# Questions tagged [transistors]

A transistor is a semiconductor device that can amplify signals and switch power. The types that are used most are BJT (Bipolar Junction Transistor), UJT (Unijunction Transistor), and MOSFET (FET, for Field Effect Transistor).

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### How to calculate the tolerance of this constant current circuit?

I am using the circuit below to provide constant current to th LEDs. I request your suggestion on how to calculate the tolerance of current (minimum and maximum variation of set current) due to ...
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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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### 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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### (NPN) Why is the collector current at saturation, less than the collector current in the forward active mode?

Why is the collector current at saturation less than the collector current in the forward active mode? Specifically, Why is $I_C < \beta I_B$ ?
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### TTL NAND gate (totem pole) current and voltage analysis

I am working on a few practice problems for my course and I am unsure about my working throughout. hoping for some feedback and guidance from the community as it's not my strongest topic. Here is ...
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### How is possible that with same Ibase there is more than one Vce?

Ok guys, look at this picture: How is possible that with the same $I_{\rm base}$ we have more than one $V_{\rm ce}$?
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### Bootstrap circuit function

I have some question about the bootstrap capacitor on a gate driver. First, the bootstrap capacitor is used because the voltage on the high-side driver's gate needs to be about 10-15 V higher than the ...
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### Why can't class A amp drive 8 ohm speaker with just one BJT?

I designed this class A amplifier. It's my first attempt at designing an amplifier with transistors. My objective is to build a guitar amplifier with tubes, but I figured that building an amp with ...
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### Calculation of base current and what decides the current through collector-emitter branch

I have the below circuit. Its not a homework material. I am understanding how to analyze the transistor circuits. Below are my questions while trying to analyze : If I am given the below circuit, ...
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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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### I don't care how a transistor works, how do I get one to work?

Every reference I can find on transistors immediately launches into theory-heavy alphabet soup. The above seems also to be assumed knowledge for reading a datasheet. I don't care; I just want to get ...
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### Switching DC with MOSFET: p-Channel or n-Channel; Low Side Load or High Side Load?

I think, it is time I understand working principle of MOSFET transistors... Suppose that; I want to switch voltage on a resistive load by a MOSFET transistor. Any control signal between -500V and +...
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### How are logic gates created electronically?

So we have AND, NOT, NAND, NOR, OR gates, but how are they created electronically/electrically? For example, what makes NOT gate reverse the value?
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### Driving DC motors with MOSFETs and a microcontroller?

I'm developing a nano quadcopter using an Atmega328 microcontroller, powered at 3.3V, and very small brushed DC motors. The average current used by these motors is about 800mA @ 3.7V. Initially, to ...
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### Schottky transistor, Not sure I understand it?

So I've been looking through and going through my Digital Computer Electronics book, and I came to this... It seems so simple and I understand the "point" of it, but I'm not sure I understand exactly ...
1 vote
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### Cant understand an ab amplifier diagram

I been trying to figure out this diagram, and Can't understand what happens from the beginning of the input , how the voltage changes in a way that "shuts off" one transistor. Also cant understand the ...
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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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### How do I make the turn off and turn on time equal in a NPN transistor?

I have a simple NPN switch, see the diagram. I feed a 100KHz square wave (TTL) to the base of this transistor and it turns on very very fast (a few nSec) but it doesn't turn off as fast, it almost ...
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### How to drive a 20mA LED From a 4mA max GPIO Pin

I have an IC that has an GPIO with which I would like to drive a LED. Since the device will be running off battery, keeping the power use low (Zero maybe) while the LED is off as a priority. The ...
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### Need help calculating resistance for transistor base

I have 3 12VDC/40A automotive relays (datasheet) that I want to use with my Arduino. Based on the tutorial I am following (link) I need transistor, resistor, and diode. I am not an electrical engineer ...
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### Is this an astable multivibrator?

In this circuit the LED flashes on and off, however could it be classified as an astable multivibrator?
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### How to sense the zero crossing of current?

