![]() ![]() Let’s review a common-emitter configuration of an NPN transistor: The common-emitter amplifier is popular due to its ability to amplify both current and voltage. When it comes to amplifiers, the transistor can be configured as a common-emitter, common-based, or common-collector to different effects. Calculating Resistance Reflection for a BJT The resulting equation provides a picture of how altering the resistance of the output affects the resistance seen from the input. It is derived by solving simple equations of the transistor amplifier circuit. The resistance reflection rule refers to the relationship between the input resistance and output resistance of a transistor. ![]() ![]() When driven in an active region, the transistor operates as an amplifier, which either amplifies the voltage, current, or both. The resistance reflection rule comes into play when a transistor is in an active region. Both types are determined by the order of how the terminals are arranged.īJTs can operate in the cut-off, saturated, and active regions. There are two types of BJTs: NPN transistors and PNP transistors. BJTs have 3 terminals, known as the collector (C), base (B), and emitter (E) terminals. The BJT is an electronic component made up of 2 P-N junctions. To better understand this formula, let’s first take a look at the BJT. This formula determines the characteristics and performance of an amplifier when set up in a particular configuration. If you’re designing an amplifier with a bipolar junction transistor (BJT), you need to understand the significance of the resistance reflection rule. The resistance reflection is an essential characteristic of BJT amplifiers Applying this rule correctly ensures that designing amplifiers is far more predictable and controllable than playing a FIFA game. The resistance reflection rule provides insight into how an amplifier will perform in a given configuration. ![]() When designing an amplifier with a BJT transistor, luckily, designers have more variables in their control than when playing a FIFA game. When such instances happen, I try to accept the outcome and move on, as there isn’t much point in worrying about things that aren’t in my control. However, with these moments of greatness also come moments of frustration, when I lose a game for reasons out of my control. In these games, I relish the moments of greatness when I can easily defeat my opponent. Even now, in my late 30s, I still enjoy battling other players in a game of 11 vs. Growing up, I was an avid fan of FIFA games. The resistance reflection rule helps designers choose the right resistor values Understand the significance of the resistance reflection rule when designing an amplifier with a BJT transistor. Learn how to calculate the resistance reflection rule. The advantage is that it makes the off transition faster since the transistor is in the "linear" region instead of in saturation.Discover what the resistance reflection rule is. The on state voltage will be a little higher since the transistor isn't fully saturated. When the collector gets low when it's nearly in saturation, it steals base current which keeps the transistor just at the edge of saturation. A Schottky diode is integrated into the transistor from base to collector. This is used to advantage in Schottky logic. One interesting thing is that the collector of a saturated transistor is actually below the base voltage. That's not very scientific and only roughly described the semiconductor physics, but it's a good enough model to keep in your mind as a first order explanation. There are extra "unused" charges in the base that take a little while to drain out. One artifact of saturation is that the transistor will be slow to turn off. A saturated transistor typically has around 200mV C-E, but that also can vary a lot by the design of the transistor and the current. Since the collector of a NPN will act like a current sink and in saturation the external circuit isn't giving it as much current as it could pass, the collector voltage will go as low as it can. ![]()
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