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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.

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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 ...
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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?
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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 ...
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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 ...
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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 ...
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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 ...
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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. ...
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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 ...
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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 ...
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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?
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75k views

How to calculate the power dissipation in a transistor?

Consider this simple CircuitLab 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 ...
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I don't quite understand this FET-BJT preamp circuit

I see this circuit a lot on electret microphone preamps, but I don't quite understand it. The FET is operated as a common source amplifier, so it has gain, inverts, and has relatively high output ...
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Tips on analyzing circuit with multiple op-amps and bjts

In a homework exercise I need to sketch the Vout-vs-Vin graph, having it first obtained analytically. I know that Ur and Vin are both nonnegative, and the circuit is shown here: It seems like there ...
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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 <...
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Schottky diode like BJT symbol

In this schematic, what do the Schottky diode like "barbs" mean on some of the BJTs where the base is connected? I haven't encountered this symbol before.
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Basic operation of a bipolar junction transistor

I've tried really hard to understand the basic operational principle of a transistor. I've referred to many books and been to forums but have never got a convincing answer. Here are the things I want ...
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Why are BJTs more reliable than MOSFETs under severe weather conditions?

I read in a textbook (Microelectronic Circuits by Sedra and Smith, pg. 494, (2010) sixth edition) that BJTs are preferred by the automotive industry due to their reliability under severe weather ...
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What's this driving circuit of LED?

I found out this driving circuit of LED somewhere as in the below picture, and I just can't figure it out what its design purpose is. There are two kinds of VDD input, 9 V and 6.5 V. I ...
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2answers
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Prevent high-side BJT saturation

I am building high-speed (10-20ns on BC847-class transistors) digital "buffer"/"inverter" out of BJTs. Scheme is attached. While I can prevent saturation of low-side BJT by adding Schottky diode, it'...
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What's the difference between NPN and PNP transistors?

Suppose that I know how an NPN transistor works. How different is a PNP transistor? What are the operational differences between a PNP and a NPN?
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1answer
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What's the usage of two NPN transistors with their bases and collectors tied together?

I'm thinking to know what is the usage of these parts of this circuit. Source It's the schematic of UT70C Digital multimeter.
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1answer
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Increasing the compliance voltage range for a variable, two-quadrant constant current pin-driver circuit

The following is for hobbyist work and I have no commercial intentions at all. Only a handful (two?) will be built. (I use these for part testing and curve generation, though with the higher voltage ...
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Why would the transistor not switch?

I was reading an example from a text book. And for this circuit above the author claims when R3 is less than 100 ohm Q3 will not switch. I couldn't figure out the "reason" why. But I verified with ...
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Why would I use 2N3904 instead of 2N4401?

2N3904 and 2N4401 appear to be very comparable parts in all specs. 2N4401 has a higher current rating, but otherwise they look to be about the same in price and everything else. Obviously it's ...
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Where does the Maximum Emitter-Base Voltage come from?

The datasheet for a BD679 transistor lists amongst the absolute maximum ratings that the "Emitter Base Voltage" has a maximum of 5v. This figure confuses me - my mental model of a (BJT) transistor ...
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Why would you attach a Diode to the base of a BJT?

I was looking at a DC BJT setup for sourcing current and came accross this I have never seen a diode attached to the base of BJTs before and was wondering what it might be used for? I believe it ...
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BJTs versus (MOS)FET to switch a load from a microcontroller

I would like to know what are the reasons for choosing bipolar junction transistor (BJT) versus a field-effect transistor (FET) (MOSFET or a JFET) to switch a load from a microcontroller. In our ...
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3answers
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Why does the current still flow through the collector eventhough the transistor circuit has no Vcc

I came across a tutorial where I thought the above BJT NPN circuit is wrong because of the conclusions made. But when I simulated this circuit I found out the conclusions were true. I was ...
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4answers
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Best transistor to use for audio amplifier

This term, we will be designing an audio amplifier. So far in our lecture, we are still at BJT and based from what I've heard, FETs will just be partly discussed unlike the thorough one on BJT. Anyway,...
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4answers
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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 ...
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7answers
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Saturated BJT Transistors. \$V_{CE} < V_{BE}\$ : AKA - Things that make you go “Hmmmm”

We use them every day and those in the know fully understand the functional characteristics of BJT transistors. There are documents and links galore explaining the operational math. There are even ...
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4answers
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Which is the objective of an Base-emitter resistor?

Can someone explain me the purpose of the resistor R2? If I remove R2, the circuit will perform the same result, won't it? simulate this circuit – Schematic created using CircuitLab
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My teacher said a BJT phototransistor doesn't have a base

But I find it to be absolutely incorrect. How can a BJT transistor even work without a base? Shouldn't a transistor without base be just a semiconductor (PP, NN)? Does there exist some special NASA-...
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Help understanding this LED lighting transistor circuit

I simulated this circuit from a reference design but I'm not entirely sure how it's working or how you would go about designing such a thing. In simulation it looks like it's designed to hold the ...
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3answers
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How is this circuit for interfacing 20V signal with 3v3 microcontroller

I have designed the following circuit to interface a 12-20V signal to a microcontroller running on 3.3 volts. The signal is either 20V or open circuit. I want the circuit to be as resilient as ...
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5answers
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What does (PNP) BJT with shorted C-E do?

I was browsing through the TI datasheet for an LM78L05 and noticed this application schematic: Note how Q2 has its collector and emitter shorted. I can't say I've ever seen that before and search ...
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What do multiple emitters/collectors imply in a BJT schematics symbol?

I sometimes come across with BJT symbols from which multiple lines come out of emitter or collector. Are such BJTs different kind of devices? What does it mean in solid-state level?
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Designing a BJT Amplifier given some constraints

I am trying to design a BJT amplifier following this model: Where the beta parameter may vary from 100 to 800, the voltage between the base and the emitter equals 0.6V (active mode), \$V_t = 25 mV\$ ...
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4answers
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Flow of holes in transistor?

Bipolar transistor are said to have both electron flow and hole flow. Movement of electrons can be understood, but holes are fixed part of the atomic/crystal structure. How can we characterize their ...
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What's the difference between small signal bipolar junction transistors (BJTs) marketed as switches vs. amplifiers?

For instance, the MMBT3904 and MMBT3906 BJTs are listed as NPN/PNP Switching Transistors, and the datasheets mention the switching times, while the BC846 and BC856 BJTs are listed as NPN/PNP general ...
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6answers
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Why do my P-channel MOSFETs keep dying in this H-bridge?

So this is my H-bridge: Every time I start using it in one direction the P-channel MOSFET and the NPN BJT which belong to the used direction die in seconds. The killed MOSFET and the BJT developing a ...
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6answers
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How does this constant current sink actually work?

I have implemented a constant current source and it works wonderfully, but I was just hoping to try and understand it a little bit more! Here is the circuit in question: I have tried doing some ...
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6answers
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Why won't this PNP Transistor trigger?

The circuit below should be taking a 3.3V signal from an MCU on MCU_LS12 and outputting a 12V high-side signal. The output is always 12V. On scoping the base to the output transistor is not being ...
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6answers
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The 'gap' between understanding of transistors and their application in real circuits

How do they even work? I'm in senior high school year and have electronics as a subject. I'm really interested in getting to understand this and take up electronics in college too. But now this seems ...
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11answers
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Why is a BJT considered “current-controlled”?

With BJTs, we can control base current using Vin (from diagram). Why do textbooks state that BJTs are current controlled when it's obvious that changing the voltage controls the current through the ...
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3answers
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Switching 12V with an active low 5V signal

I’m trying to switch 12V (max 30mA) with a signal from a MCU that I’d prefer to be active low (so the output voltage is 12V when the control signal is 0V, and 0V when the control signal is 5V). Since ...
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Using transistor as switch, why is load always on the collector

I find in reference circuits that when BJT is used as switch when it will be used in saturation mode, the load is always at the collector. For NPN the emitter is connected to the ground, for PNP the ...
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1answer
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How does this BJT transistor circuit works?

I just wanted to know a little more about this circuit about how it works a more in depth analysis I mean. I know there's some kind of differential amplifier and a Zener diode for voltage regulation ...
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2answers
771 views

Why is my NPN base so slow to turn off?

The circuit below is about as simple as it gets, yet it's not behaving like I expect. V3 is a 3.3Vpp square wave going into the base of the transistor, so I would expect to see V_Out be high when V3 ...
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2answers
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Which resistor for NPN transistor base?

Which is the way to choose a resistor for a NPN transistor base? I want to use P2N2222A as a switch, in a design as shown below. When I have voltage in base (1.8 V), I want a connection to be created ...