Questions tagged [transistors]

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

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Relay vs. Transistor?

Rather basic, I'm afraid, but when would you use a relay, and when would you use a transistor? In a relay the contacts wear out, so why are relays used at all?
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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 are there so many relays used in a car, instead of transistors?

Recently I did some work on my car electric circuits. I see many relays are used in car circuits. These relays are used for simple switching, and I wonder why these circuits are based on relays and ...
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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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Why do you need 2 resistors when hooking up a transistor as a switch

What is the point of R2 in the following diagram: I get that R1 controls the current to the Base, but what does R2 do?
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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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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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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 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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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 can “purely” electrical circuits emit sound?

Moving membranes or piezoelectric materials obviously produce sound waves, but how can "purely" electrical circuits such as transformers or DCDC choppers (and others) often have an audible noise? Is ...
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Why do CPUs need so much current?

I know that a simple CPU (like Intel or AMD) can consume 45-140 W and that many CPUs operate at 1.2 V, 1.25 V, etc. So, assuming a CPU operating at 1.25 V and having TDP of 80 W... it uses 64 Amps (a ...
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Why would an op-amp use BJTs over MOSFETs?

I've always been told that ideally, op-amps have infinite input impedance. So when I was looking at the LM741's transistor level schematic, I was confused when they used BJTs instead of MOSFETs. ...
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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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Why don't we use GaN transistors everywhere?

There has been a lot of research around GaN transistors, proving that they have a very low on-resistance, low gate-charge and are very effective at high temperatures. So why are we still mostly ...
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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 do we need so many transistors?

Transistors serve multiple purposes in an electrical circuit, i.e switches, to amplify electronic signals, allowing you to control current etc... However, I recently read about Moore's law, among ...
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Which configuration is better for pulling down an NPN transistor's base?

I was discussing on pull down resistors with a colleague of mine. Here are the two configurations for transistor as a switch. The input signal can either come from a microcontroller or an another ...
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Darlingtons, MOSFETS, and bipolar junctions

I realize there are dozens upon dozens of kinds of transistors, each used for specific purposes. Sometimes there is great overlap; multiple kinds may work in a given circuit, but at times only a ...
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“Starter Kit for electronics” - What to buy?

I'm going to buy some things for back to the electronic world. Starting with an Resistor Assorted Kit. Need to buy some kits containing the most common capacitors and transistors... what to buy??? [...
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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 ...
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What's the purpose of these two diodes in this circuit?

What's to point of using these two diodes? Can't you just replace them with a resistor?
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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 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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Connect load to collector or emitter of transistor

When setting up a transistor as a switch is there any difference between putting the load on the collector or emitter? As far as I can see the only difference is in calculating the Vbe i.e. ...
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What do the arrows in transistor symbols point to (and from)?

Transistor symbols are often drawn with arrows pointing in one direction or the other, depending on type, like in the following symbols: But what is the arrow actually pointing at? And from where are ...
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How can a high-voltage transistor be in such small packaging?

For example: STN0214 - Very high voltage NPN power transistor It is said to accept more than 1 kV between its collector and emitter. It comes in a SOT-223 package (3 pins plus a tab). With a ...
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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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Explain what is a transistor in plain English!

Can someone care to explain - in plain English - what a transistor is and how it works. I know computers are full of these but don't know much about them. P.S. I know about this site from another one ...
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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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Is transistor the only electronic component on a CPU?

I have been reading about CPUs recently and came to know that all logical blocks and memory on CPU can be made out of transistors. So is it the only electronic component on CPU? Edit(Made after first ...
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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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How do I know if which leg is the emitter or the collector? (Transistor)

I'm new to electronics and here's a newbie question that I would like to ask: How do I know if which leg is the emitter or the collector in a Transistor (For both PNP and NPN) using only an analog ...
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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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Is MOSFET gate threshold voltage a limit or minimal “Full-on” switching voltage?

I've been shopping for some mosfet transistors, for a starter kit, and noticed listings that state a mosfet is suitable for 5v logic, but the data sheets says Gate Threshold is 1-2v. 4v gated mosfets,...
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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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Why are two transistors often used instead of one?

Many circuit designs I see with transistors use two transistors chained together instead of just using one transistor. Case in point: This circuit is designed to allow a device with a 3.3V UART to ...
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Why is a MOSFET triggered by Vgs and not Vgd?

Looking carefully at this diagram of one type of MOSFET: (found in this application note) We can see the device is virtually symmetrical. What makes the gate reference itself to the source and not ...
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How can I design a circuit that will turn on when a wire is cut?

I'm trying to build my own cable lock alarm (for science!) and am having trouble figuring out a way to detect that a wire has been cut. The circuit has to Use no power at idle (or at least, very ...
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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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TO-92 not getting hot enough

I am controlling a DC fan by an AVR MCU and I am curious about the thermal characteristics of a 2N3904 NPN transistor over which the fan is connected. Reading the transistor's datasheet, I find the ...
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Equivalent circuit of a solid state relay

I want to switch 50V AC voltage. Maximum drained current will be 5A. Frequency is 50Hz. Switching speed is not important, can be real slow, it is not a problem in my application. I wanted to use ...
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What does “the resistance looking into” mean?

See this circuit, from Sedra & Smith microelectronics, 6th edition, page 287: It says that the resistance between the gate and source looking into the source is 1/gm, but that the resistance ...
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Unnecessary pull down resistors on BJT and FET transistors?

I commonly see weak pull down resistors at the base of NPN transistors. Many electronic sites even recommend doing such things, usually specifying the value as something like 10x the base current ...
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Basic questions about transistor amplification

Can anyone explain how a transistor can amplify voltage or current? According to me, amplification means - You send in something small, it comes out bigger. Say for example, I want to amplify a sound ...
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Simple Audio amplifier Using Transistors

I would like to build a simple audio amplifier using transistors. I know there are IC designs specifically for the task. But i want to use transistors so i can learn how to use them for amplification. ...
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What's the difference between MOSFETs and BJTs (from a circuit analysis perspective)?

When analyzing circuits with transistors in them, when does it make a difference whether they are MOSFETs or BJTs?
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What is the difference between SOT 23 and SOT 23-3 package types?

I am faced with two types of packaging: SOT 23 SOT 23-3 Is there some difference between them? Based an article in Wikipedia it seems like there are no differences.
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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 ...