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).

The transistor is a family of semiconductor devices, usually with three terminals. It is basically a semiconductor version of a triode. The word transistor comes from "transfer-resistor", since it can amplify and transfer a signal from a low-resistance circuit to a high-resistance circuit.

We can think of a transistor as a variable resistor whose resistance can be varied by an electrical signal.

BJTs are current-controlled current sources, whereas FETs are voltage-controlled current sources. Both can be used as an amplifier, or as a switch.

There are three basic circuit patterns around a BJT transistor:

  1. common emitter circuit
  2. common base circuit
  3. common collector circuit

Further reading:

Related tags:

7762 questions
103
votes
8 answers

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
  • 4,980
  • 13
  • 52
  • 56
92
votes
5 answers

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?
hauntsaninja
  • 1,023
  • 1
  • 7
  • 5
73
votes
2 answers

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
  • 3,557
  • 3
  • 33
  • 37
69
votes
9 answers

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 not on transistors or other electronic components…
Al Kepp
  • 3,071
  • 7
  • 30
  • 32
68
votes
9 answers

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
  • 4,980
  • 13
  • 52
  • 56
60
votes
5 answers

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 transistor…
Phil Frost
  • 58,448
  • 21
  • 155
  • 275
58
votes
7 answers

Why don't we make CPUs with 1000s of layers to make use of space in the third dimension?

I am wondering why don't we make processors such as CPUs with 1000s of stacked layers to make use of space in the third dimension now that we have three-dimensional transistors. To be clear I'm referencing making something of a rectangular-prism…
Ethan
  • 719
  • 1
  • 7
  • 9
57
votes
5 answers

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 one to work. I understand there's some…
Robert Atkins
  • 2,267
  • 5
  • 26
  • 44
53
votes
5 answers

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?
Tyler DeWitt
  • 749
  • 1
  • 7
  • 9
50
votes
8 answers

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?
user186424
49
votes
9 answers

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 digital output to drive a load, or from an analogue…
abdullah kahraman
  • 6,050
  • 8
  • 65
  • 105
48
votes
5 answers

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. Wouldn't using a BJT result in current flowing into…
Nate
  • 1,718
  • 3
  • 16
  • 19
47
votes
3 answers

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 +500V can be easily generated. The transistor models…
hkBattousai
  • 14,423
  • 39
  • 122
  • 206
46
votes
2 answers

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 lot of amps). Why does a CPU need more than 1 A…
Leonardo Bosquett
  • 619
  • 1
  • 5
  • 5
46
votes
3 answers

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
  • 509
  • 4
  • 6
1
2 3
…
99 100