BJT Open Collector Outputs are really starting to show up more and more in the world of digital chip design, operational amplifiers, and those advanced micro-controller projects we love to tinker with, like Arduino.
We often use the BJT open collector outputs to connect with other circuits or to power high-current loads such as indicator lamps and relays, especially when those loads might not wok correctly with the electrical characteristics of our control circuits.
Here we will learn more about what “open-collector” actually means and how we can put it into practice in our circuit schematics.
From what we have covered in our earlier lessons, we know that both NPN and PNP bipolar junction transistors are three-terminal devices.
We can easily spot these three terminals as the Emitter, the Base, and the Collector.
Now bipolar transistors are pretty versatile, as they can work as an amplifier, which means they take an input signal and boost it to a higher amplitude for the output, or they can act as a solid-state electronic switch that we typically use for simple “ON/OFF” control.
As we know, the Bipolar Junction Transistor (BJT) is a device with three terminals. This allows us to setup and operate it in one of three different switching modes. These modes are called Common Base (CB), Common Emitter (CE), and Common Collector (CC).
Among these, the “Common Emitter” configuration is definitely the most widely used setup when it comes to amplification in the active region or for switching in the cut-off or saturation regions. This is the particular transistor configuration we are going to focus on in this tutorial about open collector outputs.
Now let us take a moment to consider the standard common emitter amplifier configuration that’s displayed below.
Analyzing the Common Emitter Configuration
In this single-stage common emitter (CE) configuration, we can see a resistor hooked up between the collector terminal of the transistor and the positive supply rail VCC.
The input signal is fed into the base-emitter junction of the transistor while the emitter terminal is connected directly to ground. This grounding of the emitter is why we call it the “common emitter” configuration, it is shared as a reference point.
To turn the transistor “ON” a bias current, IB, flows into the base of the NPN transistor through a base resistor RB. The output signal is then taken from between the collector and emitter terminals. What is interesting here is that the output signal ends up being 180° out of phase with the input signal which is a hallmark of this setup.
This configuration lets us control the transistor’s collector current IC, across its full range, from zero current when it is in the cut-off, to its maximum current in saturation.
This standard arrangement for a common emitter stage can be biased to work either as a class-A amplifier for signal amplification or as a simple ON/OFF switch for logic operations. It is a classic setup that we see over and over again in practical circuits.
But we have a situation here since both the transistor and its collector load resistance are all hooked up to one common supply voltage.
Now, this collector resistor RC, plays a pretty important role. It is there to let the voltage at the collector VC, change its value when we put an input signal at the base terminal of the transistor.
This is what enables the transistor to crank out an amplified output signal. Without RC in the picture, we would find that the voltage at the collector terminal would just stick at whatever the supply voltage is, and that would not be very useful!
Now remember the bipolar junction transistors? They can operate between their cutoff and saturation regions.
This happens when the base-emitter voltage or VBE, is way less than 0.7 volts (which means there is zero base current) or when it is way more than 0.7 volts (which indicates maximum base current), depending on what we are trying to do.
In this setup we can use an NPN bipolar transistor as an electronic switch that does this cool thing called inversion.
Here is how it works, when the transistor is “OFF,” its collector terminal—and therefore VCE—sits at a “HIGH” level, which is basically at VCC.
But when we flip it to “ON” (meaning it is conducting), the output across VCE drops down to “LOW.” This switching behavior is actually opposite to what we might want if we are trying to control something like a relay, solenoid, or lamp.
Now if we want to tackle this inversion issue with the transistor’s switching state, one way we can do that is by completely removing the collector resistor RC.
This way we can connect the transistor’s collector terminal directly to some external load. When we set it up like this, it gives us our well known open collector output configuration.
Understanding an Open Collector Output Circuit using an NPN BJT
Now let us analyze what happens when we use an NPN bipolar transistor in an Open Collector (OC or o/c) configuration.
Basically when we set it up this way, the transistor can switch between being fully-ON and fully-OFF, which means it’s acting like a solid-state electronic switch.
Here is how it works: when we do not apply any base bias voltage at all, the transistor is just sitting there in its fully-OFF state. But as soon as we apply a suitable base bias voltage, it instantly flips to fully-ON!
So when we are operating the transistor between its cut-off region (which is the OFF state) and its saturation region (the ON state), it is important to note that it is not working as an amplifying device like it would if we were using it in its active region.
Now one of the cool things about switching the transistor between cut-off and saturation is that it gives us open collector outputs the ability to drive external loads that need higher voltages and/or currents than what we could manage with a common emitter configuration.
The only thing we really have to keep in mind is that the switching transistor has specific limits on how much maximum voltage and current it can handle.
Advantages and Disadvantages
Now let us talk about the advantages of open collector outputs. One of the best things we can do with them is that we can get any output switching voltage we want just by pulling up the collector terminal to a single positive supply like we did before. Or if we need to, we can power the load from a totally separate supply rail.
