenoid or lamp, for example.

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.

NPN Open Collector Output

When an NPN bipolar transistor is operated in an Open Collector (OC or o/c) configuration, it is operated between being fully-ON, or fully-OFF, thus acting as an electronic solid-state switch. 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 its open collector output 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 an open collector output 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 the open collector output 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 de-energised the output must be pulled HIGH again by the use of an external “pull-up resistor” connected between its collector terminal and the supply voltage to stop the open collector terminal from floating about between HIGH (+V) and LOW (0V) when the transistor is OFF. The value of this pull-up resistor isn’t critical and will depend somewhat on the load current value required at the output, with resistive values ranging from of a few hundred to a few thousand ohms being typical. Thus for an NPN bipolar transistor, its open-collector outputs are current sinking outputs only.

Open Collector Transistor Circuit

open collector transistor circuit

The above image shows the typical arrangement of an open collector switching circuit which is useful for driving electromechanical type devices as well as many other switching applications. The NPN transistors base driving circuit could be any suitable analogue or digital circuit. The transistor’s collector is connected to the load to be switched, with the transistors emitter terminal connected directly to ground.

For an NPN-type open collector output, when a control signal is applied to the base of the transistor it turns ON, and the output, which is connected to the collector terminal, is pulled down to ground potential through the now conducting transistor junctions energising the connected load and turning it ON. Thus the transistor switches and passes the load current, IL which is determined using Ohm’s Law as:

Load Current, Iload  =  Voltage across load / Resistance of load

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.

Open Collector Output Example No1

A +5 volt digital output pin from an Arduino board is required to drive an electromechanical relay as part of a school project. If the relay’s coil is rated at 12 VDC, 100Ω and an NPN transistor used in its open collector configuration has a DC current gain (Beta) value of 50, calculate the base resistor required to operate the relay coil.

The current through the coil can be calculated using Ohm’s law as: I = V/R

bipolar transistor base current

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.

The voltage drop across the base-emitter junction (VBE) when the transistor is fully ON will be 0.7 volts. Thus the value of base resistor, RB required is calculated as:

transistor base resistor

Then the open-collector transistor circuit would be:

Open Collector Circuit

open collector circuit

While the NPN open collector transistor circuit produces a “current-sinking” output, that is the NPN transistors open collector terminal will sink the current to ground (0V), a PNP-type transistor can also be used in an open collector configuration to produce what is called a “current-sourcing” output.

PNP Open Collector Output

We have seen above that the main characteristic of an open-collector output is that the load signal is actively “pulled down” to ground level by the switching action of the NPN bipolar transistor when fully ON, and passively pulled back up again when OFF producing a current sink output. But we can create the opposite switching condition by using the open collector output of a PNP bipolar transistor to actively switch its output towards a voltage supply rail and use an externally connected “pull-down” resistor to passively pull the output low again when OFF.

For a PNP-type open collector output it is only possible for the transistor to switch the output HIGH to the supply rail, so its output terminal must be passively pulled “LOW” again by an externally connected “pull-down” resistor as shown.

Open Collector PNP Transistor Circuit

open collector pnp transistor circuit

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.

As well as using bipolar transistors in their open collector configuration, it is also possible to use n-channel and p-channel enhancement mode MOSFETs or IGBTs in their open source configuration. Unlike the bipolar junction transistor (BJT), which requires a base current to drive the transistor into saturation, the normally-open (enhancement) MOSFET requires a suitable voltage applied to its gate (G) terminal. The MOSFET’s source (S) terminal is connected directly to ground or the supply rail, while the open-drain (D) terminal is connected to the external load.

The use of MOSFETs (or IGBTs) as open-drain, (OD) devices follow the same requirements as for open-collector outputs, (OC) when drivng power loads, or loads connected to a higher voltage supply, in that the use of pull-up or pull-down resistors applies. The only difference being the MOSFETs channel thermal power rating and static voltage protection.

Open Drain Enhancement MOSFET Configuration

open drain mosfet configuration

Open Collector Outputs Summary

We have seen here in this tutorial about the open collector output that it can provide a current sink or current source output depending on the type of bipolar transistor, NPN-type or PNP-type, used.

When an NPN-type transistor is in its “ON” state, it will provide or “sink” a path to ground. When in the “OFF” state, its output terminal may float unless the open-collector output is connected via a pull-up resistor to the positive supply voltage. The reverse is true of a PNP-type transistor. When it is in its “ON” state, it will supply or “source” a path from the supply rail. When in the “OFF” state, its output terminal may float unless the open-collector output is connected via a pull-down resistor to ground (0V).

The advantage of open-collector outputs, or open-drain outputs is that the load to be switched or controlled can be connected to a voltage supply which is independant, and/or different from the supply voltage used by the controlling circuit, and that they can “sink” or “source” an externally-supplied voltage depending upon whether its to ground, or source. The only limit is the maximum allowable voltage and current ratings of the output switching transistor or e-MOSFET.


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