Open collector outputs are becoming increasingly common in applications such as digital chip design, operational amplifiers and microcontroller (Arduino) type applications to interface with other circuits or drive high current loads such as indicator lamps and relays.
From our previous training, we know that a bipolar connection transistor is a 3-terminal device, whether it is an NPN type or pnp type. These three terminals are defined as emitters, bases and collectors. We can use bipolar transistors to work as an amplifier, that is, the output signal has a larger amplitude than the input signal. More commonly, solid state "on/off" type is used as an electronic switch.
Since the Bipolar connection transistor (Bjt) is a 3-terminal device, it can be configured and operated in one of three different switching modes. These are common base (CB), common emitter (CE) and common collector (CC), and the "common emitter" configuration is the most common transistor process when used for amplification (active zone) or switching (cutting or saturation zones). So this is the transistor configuration that we will look at in this tutorial about open collector outputs.
The standard common emitter amplifier configuration shown below is provided.
Common Emitter Configuration
In this single-stage joint emitter configuration, a resistance is connected between the collector terminal of the transistor and the positive feed rail VCC. The input signal is applied between the base and emitter connection of the transistors, and the terminal of the emitter is connected directly to the soil. Therefore, the descriptive term is "common emitter" (CE).
The IB biasing current required to "turn on" the transistor is fed directly into the base of the NPN transistor via rb base resistance with an inverted output signal of 180o-phase according to the input signal received between the collector and emitter terminals. This allows transistors to control collector current between zero (cutting) and some maximum value (saturation). This is the standard arrangement for the common emitter configuration, which is biasing to work as a Class A amplifier or as a logical On/Off switch.
The problem here is that both the transistor and the collector load resistance depend on a common supply voltage. Collector resistance, RC is used here to allow the collector voltage to change the value in response to an input signal applied to the base terminal of the VC transistors. Thus, it allows the transistor to produce a reinforced output signal. As without RC, the voltage in the collector terminal will always be equal to the feed voltage.
As already mentioned, a bipolar connection transistor can be operated between cutting and saturation zones when VBE is much less than 0.7 volts (zero base current) or much larger than 0.7 volts (maximum base current), respectively. In this way, when the PNP bipolar transistor transitor is OFF", the collector terminal and thus the free training can be used as an electronic switch without performing the reversal process because it is HIGH " at the VCC level.
One way to overcome this inversion of the switching state of transistors is to completely remove the collector resistance, RC and have a transistor collector terminal to connect to external loads of the base. This type of setup produces what is often 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 completely on or off, thus functioning as an electronic solid state switch. That is, when the base biasing voltage is not applied, the transistor will be completely turned off, and when a suitable base biasing voltage is applied, the transistor will be completely on. Therefore, when the transistor is operated between the cutting (closed) and saturation zones (on), it does not work as an amplification device, as it is when checked in its active zone.
Switching the transistor between cutting and saturation allows the open collector output to drive externally connected loads that require higher voltages and/or currents than the previous common emitter configuration allowed. The only limit is the maximum permissible voltage and/or current values of the actual switching transistor.
The advantage of then open collector output is that any output switching voltage can be achieved by pulling the collector terminal into a single positive feed as before or feeding the load through a separate supply rail. For example, you might want to use a +5 volt logic gate or a low-current lamp or relay that requires a +12 volt feed from the output of the Arduino, Raspberry-Pi output pin.
The disadvantage, however, is that when using open collector output to change digital signals, doors or inputs of electronic circuits, an externally connected tensile resistance is usually required, since the transistor collector terminal does not have output drive capacity. This is because for an NPN transistor it can only draw output to low soil (0V) when energized. It cannot return when it is closed or push back high again.
When the power is cut, the output must be pulled high again using an external "pull-up resistance" connected between the collector terminal and the supply voltage to stop the open collector terminal from swimming between high (+v) and low (0V) when the transistor is off. The value of this pull-up resistance is not critical and will depend slightly on the load current value required at the output.
Open Collector Transistor Circuit
The image above 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 basic drive circuit of NPN transistors can be any suitable analog or digital circuit. The collector of the transistor is connected to the load to be switched when the emitter terminal of the transistors is connected directly to the soil.
For an NPN type open collector output, a control signal is applied to the transistor's base, and the output connected to the collector terminal is drawn to the soil potential through the now conductive transistor junctions that energize and open the connected load. Thus, the transistor changes and passes the load current determined by using the Ohm Act:
When the positive base drive of transistors is removed (off), the NPN transistor stops conductivity and load, which can be relay coil, solenoid, small dc motor, lamp, etc. without energy and closes at the same time. The Output transistor can then be used to control an externally connected load, since the current absorbent switching motion of the open collector of NPN transistors functions as an open circuit (off) or short circuit (on).
The advantage here is that the collector load does not need to be connected to the same voltage potential as the drive circuit of the transistors. Because it can use a lower or higher voltage potential, such as 12 volts or 30 volts DC. In addition, the same simple digital or analog circuit can be used to change many different loads by changing the output transistor. For example, use 6 VDC (2n3904 transistor) at 10ma or 40 VDC (2n3506 transistor) in 3 amps, or even an open collector Darlington transistor.
Open Collector Output Example
To drive an electromechanical relay as part of a school project requires a digital output pin of + 5 volts from a arduino card. If the relay coil is rated 12 VDC, 100Ω, and an NPN transistor used in the open collector configuration has a 50 DC current gain (Beta) value, calculate the base resistance required to operate the relay coil.
The current along the coil can be calculated using the Ohm Act as follows: I = V / R
Therefore, a base current of 2.4 ma is required for an NPN transistor with a 50 DC current gain, ignoring the collector-emitter saturation voltage (VCE(sat)) of approximately 0.2 volts. Remember that a transistor DC current gain is a specification of how much base current is required to produce the resulting collector current.
Voltage Drop will be 0.7 volts along the base-emitter junction (VBE) when the transistor is fully open. Thus, the value of the required RB base resistance is calculated as follows:
Open Collector Circuit
While the NPN open collector transistor circuit produces a "current absorber" output, the open collector terminal of NPN transistors immerses the current in the ground (0V), while the PNP type transistor can also be used in open collector configuration to produce what is called the "current source" output.
PNP Open Collector Output
Above, we found that the main characteristic of an open collector output is that the load signal is actively "pulled down" to ground level by switching the NPN bipolar transistor when fully open, and passively pulls up again when producing a current sink output. But by using the open collector output of a PNP bipolar transistor to actively change its output towards a voltage supply rail, we can create the opposite switching condition and use an externally connected "pull down" resistance to passively pull the output.
For an open collector output of type PNP, it is possible for the transistor to change the output to high on the feed rail. Therefore, the output terminal should be passively pulled "low" again with an externally connected "pull down" resistance, as shown.
Open Collector PNP Transistor Circuit
We can see that lowering the transistor to LOW or HIGH depending on the type of transistor. The type of output transistor used and therefore the switching action produces a current absorber or a current source condition.
In addition to using bipolar transistors in open collector configurations, it is also possible to use n-channel and p-channel development mode Mosfets or Igbts in open source configurations. Unlike the bipolar connection transistor (bjt), which requires a base current to drive the transistor to saturation, normally open (enhancement) MOSFET requires a suitable voltage applied to the gate (G) terminal. The welding (s) terminal of MOSFET is connected directly to the soil or feed rail, while the drain (D) terminal is connected to the external load.
The use of mosfets (or Igbts) as drain, (OD) devices follows the same requirements as for open collector outputs, (OC) power loads or loads connected to a higher voltage source. The only difference is the nominal thermal power and static voltage protection of the MOSFET channel.