The problem here is that both the transistor and its collector load resistance are linked together to one common supply voltage. 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. This allows the transistors collector current to be controlled between zero (cut-off) and some maximum value (saturation). These being Common Base (CB), Common Emitter (CE), and Common Collector (CC). Since the Bipolar Junction Transistor (BJT) is a 3-terminal device, it can be configured and operated in one of three different switching modes. 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. These three terminals are identified as being the Emitter, the Base, and the Collector. 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. But what does “open-collector” mean, and how can we use it within our circuit designs.
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.
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