12/3/2023 0 Comments Emitter base voltage![]() ![]() Some off the cuff rules that usually work so long as your bias resistors allow enough current flow generally means that you treat R1 and R2 as a voltage divider and find an approximate DC setpoint from that. If your input voltage source has an output impedance somewhere in between the two extreme cases I give (0 and infinite impedance) then you'll need to add that into the circuit and start crunching the numbers/equations. ![]() In that case, what the others post is correct. In that case, the bias resistors you see in your diagram are what will determine the DC set point of Vb is. This means that DC will be blocked and AC will pass through. In that case, all of the bias resistors really don't matter because the input voltage will drown out their effect.Īlternately, something like this is commonly driven by a capacitively coupled source. The emitter, which is the first letter in the NPN sequence, is connected to the negative side of the battery while the base, which is the second letter (NPN), is connected to the positive side. If it's low, then it's as if you're applying a true voltage source to Vb. If an op-amp or a similar amplifier stage drives Vb, then the output impedance should be quite low. Without it you can't really know for sure. The big difference is that, in the transistor, the lion's share of the emitter current is redirected to the collector, as long as the voltage on the collector (say, relative to the emitter) is. The -6 V minimum comes from the maximum emitter-base voltage, and 0.8 V comes from the fact that the maximum collector current is 200 mA and the above chart shows that the collector current is 100 mA at a base-emitter voltage of 0.8 V. However, for sake of convenience, it is customary to show emitter and collector to be of equal size.You would really need to know the output resistance of the input voltage source to Vb. The base-emitter junction has most characteristics of a diode in a sense that the emitter current increases exponentially with the base-emitter voltage. In the circuit you show (where the emitter is at ground), the voltage at the base can vary from -6 V to 0.8 V. For an emitter-base voltage VEB 600 mV, calculate the emitter-collector voltage. Due to this difference, collector and emitter are not interchangeable. In the circuit shown in the figure, the BJT has a current gain () of 50. In most of the transistor terminals, the collector region is made physically larger than emitter region because collector is to dissipate much power. As the resistance of emitter-base junction is very small as compared to that of collector-base junction, therefore, the forward bias applied to the emitter-base junction is usually very small whereas the reverse bias on the collector-base junction is much large. The resistor needs to be large enough to effectively limit the current, but small enough to feed the base enough current. The value of the resistor, and voltage across it, will set the current. The forward-biased emitter-base junction offers low resistance to the emitter current whereas the collector-base junction, which is reverse biased, offers high resistance to the collector current. The base-emitter node can get its happy voltage drop of 0.6V, and the resistor can drop the remaining voltage. ![]() Base – It is the middle section of the transistor and is very lightly doped to reduce the recombination within the base so as to increase collector current and is very thin (of the order of μm) in comparison to either emitter or collector so that it may pass most of the injected charge carriers to the collector.īase forms two junctions i.e., emitter junction and collector junction, each having its own barrier voltage. Large in size to withstand the temperature generated at the collector.ģ. It is moderately doped to avoid the chances of mesh formation even after taking the carriers from the emitter. Collector is always reverse biased so as to remove the charge carriers away from its junction with the base. Collector – It is the right hand section of the transistor and its main function is to collect majority charge carriers. ![]()
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