95 votes
Accepted

Why are there so many relays used in a car, instead of transistors?

Relays are much more stable temperature-wise: a sealed relay has essentially the same characteristics at -30°C and +70°C, both temperatures being common for cars. A transistor works quite differently ...
Dmitry Grigoryev's user avatar
80 votes
Accepted

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

The two killer reasons are yield, and heat. Yield. Every time you do a process step, you get less than 100% perfection. Let's say you get 99% perfection per step. In a process with 20 steps, you would ...
Neil_UK's user avatar
  • 165k
76 votes

Why are NAND gates used to make AND gates in computers?

In order to get non-inverting operation for logic (i.e., AND or OR vs. NAND or NOR), you need to operate the transistors in common-drain mode, also known as "source follower" mode. Among the problems ...
Dave Tweed's user avatar
  • 172k
73 votes
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Why do CPUs need so much current?

CPUs are not 'simple' by any stretch of the imagination. Because they have a few billion transistors, each one of which will have some small leakage at idle and has to charge and discharge gate and ...
alex.forencich's user avatar
66 votes
Accepted

What's the purpose of these two diodes in this circuit?

This is a crude current source. The two diodes are to create a voltage drop of approximately 1.4v, which is then applied across the transistor VBE and R21. This gives approximately 0.7v on R21, or ...
Neil_UK's user avatar
  • 165k
56 votes
Accepted

Why use two transistors driving LEDs when one would work?

It's a constant-current circuit. The current should be about 0.7/68 ~=10mA. simulate this circuit – Schematic created using CircuitLab Your circuit with one transistor would allow a highly ...
Spehro Pefhany's user avatar
53 votes

When is a MOSFET more appropriate as a switch than a BJT?

When is a MOSFET more appropriate as a switch than a BJT? Answer: 1) a MOSFET is better than a BJT when: When you need really low power. MOSFETs are voltage-controlled. So, you can just charge ...
Gabriel Staples's user avatar
53 votes
Accepted

Why don't we use GaN transistors everywhere?

I've been using GaN extensively since 2013 or so, primarily for a niche application that can easily benefit from one huge advantage GaN has over Si -- radiation tolerance. There's no gate-oxide to ...
Krunal Desai's user avatar
  • 6,306
53 votes

Why would an AND gate need six transistors?

Your classmate is wrongly treating the transistors in your circuit as magical devices whose behaviour is completely controlled by something that appears at the gate and only the gate. They are failing ...
DKNguyen's user avatar
  • 56.4k
52 votes
Accepted

Is transistor the only electronic component on a CPU?

The logical blocks and memories can be made out of only transistors. The important question is: are all of the circuits on CPUs logical blocks and memories, or is there anything else? The answer ...
Justin's user avatar
  • 5,974
48 votes

Why are there so many relays used in a car, instead of transistors?

A car is (still, in the modern era) an electrically harsh environment. The "12V supply" is typically 13.5 to 15 V, and may occasionally spike to 80V or more. There may also be some high-frequency junk ...
Ecnerwal's user avatar
  • 5,614
48 votes
Accepted

What's the intuition for reading transistor/resistor circuits?

What's a step by step way of thinking about the flow of electrons in such a circuit? How can we tell it would provide constant current for instance? The first step is to forget about flow of ...
Transistor's user avatar
  • 175k
47 votes
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What is the difference between SOT 23 and SOT 23-3 package types?

In the beginning of the surface mount era was SOT 23, which had three pins. It was used mainly to replace the very popular TO-18 and TO-92 3 pin leaded transistor packages, the name means Small ...
Neil_UK's user avatar
  • 165k
45 votes
Accepted

How does a transistor act as an amplifier considering energy conservation?

The base current in a transistor controls the collector current. The energy comes from the power supply. It is not generated within the transistor.
Peter Bennett's user avatar
41 votes

In this transistor circuit, what is the purpose of the diode across the motor?

