Open-collector outputs are increasingly common in digital chip designs, operational amplifiers, and microcontroller (Arduino) type applications for connecting with other circuits or driving high-current load control circuits such as leds and relays that may be incompatible with electrical characteristics. But what does "open collector" mean, and how can we use it in circuit design?
As we know from our previous tutorials, bipolar junction transistors, whether NPN or PNP, are three-terminal devices. The three terminals are identified as Emitter, Base, and Collector.
We can use bipolar transistors to operate as amplifiers, i.e. the amplitude of the output signal is greater than the input signal, or, more commonly, as solid-state "on/off" type electronic switches.
Because the bipolar junction transistor (BJT) is a 3-terminal device, it can be configured and operated in one of three different switching modes. They are common base (CB), common emitter (CE), and common collector (CC).
When used for amplification (active region) or switching (cut-off or saturation region), the "common emitter" configuration is by far the most common transistor configuration. This is the transistor configuration we will see in this tutorial regarding the open collector output.
Consider the standard common emitter amplifier configuration shown below.
In this single-stage common-emitter configuration, the resistance is connected between the collector terminal of the transistor and the positive power rail V CC. The input signal is applied between the transistor base and the emitter junction, and the emitter terminal is directly grounded. Hence the descriptive term "common emitter" (CE).
The bias current I B required to "turn on" the transistor is fed directly into the base of the NPN transistor through the base resistance R B, and the output signal is reversed 180 ° relative to the input signal, taken from the collector and emitter terminals.
This allows the transistor collector current to be controlled between zero (cut-off) and some maximum value (saturation). This is standard in common emitter configurations and can be biased either 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 connected to a common supply voltage. The collector resistance R C is used here to allow the collector voltage V C to change in response to the input signal applied to the base terminal of the transistor, thus allowing the transistor to produce an amplified output signal. Since there is no R C, the voltage on the collector terminal will always be equal to the supply voltage.
As mentioned earlier, when the VBE is much less than 0.7 volts (zero base current) or much more than 0.7 volts (maximum base current), a bipolar junction transistor can operate between its cutoff and saturation regions.