For instance, let us say we want to drive a low-current lamp or relay that needs a +12 volt supply. We could easily do this using the output from a +5 volt logic gate or even an Arduino or Raspberry Pi output pin. This is really a useful advantage.
But here is an issue. The downside of using open collector outputs for switching digital signals, gates, or inputs in electronic circuits is that we usually need to connect an external pull-up resistor. Why? It is because the collector terminal of the transistor does not have any output drive capacity on its own.
You see, for an NPN transistor, it can only pull the output LOW to ground (which is 0V) when it is energized, but it can not push it back HIGH again when it is in the OFF state.
So when the transistor is switched OFF, its collector output just hangs there, floating between HIGH and LOW which is not what we want.
This means that when the transistor is de-energized, we need to pull the output back HIGH again using this external “pull-up resistor.”
We connect this resistor between the collector terminal and the supply voltage to keep that open collector terminal from floating around between HIGH (+V) and LOW (0V) when the transistor is OFF.
Now about that pull-up resistor, its value is not super critical, but it does depend on how much load current we need for the output stage.
Typically you will find the resistive values ranging from a few hundred ohms to a few thousand ohms being used.
So, actually, for an NPN bipolar transistor, its open-collector outputs work just like a current sinking outputs only.
Analyzing a Practical Open Collector BJT Circuit
If we take a look at the image above, it shows us the typical setup of an open collector switching circuit.
This kind of arrangement is super handy for driving electromechanical devices and it can also be used in a bunch of other switching applications.
Now when we talk about the NPN transistor’s base driving circuit, it could really be any suitable analog or digital circuit that we want to use.
In this setup we connect the transistor’s collector to the load that we want to switch on and off.
Meanwhile the emitter terminal of the transistor is hooked directly to ground.
Now for an NPN-type open collector output, when we apply a control signal to the base of the transistor, it turns ON.
This means that the output connected to the collector terminal gets pulled down to ground potential thanks to the transistor junctions that are now conducting.
As a result, this energizes the connected load and turns it ON!
So basically, what is happening here is that the transistor is switching and allowing the load current (which we call IL) to flow through.
We can calculate this load current using Ohm’s Law with this simple formula:
Load Current, Iload = Voltage across load / Resistance of load
Now, if we remove the positive base voltage from the transistor, which basically means we are turning it OFF, the NPN transistor stops conducting electricity.
As a result, whatever load we have connected—whether it is a relay coil, a solenoid, a small DC motor, a lamp, or something else—gets de-energized and also turns OFF.
This looks pretty straightforward, when the transistor is not active anymore, the load it was powering goes dark or stops working.
Now, what’s cool about this setup is that the output transistor can be used to control an externally connected load also.
The current-sink switching action of the NPN transistor’s open collector means that it can act like either an open circuit (which is the OFF state) or a short circuit (which is the ON state).
This gives us some flexibility in how we use it.
One of the big advantages here is that we do not have to connect the collector load to the same voltage potential as the driving circuit for the transistor.
This means that we can use different voltage levels for our loads.
For instance, we might want to use 12 volts or even 30 volts DC for our load without any issues!
Plus, we can keep things super simple because we can use the same basic digital or analog circuit to switch a variety of different loads.
All we need to do is swap out the output transistor for one that suits our needs. For example if we want to handle 6 VDC at 10mA, we could then use a BC547 transistor.
Or if we need something more powerful, like 40 VDC at 3 amperes, then we could go with a 2N3055 transistor.
We could even opt for an open collector Darlington transistor if that fits our project better.
Solving a BJT Open Collector Output Problem #1
Let say we want to use a +5 volt digital output pin from an Arduino board to drive an electromechanical relay for our project.
Now, this relay has a coil that’s rated at 12 VDC with a resistance of 150 ohms. To make this work, we are going to use an NPN transistor in its open collector configuration.
This setup is pretty common when we want to control higher voltage devices with a lower voltage signal.
Now our NPN transistor has a DC current gain Beta, and in this case, it is given as 60. This means that for every unit of current we apply to the base of the transistor, it can allow 60 units of current to flow from the collector to the emitter.
So, we first have to calculate the current that will flow through the relay coil when it is energized.
We can use Ohm’s Law for this which tells us that current (I) is equal to voltage (V) divided by resistance (R).
Since the relay coil is rated at 12 VDC and has a resistance of 150 ohms, we can plug those numbers into our formula:
I = V / R
So in our case:
I = 12 V / 150 Ω = 0.8 A
This means that when the relay is activated, we’ll have a current of 0.8 A flowing through the coil.
Next up, since our transistor has a Beta value of 60, we can find out how much base current (Ib) we need to provide in order to allow that collector current (IC) of 0.8 A to flow through the relay. We can use the formula:
IC = Beta × Ib
Rearranging this gives us:
Ib = IC / Beta
Now plugging in our values:
Ib = 0.8 A / 60 = 0.0013 A, or 1.3 mA
So now we know we need to supply about 1.3 mA of base current to get our relay working properly.
Finally let us calculate the base resistor (Rb) needed.