I am trying to sense the power factor angle using microcontroller. Now the mechanism that I have thought it that Measure the zero crossing of voltage When the voltage zero is sensed, start the ...
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### Understand 2 transistors (NPN & PNP) flasher circuit working

I implemented this circuit on a breadboard and it works. However on Proteus ISIS it doesn't (I have to change the 330k to 1500k for it to work on Proteus). I have two questions related to it. 1- Why ...
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### Selecting a MOSFET for driving load from logic

I'm looking to drive a magnetic door lock from an Arduino. I've found a question about driving a solenoid from an Arduino, which includes a circuit that looks perfect for this kind of situtation: ...
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### How to invert a digital signal

I need a way to invert a digital signal i.e. if the input is high, I want the output to be low and if the input is low I want the output to be high. I think this can be accomplished with a single ...
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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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### 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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### Bjt base current calculation

In a bjt transistor that is used as a switch (maximum saturation, Vce minimum as possible), what is the minimum base current that is required for a corresponding desirable collector current?
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### How does early voltage affect collector current?

The transistor in the following circuit is said to feature $I_{C} = 1.0mA$ at $V_{BE} = 0.7V$ and $V_{CE} = 10V$. If $V_{A} = 50V$ and $\beta = 50$, find $V_{B}$ and $V_{C}$. simulate ...
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### Why are NAND gates used to make AND gates in computers?

Why is this a standard for AND gates when it could be made with two FETs and a resistor instead?
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### What is the difference between emitter and collector for BJTs?

the (surely simplistic) model of a bipolar junction transistor one is taught in foundational physics course appears to be symmetric. - So, what is the difference between the collector and the emitter ...
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### Choosing the right transistor for a switching circuit

I want to control a siren speaker (has a built-in driver) with an Arduino. The siren needs 12V and consumes about 480mA. The Arduino output pin is 5V and can source 40mA maximum. I would like to use ...
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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 ...
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### P-Channel MOSFET high side switch

I am trying to reduce the power dissipation of a P-Channel MOSFET high side switch. So my question is: is there any way in which this circuit can be modified so that the P-Channel MOSFET will ...
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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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### Using NPN transistor as switch

Before I ask my question, I should say that I am very new to working with electronics and I may not quite know how to describe my problem. I am trying to use a pin on my msp430 microcontroller I have ...
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### Motor driver using only a 2N2222 transistor?

Is it possible to build a motor driver using only a 2N2222 transistor? If yes, then how?
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### MOSFET in reverse polarity protection

I have such a reverse polarity protection circuit for the power supply: The MOSFET used has a maximum VGS of 10 V, so that's why there's a Zener diode of 7.5 V. But what happens if I ...
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### Totem-pole bipolar circuit explanation

I came across the following circuit in my text book: Summary of the explanation given in my text book: When control input Q3 is low, Q1 is on and output is HI. When control input Q3 is high, Q2 is on ...
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### What's the difference between field-effect transistors (FETs) marketed as switches vs. amplifiers?

For instance, the J108 JFET is listed as "N-Channel Switch", and the datasheet mentions the RDS on resistance, while the J201 JFET is listed as "N-Channel General Purpose Amplifier" (and the on-...
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### Why does MOSFET pinch-off occur?

This question is about enhanced n-type MOSFETs. From what I understand, an inversion layer is formed underneath the insulating layer below the gate of the MOSFET when a voltage is applied to the gate. ...
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### Can a thyristor be built from two transistors?

Supposedly, an SCR/thyristor is just a simple, four layer PNPN semiconductor. If that's the case.. When a circuit calls for a SCR/thyristor, and there isn't one available, can it be substituted by (...
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### Where should I put the kickback diode in a transistor switch?

When driving inductive loads with transistors, we use kickback diodes. What I understand is a kickback diode supplies a path for the inductive charge to discharge. Also, an inductor will try to ...
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### Are reverse biased transistors stable?

The usual circuit (found widely across the internet) for a true random number generator's (TRNG) entropy source is a reverse biased transistor like so: Q1 is typically a 2N3904 as it has been found ...
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### Switching 9V using a NPN transistor and an Arduino

First of all, I'd like to say I'm not an Electrical engineer so please bear with me. I haven't got a sketch for what I'm trying to achieve but hopefully I can explain it ok. I have an NPN transistor (...
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### How do we create current sources?

Constant current sources (simply, current sources) are useful and necessary devices. They are widely used to drive LEDs, charge capacitors and batteries, to bias differential amplifiers, etc. The only ...
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### BJT base connected to ground and still operates?

When in university I used to get confused how a transistor (NPN) can draw current from ground. I'm now studying power amplifiers and again, this same configuration appears. How does Ir draw current ...
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### Lighting either of two LEDs, or none, based on digital input

I ran into a problem which I tought was extremely simple, and after giving it some thought for more than 2 hours I had to recognise it actually is quite complicated. The problem is the following: I ...
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