The diode in this configuration is called a "flyback" diode. A motor consists of a coil of wire which is effectively an inductor (and an electromagnet). As the motor spins, the coil will be turned on ...
Los Frijoles's user avatar
  • 2,252
41 votes
Accepted

How can I use a 2N2222 transistor as a temperature sensor?

TMP36 is specifically a temperature sensor, not an NPN transistor like the 2N2222. You might have them confused because they both can come in TO-92 package. It is possible to look at the various ...
crossroad's user avatar
  • 1,400
39 votes
Accepted

What is this unusual NPN-PNP transistor pair?

These transistors are devices which are highly optimized for use in ultra-low-cost and high-volume consumer products- off-line fluorescent lamp ballasts. The anti-saturation network (for fast ...
Spehro Pefhany's user avatar
36 votes

Why don't we use GaN transistors everywhere?

it surely must compensate if it's used in IC's Well no, it does not for several reasons: GaN transistors cannot easily be made in today's IC manufacturing processes Not every application needs the ...
Bimpelrekkie's user avatar
  • 80.8k
36 votes
Accepted

What does this transistor circuit do?

The circuit topology is that of a capacitance multiplier designed to remove ripple from a 'dirty' regulated power supply. The name stems from the fact that the value of the capacitance seen from the ...
Sredni Vashtar's user avatar
34 votes
Accepted

Is it good practice to run large amounts of current through a MOSFET?

Why can a thick copper wire handle a large current? Because it has a low resistance. As long as you keep the resistance low (switch the MOSFET fully on, for example use Vgs = 10 V as in the datasheet ...
Bimpelrekkie's user avatar
  • 80.8k
34 votes
Accepted

Why would an AND gate need six transistors?

In the logic gate level digital design abstraction, inputs are assumed to switch from logic HIGH to logic LOW and vice-versa instantaneously. This is done to simplify logic design. However, in the ...
Shashank V M's user avatar
  • 2,311
34 votes
Accepted

Why do CMOS NOT gate designs differ from BJT NOT gate designs?

However, I've always been perplexed as to why people do not use the former method for chip manufacturing. In chip design, resistors take up more area than transistors, and area is one of the main ...
The Photon's user avatar
  • 129k
33 votes
Accepted

TO-92 not getting hot enough

The 0.83 W is the power going into the fan, not the transistor. The power dissipated by the transistor is essentially the same current value, but multiplied by the voltage from C to E, which is ...
Dave Tweed's user avatar
  • 172k
33 votes

2N2222 burning out in the lab at 22 V

The only possible answer is: you don't have that circuit. So one of the following must be true: Your transistor isn't actually 2N2222, Your circuit is not wired as you think, Your voltage isn't 22 V, ...
jonathanjo's user avatar
  • 13.9k
30 votes
Accepted

Can you make a CPU out of electronic components drawn by hand on paper?

If the gain of a single inverter is less than unity, then it will not be possible to combine any significant number of gates together to build a larger circuit. The signal levels will just peter out. ...
Dave Tweed's user avatar
  • 172k
30 votes

Can two BJT transistors work as a full bridge rectifier?

That would be something like this... This is kind of a one-line proof, or proof by diagram; if you understand BJT operation. tl;dr It's not useful. Using common transistors, the transistor effect (...
Tim Williams's user avatar
  • 32.1k
29 votes

Is transistor the only electronic component on a CPU?

I think you have to look at it the other way around: CPUs are made with steps (implantation, lithography, etching, deposition of materials). If you design the steps and layers a certain way you get a ...
rienafairefr's user avatar
29 votes
Accepted

Effect of bootstrapping in amplifier circuit

You've framed some good questions and I've upped you for that. To address (1) and (2), let me avoid the small-signal linearization model and just have you look squarely at the circuit itself, as it ...
jonk's user avatar
  • 77.8k
29 votes
Accepted

H-Bridge with emitter followers

That is called an "H-Bridge." It is often used to drive motors forwards as well as backwards. In your case, it allows you to generate a magnetic field whose polarity and intensity you can vary ...
JRE's user avatar
  • 71k

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