Open collector outputs are increasingly common in digital chip designs, operational amplifiers, and microcontroller (Arduino) type applications to interface with other circuits or to drive high current load control circuits such as indicator lights and relays that may not be electrically compatible . But what does “open collector” mean and how do we use it in circuit design.
We know from our previous tutorials that a Bipolar Junction Transistor, either NPN or PNP type, is a three-terminal device. These three terminals are identified as Emitter, Base and Collector.
We can use bipolar transistors to operate as amplifiers, where the output signal has a greater amplitude than the input signal, or, more commonly, as solid-state “on/off” type electronic switches.
Because a bipolar junction 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).
The “common emitter” configuration is by far the most common transistor configuration when used for amplification (active region) or switching (cutoff or saturation region). This is the transistor configuration we will look at in this tutorial with respect to the open collector output.
Consider the standard common emitter amplifier configuration shown below.
Common Emitter Configuration
Application Introduction of Open Collector Output in High Current Load Control Circuit
In this single-stage common-emitter configuration, the resistor is connected between the collector terminal of the transistor and the positive supply rail, V CC . The input signal is applied between the base and emitter junction of the transistor, with the emitter terminal directly connected to ground. Hence the descriptive term “common emitter” (CE).
The bias current IB required to turn the transistor “on” is fed directly into the base of the NPN transistor through the base resistor RB, and the output signal, 180° out of phase with respect to the input signal, is taken from the collector and emitter terminals.
This allows the transistor collector current to be controlled between zero (cutoff) and some maximum value (saturation). This is standard in a common emitter configuration and can either be biased as a Class A amplifier or as a logic on/off switch.
The problem here is that both the transistors and their collector load resistors are connected to a common supply voltage. The collector resistor RC is used here to allow the collector voltage VC to change value in response to the input signal applied to the base terminal of the transistor, thereby allowing the transistor to produce an amplified output signal. Since there is no RC, the voltage on the collector terminal will always be equal to the supply voltage.
As mentioned earlier, a bipolar junction transistor can operate between its cutoff and saturation regions when V BE is much less than 0.7 volts (zero base current) or much greater than 0.7 volts (maximum base current).
In this way, an NPN bipolar transistor can be used as an electronic switch that performs inverting operation, since when the transistor is in the “off” state, its collector terminal, and therefore V CE , is “high” at the V CC level. level”, while when it is “ON” the output on (conducting) V CE will be “LOW”, for example, if we want to control a relay, solenoid or lamp, this is the opposite switching condition.
One way to overcome transistor switching state inversion is to remove the collector resistor RC entirely and make the transistor collector terminal available for connection to some external load. This type of setup produces what is commonly referred to as 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 operates between 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 completely off, and when a suitable base bias voltage is applied, the transistor will be fully on. Therefore, when a transistor is operating between its cutoff region (OFF) and saturation region (ON), it does not behave as an amplifying device as it does when its active region is controlled.
Switching of the transistor between cutoff and saturation allows the open collector output the ability to drive externally connected loads that require higher voltages and/or currents than previously allowed by the common emitter configuration. The only limitation is the maximum allowable voltage and/or current value of the actual switching transistor.
The advantage of an open collector output then is that any output switching voltage can be obtained simply by pulling up the collector terminal to a 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 +12 volt power from the output of a +5 volt logic gate or an Arduino, Raspberry-Pi output pin.
But its disadvantage is that when the open-collector output is used to switch the input terminals of digital signals, gate circuits or electronic circuits, since the collector terminal of the triode has no output drive capability, an external pull-up resistor is generally required. This is because with an NPN transistor, it can only pull the output low to ground (0V) when powered on, and cannot return or push it back high again when it is off.
When powered down, the output must be pulled high again using an external “pull-up resistor” connected between its collector terminal and the supply voltage to prevent the open collector terminal from switching between high (+V) and low. Float between (0V) when the transistor is off.
The value of this pull-up resistor is not critical and depends somewhat on the value of load current required at the output, typically ranging from a few hundred to a few thousand ohms. Therefore, for an NPN bipolar transistor, its open collector output is only a current sinking output.