Since we are using a +5 volt output from the Arduino and we need about 1.3 mA flowing through the base resistor, we can use Ohm’s Law again.
First we need to figure out what voltage will be across the resistor when the transistor is turned ON.
The base-emitter voltage drop (Vbe) for an NPN transistor is typically around 0.7 volts when it is conducting.
So if we subtract this from our +5 volts output from the Arduino, we get:
Voltage across Rb = Vout – Vbe = 5 V – 0.7 V = 4.3 V
Now we can use Ohm’s Law once more to find out what resistor value we need:
Rb = Voltage across Rb / Ib
Plugging in our values gives us:
Rb = 4.3 V / 0.0013 A = 3,307.69 ohms
Since resistors come in standard values, we could round this up to a commonly available resistor value like 3k kΩ or 3.3 kΩ, depending on what may be available.
Circuit Diagram using the above Results
When we use an NPN transistor in this setup, it produces what we call a “current-sinking” output. This basically means that the open collector terminal of the NPN transistor will sink the current down to ground, which is 0V.
But here is something interesting, we can also use a PNP-type transistor in an open collector configuration and this will give us what is known as a “current-sourcing” output.
PNP Open Collector Output
Now we have already talked about how the main feature of an open collector output is that when the NPN bipolar transistor is fully ON, it actively pulls the load signal down to ground level.
Then when it is turned OFF, that signal gets passively pulled back up again through a pull up resistor, creating that current sink output we mentioned earlier.
But what if we want to flip the above condition around? We can easily do that simply by using the open collector output of a PNP bipolar transistor.
In this case, the PNP transistor actively switches its output towards a voltage supply rail.
When it is OFF, we need to use an externally connected “pull-down” resistor to passively pull the output low again. This setup allows us to create the opposite switching condition compared to what we see with the NPN configuration.
So with a PNP-type open collector output, the transistor can only switch the output HIGH to connect it to the supply rail.
However when we want to bring that output back down, we rely on that external “pull-down” resistor to do the job.
This means that whenever the PNP transistor is turned OFF, it is this resistor that helps pull the output terminal back down to a low state.
Analyzing an Open Collector PNP BJT Circuit
When we look at the configurations for either an NPN-type or a PNP-type open collector output, we notice that these setups can only actively pull their output LOW to ground or HIGH to a supply rail, depending on which type of transistor we are using and whether it is turned ON.
The collector terminal of these transistors needs some help to be pulled up or down passively.
This is where we bring in the pull-up or pull-down resistors to connect them to the output terminal, especially if the load we are working with is not capable of doing this on its own.
So depending on the type of output transistor we choose, we end up with either a current “sink” condition (in the case of the NPN) or a current “source” condition (for the PNP). This switching action really defines how our circuit behaves.
But in addition to using bipolar transistors in their open collector configuration, we also have the option to use n-channel and p-channel enhancement mode MOSFETs or even IGBTs in their open source configuration.
When we are working with bipolar junction transistors (BJTs), they need a base current to push the transistor into saturation.
On the contrary, if we ae using a normally-open (enhancement) MOSFET, then we just need to apply a suitable voltage to its gate (G) terminal to get things going.
Current is no longer crucial. In this setup, the source (S) terminal of the MOSFET is typically connected directly to ground or to the supply rail, while the open-drain (D) terminal connects to whatever external load we are working with.
Now when it comes to using MOSFETs (or even IGBTs) as open-drain (OD) devices, they have similar requirements to those of BJT open-collector outputs (OC), especially when we are driving power loads or connecting to a higher voltage supply.
This means that we still need to think about using pull-up or pull-down resistors in our circuit design.
The main difference here lies in the thermal power rating of the MOSFET’s channel and its ability to handle static voltage protection.
Conclusions
In this tutorial we have explored the concept of transistor open collector outputs, and one of the important things we learned is that they can provide either a current sink type or a current source type output.
This really depends on the type of bipolar transistor we decide to use, whether it is an NPN-type or a PNP-type.
When we have an NPN-type transistor in its “ON” state, it essentially creates a path for current to flow down to ground, which is what we mean by “sinking” the current.
However when this transistor is in its “OFF” state, we might find that its output terminal can float unless we connect the open collector output to the positive supply voltage through a pull-up resistor. This connection helps keep everything stable.
Now if we reverse the conditions by using a PNP-type transistor, we see the opposite behavior.
When this type of transistor is in its “ON” state, it supplies or “sources” a path from the supply rail to whatever load we are controlling.
But just like with the NPN, when it is in the “OFF” state, its output terminal can also float unless we connect it via a pull-down resistor to ground (which is 0V).
One of the big advantages of using BJT open collector outputs, or FET open drain outputs, is that we can connect the load we want to switch or control to a voltage supply that is independent from, or even different than, the supply voltage used by the controlling circuit.
This flexibility means that these outputs can either “sink” or “source” an externally supplied voltage depending on whether they are connected to ground or sourcing from the supply rail.
The only real limitation we need to keep in mind is the maximum allowable voltage and current ratings for the output switching transistor or the e-MOSFET.
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