The flow rate is is capped by the size of the flood gate. In the same way you can throw open the flood gates but that doesn't mean you can pass an infinite amount of water through the flood gates. It is amplifying so hard that it can't amplify any further. When acting as a closed switch (conducting) it is acting as an amplifier trying to amplify the largest signal it can. When acting as an open (non-conducting) switch it is acting as an amplifier amplifying a signal of zero. Anything in between full closed and fully opened is throttling the flow of water somehow, but when fully open or fully close it's just acting as a switch to block or pass for the water. Imagine you use your weak little fingers to push some buttons to control a massive flood gate. Image source: Output Characteristics Curves of a Typical Bipolar Transistor from Electronics TutorialsĪcting as a closed or open switch is just an extension of it acting as an amplifier at its limits. Because at that region collector current changes with base current.Īnd the cutoff and saturation condition acts as a switch. The range between the cutoff and saturation can be used as an amplifier. At that point, your transistor is fully off. For a certain base current, the collector current will be almost equal to zero. Similarly, if you reduce the base current, the collector current will decrease. As soon as the transistor reaches saturation, it is fully on. So with the increase in base current, the collector current will increase until it reaches saturation. And the current at that condition is called saturation current, which is defined as:īut in real life collector-emitter voltage will never be zero. And the condition when the collector & emitter works like a short circuit is called the saturation condition. This is the maximum value of the current that can be passed through the resistance if the collector & emitter works as a short circuit. Now, let's assume, you have connected the collector and emitter through a copper wire (i.e. For example, a 0.01mA increase in base current has caused a 10mA increase in collector current. We know that for a BJT the collector current increases with the increase in base current. Simulate this circuit – Schematic created using CircuitLab
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