![]() ![]() When de-energised the output must be pulled HIGH again by the use of an external “pull-up resistor” connected between its collector terminal and the supply voltage to stop the open collector terminal from floating about between HIGH (+V) and LOW (0V) when the transistor is OFF. This is because for an NPN transistor, it can only pull the output LOW to ground (0V) when energised, it cannot return or push it back HIGH again when it is in the OFF state. However, the disadvantage is that when using open collector outputs to switch digital signals, gates, or inputs of electronic circuits, an externally connected pull-up resistor is generally required as the collector terminal of the transistor has no output drive capacity. For example, you might want to drive a low-current lamp or relay that requires a +12 volt supply from the output of a +5 volt logic gate or Arduino, Raspberry-Pi output pin. Then the advantage of open collector outputs is that any output switching voltage can be obtained simply by pulling up the collector terminal to the single positive supply as before, or by powering the load from a separate supply rail. The only limit is the maximum allowable voltage and/or current values of the actual switching transistor. The switching of the transistor between cut-off and saturation allows for the open collector outputs the capability of driving external connected loads which require higher voltages and/or currents than allowed by the previous common emitter configuration. So when the transistor is operated between its cut-off (OFF) and saturation regions (ON), it does not operate as an amplifying device as it would do if controlled in its active region. ![]() That is with no base bias voltage applied, the transistor will be fully-OFF, and when a suitable base bias voltage is applied, the transistor will be fully-ON. When an NPN bipolar transistor is operated in an Open Collector (OC or o/c) configuration, it is operated between being fully-ON, or fully-OFF, thus acting as an electronic solid-state switch. This type of set-up produces what is commonly called an open collector output configuration. One way to overcome this inversion of the transistors switching state is to remove the collector resistor, R C completely and have the transistors collector terminal available to be connected to some external load. In this way the NPN bipolar transistor can be used as an electronic switch performing the operation of inversion, because when the transitor is “OFF”, its collector terminal, and thus V CE, is “HIGH” at V CC level, and when it is “ON”, (conducting) the output taken across V CE will be “LOW”, which is the opposite switching conditions if we want to control a relay, solenoid or lamp, for example. As without R C the voltage on the collector terminal would always be equal to supply voltage.Īs mentioned earlier, a bipolar junction transistor can be operated between it cut-off and saturation regions when V BE is much less than 0.7 volts (zero base current), or when it is much greater than 0.7 volts (maximum base current) respectively. The collector resistor, R C is used here to allow the collectors voltage, V C to change value in response to an input signal applied to the transistors base terminal, thus allowing the transistor to produce an amplified output signal. 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. ![]()
0 Comments
Leave a Reply.AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |