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Q&A

Posts tagged BJT

Synonyms: bipolar-junction-transistor bipolar-transistor bipolar-junction-transistors bipolar-transistors

Subtag of transistor

Anything related to BJTs (Bipolar Junction Transistors). A BJT is an active electronic component based on semiconductors usually used as an amplifier or a switch.

They come in two varieties, NPN and PNP, whose behavior is specular wrt. the polarities of currents and voltages.
Tag Wiki

BJTs are usually 3-terminal semiconductor devices (although there are some exceptions for BJTs used inside integrated circuits).

Their terminals are called Emitter (E), Collector (D) and Base (B). Roughly speaking, emitter and collector are the "output" terminals, whereas the base is the "input" terminal.

They come in two varieties, called NPN and PNP, depending on the layering of their internal structure. This latter is built around two tightly coupled PN junctions, formed sandwiching a thin layer of P semiconductor between two layers of N semiconductor (for NPN types) or vice-versa (for PNP types). The middle thin layer is externally connected to the base terminal, whereas the other two layers constitute the emitter and the collector.

The basic behavior is that the current flowing through collector (IC) and emitter (IE ≈ IC) is controlled by the voltage across base and emitter (VBE) through an exponential relation (see Ebers-Moll equations). During its operation, the BE junction behaves essentially like a forward biased semiconductor diode, with a base current (IB) depending exponentially on VBE.

Note: although in many elementary descriptions it is said that the output current IC is controlled by the base current IB, this is only true on some conditions and is subjected to large approximations.

BJTs are most commonly used as amplifiers and electronically-controlled switches.

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Q&A Calculating base voltage of NPN transistor

I'm having trouble using the nodal equations to determine the base voltage Vb1 in the circuit below. I've attempted to write seven equations below, but not able to fit them in entirely. (12-Vx)...

2 answers  ·  posted 1mo ago by kadamrohan16‭  ·  last activity 1mo ago by LvW‭

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Q&A How to change the polarity of an input using a single switch?

I have an mcu pin and I want to sense a voltage up to 24V to 36V. I want to connect some outputs on the board I'm designing and I don't know whether those outputs will provide a voltage or will be...

1 answer  ·  posted 1y ago by DeadMouse‭  ·  edited 10mo ago by Lorenzo Donati‭

Question microcontroller BJT
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Q&A Is a BJT 3-transistor Wilson mirror faster than a simple mirror?

My question is trying to understand if there is an effect on the switching time, on both turn-on and turn-off, between the following configurations. A mirror set up to switch a current, and there ...

1 answer  ·  posted 3y ago by Pete W‭  ·  edited 10mo ago by Lorenzo Donati‭

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Q&A Thévenins Theorem for Transistor Circuit

I am calculating the base current of transistor Q1. To simplify the circuit equations, I applied the Thévenin's theorem twice and converted V1, R1 and R2 into Vth and Rth, as shown in the image. ...

2 answers  ·  posted 2y ago by kadamrohan16‭  ·  edited 10mo ago by Lorenzo Donati‭

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Q&A PNP Darlington pair as a current limiter

I am trying to analyse what is controlling the load current in the below Darlington configuration. Is it the base current of transistor Q3 which is controlling current through R14 / emitter curr...

1 answer  ·  posted 2y ago by kadamrohan16‭  ·  edited 10mo ago by Lorenzo Donati‭

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Q&A Thermal stability coefficient [closed]

The thermal stability coefficient is defined to be: $S = \frac{dI_{C}}{dI_{CBO}}$ where $I_{CBO}$ is the reverse current of the BJT when voltage isnt applied to the base of the BJT. In case of a ...

0 answers  ·  posted 1y ago by MissMulan‭  ·  closed 11mo ago by Nick Alexeev‭

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Q&A BJT common emitter amplifier equivalent circuit (π hybrid model) [closed]

Im studying the equivalent model of a BJT common emitter amplifier at high frequencies.At university we tought that the BJT at high frequencies looks something like this: However when designing ...

0 answers  ·  posted 1y ago by MissMulan‭  ·  closed 1y ago by Olin Lathrop‭

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Q&A Small signal analysis of BJT in saturation region

The small signal analysis of a BJT when the BJT is in amplification mode: is easy.However what happens when the BJT is in saturation mode? Can we replace the dependent current source with a r...

1 answer  ·  posted 2y ago by MissMulan‭  ·  edited 2y ago by MissMulan‭

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Q&A Re model of transistor is the same in 2 different configuration

I was studying the Re model of a transistor: But the Re model for 2 different configurations (common emitter and common base )turns out to be the same! I don't know why I am really confused.How...

2 answers  ·  posted 2y ago by MissMulan‭  ·  last activity 2y ago by LvW‭

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Q&A Help with differential to single ended voltage converter

Hello I don't understand how this differential to single-ended voltage converter can work. What I don't understand is why we place a current mirror(a current mirror copies the current flowing thr...

3 answers  ·  posted 3y ago by MissMulan‭  ·  last activity 2y ago by Circuit fantasist‭