Electrical Engineering Stack Exchange
Electrical Engineering Stack Exchange

Questions tagged [bjt]

BJT stands for Bipolar Junction Transistor. It is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications.

More information on BJT transistors can be found on this Wikipedia article.

further reading

Bipolar Transistor Cookbook part 1,... , part 8. Series of articles in Nuts & Volts magazine (ca 2004).

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When is a MOSFET more appropriate as a switch than a BJT?

In my experimentation, I've used only BJTs as switches (for turning on and off things like LEDs and such) for my MCU outputs. I've been repeatedly told, however, that N-channel enhancement-mode MOSFETs are a better choice for switches (see here and…
Mark
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When to use what transistor

So there are several types of transistors: BJT JFET MOSFET Combine all that with the various flavors of each (NPN, PNP, enhancement mode, depletion mode, HEXFET, etc) and you've got a wide array of parts, many of which are capable of accomplishing…
Joel B
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Why can't two series-connected diodes act as a BJT?

Another form of the question is: will joining two diodes with wires (pn-np) make a transistor-equivalent? I read that they are not equivalent, but why?
user23564
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How do I saturate an NPN transistor?

I understand that in "saturation mode", a BJT functions as a simple switch. I've used this before driving LEDs, but I'm not sure I understand clearly how I got the transistor into that state. Does a BJT become saturated by raising Vbe above a…
Mark
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Why would one drive LEDs with a common emitter?

I've seen tutorials aimed at beginners suggest the way to drive an LED from something without enough current drive is this: (option A) but why not this: (option B) Option B seems to have some advantages over option A: fewer components the…
Phil Frost
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Basic transistor questions

I've created the circuit shown. I'm using a 9V battery (actually throwing out 9.53V) and 5V coming from an Arduino to test with both 9 and 5 volts. The transistor is a BC 548B (datasheet I'm using is here). simulate this circuit – Schematic created…
hojkoff
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BJT in Reverse Active Mode of Operation

What will happen if for a BJT transistor, it's emitter terminal is treated as collector and collector as emitter in a common emitter amplifier circuit?
Ravi Upadhyay
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What is this unusual NPN-PNP transistor pair?

I was looking for the datasheet for a transistor I'd discovered during some reverse engineering, a DK52T. Unfortunately I can't find its datasheet, but I found one for the DK52D. See the image below; this is some kind of weird combination NPN-PNP…
parkside
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Why pull base of BJT switch?

I often notice usage pull resistor for base of bipolar transistor, e.g. \$R2\$ here: Why it's used? I understand pull resistors for FETs, because of gate's high impedance an EMI can easy switch it. But BJT need base's current to open and, I think,…
user54579
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How to calculate the power dissipation in a transistor?

Consider this simple sketch of a circuit, a current source: I'm not sure how to calculate the power dissipation across the transistor. I'm taking a class in electronics and have the following equation in my notes (not sure if it helps): $$P =…
David Chouinard
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What is the objective of a base-emitter resistor?

What is the purpose of the resistor \$R_2\$? If I remove \$R_2\$, the circuit will perform the same result, won't it? simulate this circuit – Schematic created using CircuitLab
ferdepe
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When are BJTs used in common base configuration?

BJTs are mostly used in common emitter, sometimes common collector configuration. I rarely see common base. When would you use a BJT in common base? For instance, what parameters are different from common emitter?
The Resistance
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Do engineers still design for volume using discrete through-hole bipolar transistors?

As a hobbyist who likes plugging BJTs and discrete components into solderless breadboard for entertainment and education, I recognise that people like me are an insufficient market to make it worthwhile for manufacturers to continue production. I've…
RedGrittyBrick
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Effect of bootstrapping in amplifier circuit

I am trying to understand this "bootstrap bias" amplifier circuit. The picture below is adapted from the book "Transistor Techniques" by G. J. Ritchie: This circuit is a variation of the "voltage divider bias", with the addition of the…
favq
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How do BJT transistors work in a saturated state?

This is what I know about NPN BJTs (Bipolar Junction Transistors): The Base-Emitter current is amplified HFE times at Collector-Emitter, so that Ice = Ibe * HFE Vbe is the voltage between Base-Emitter, and, like any diode, is usually around 0,65V.…
Denilson Sá Maia
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