ĬircuitLab is an in-browser schematic capture and circuit simulation software tool to help you rapidly design and analyze analog and digital electronics systems. Saturation, as the name might imply, is where the base current has increased well beyond the point. Worth a google of how a transistor works. Pity you can't buy behavioural devices Farnell or Radio Shack. As is seen in 2, this implies Vout is equal to 10 volts. At saturation the voltage Vce is at its minimum, 100mV or whatever your data sheet shows. It is often instructive to replace "real" models with simplified behavioural models to strip away the complexity and see underlying function. However the simple models show the underlying behaviour and for some things, are often all you need to see how a circuit behaves. With the voltage and as shown, the Base-Emitter (B-E) junction is forward biased and the Base-Collector (B-C) junction is reverse biased. If you look at the references in the links above you'll see that bjt models rapidly get more complex as you try to account for more detailed physical behaviour and the effects junction capacitances and electron hole transit times across junctions and through the material. Here's a slightly more sophisticated version which crudely models Vbe and Vcesat: That simple model breaks down as soon as the collector voltage gets close to the emitter voltage: a real bjt saturates, the collector voltage cannot get closer to the emitter voltage and the collector current no longer increases in proportion to base current (small signal hfe tends to zero) but in this simple model the collector current carries on increasing and pulls the collector below the emitter because a CCCS is an ideal current source whereas a bjt is not. Once the collector drops below the base voltage, you are in saturation, and base current will start to increase (if you hold the base voltage constant). You put current into the base and you get hFE times the base current out of the collector. In saturation Vbc will be a bit lower than Vbe because of the geometry and doping concentrations in the various regions and there will be net current flow from collector to emitter.
Then rub those observations up against all the stuff you're reading.Ī very simple model of the bipolar transistor is that it is a current controlled current sink or CCCS (for NPN, it's a source for a PNP).
Vbe saturation voltage series#
Try also inserting a resistor in series with the emitter to ground. At this point, the base current is maximum and so is the collector current. The collector current for saturation is as much a feature of the external circuit as it is of the transistor parameters.Īnd try changing Vcc and R1 to get a feel for how things behave. The point where the load line intersects the IB IB(sat) curve is called saturation. You need to understand the basics of how a transistor works.
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