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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4
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2answers
723 views

Collector current in linear region

We all know that when the transistor is in its linear or active region, collector current does not increase further. At this point, the base collector junction is reverse bias. But how come there is ...
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3answers
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Importance of constant collector current (BJT)

I was reading these notes and I have a couple of questions. What is the importance of the constant collector current in varying voltages across collector and emitter? How does it really relate/help ...
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3answers
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Driving a large 7 segment display using a micro-controller

I need to drive a large seven segment display (segment length is a about 2 inches) using a micro-controller (PIC 16F877). I've driven normal seven segments directly from PICs without issues, but I ...
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2answers
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CMOS inverter with gate of PMOS transistor always grounded

If there is a CMOS inverter such that the gate of the PMOS transistor is always attached to the ground and the input voltage is only applied to the gate of NMOS, then how would the inverter behave, as ...
2
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2answers
895 views

How to explain the difference in modeling small signals for BJT and JFET devices?

I know there is a difference between modeling BJT and JFET devices for small signals, but I would really like to know, in depth, how to explain this difference.
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2answers
675 views

When is an input signal considered small for BJT and JFET devices?

I can't find a detailed answer to this question. What is the condition for a signal to be considered small, so that you have to use small signal modeling? Also how is this condition derived for both ...
5
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1answer
11k views

Current limiters using transistors and diodes

I have been experimenting with simulating current limiting circuits. I am trying to limit current to ~500mA given a fixed 4.8V source. I started a question based on a conventional 2-transistor current ...
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3answers
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Help needed optimising simple current limiting circuit

I have been experimenting with simulating current limiting circuits. I am trying to limit current to ~500mA given a fixed 4.8V source. I have started using a circuit like the one found on this ...
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2answers
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Is a large hFE value a sign of damage?

I have been testing a couple of BFY50 transistors that I salvaged using my multimeter. One of them reports an hFE of ~109 (this looks consistent with the data sheet value of 112). The multimeter ...
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2answers
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Transistor layout for AOI gate

This is more of an academic exercise than something practical. I've uploaded the image from a solution manual to a problem I was working on. I can't seem to agree with what they've done. Also I can't ...
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4answers
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Replacing a switch with a transistor

I am working on a project on the Raspberry Pi, it is a bit experimental as I am learning the electronics side as I go along... please edit out any problems with terminology. My aim is to replace a ...
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3answers
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Basic PNP transistor on a microprocessor output pin

I have a 5v microprocessor that needs a PNP transistor between it and the common anode of an LED display, as it can't source enough current to drive all of the LED's without burning up. I understand ...
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2answers
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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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In a circuit being switched by an NPN transistor, do power supply and input need the same ground?

I'm trying to make a circuit that will allow me to switch on a relay that will turn on an LED. However, the relay is rated for 12 V, and I only have an input of 5 V, so I'm using an NPN transistor. to ...
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3answers
7k views

Why would an IR remote circuit use one resistor/transistor pair for each LED?

I've done some reverse engineering work on the protcol for a remote that... sucks. I'm replacing it with one of my own design, but while I've improved on the interface, the range and view angle of my ...
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4answers
16k views

Simple design to switch 24v from PIC

If I were wanting to drive i.e. industrial relays which require 24v from a Microchip PIC what would I need to look at in terms of a general transistor circuit (and it's pitfalls when driving such a ...
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1answer
731 views

How to dual power PIC18F4550?

In the datasheet there is this shematic how to dual power 18F4550. However they don't specify which transistor to use and what resistor values have to be. For capacitor I use 100uF (diode side) and ...
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1answer
2k views

How to increase current drive of Charlieplexed LED array?

I am driving 15 LED's with a Charlieplexed circuit (using an Atmel ATTiny13). I've reached the current limit per I/O pin of 40mA and would like to drive the LED's higher (they're rated at 50mA). Is ...
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4answers
7k views

Replacement Transistor for Ultrasound Cleaner

I'm trying to repair the generator board on a Sonix IV ultrasound cleaner. I need to find a replacement for a transistor (in a TO-3 package), but the manufacturer uses an in-house part id, so I'm ...
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1answer
412 views

On/Off with IR Reciever

I want to use a radio shack 38kHz IR receiver to make basically an on/off switch. All I need to do is that when the receiver "sees" any signal from a remote is switch on a 9v supply on. I am trying to ...
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4answers
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4026 + 7-segment display: do I need transistors?

I'm a beginner and, as an exercise, I'm building a digital clock using without a microcontroller. The main IC that I'm using is a 4026, which counts and converts do 7-segment led format. The leds in ...
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3answers
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P-Channel MOSFET Drain-Source Polarity in Power Switch Application

I'm looking at using the LTC4412 power switch in an application. From the datasheet, here is the typical application circuit: : In the application circuit, an FDN306P p-channel MOSFET is used. I'm ...
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2answers
36k views

List of general purpose transistors [closed]

It's been more than 20 years since I've done electronic design and I'm sure I've forgotten nearly everything. Now I have a purpose and want to start with a list of general purpose, readily obtainable ...
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1answer
589 views

Loop through a vector in ngspice

ngspice is not capable of performing a DC analysis while sweeping transistor length. I'm trying to do a workaround by creating a vector, looping through it, and altering ...
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3answers
10k views

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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5answers
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Why is there a delay in a common anode 7-segment display?

I have a common anode 4-digit 7-segment display. I have each segment cathode connected to a 330-ohm resistor, and the resistor is connected to a shift register. Each segment cathode is connected to ...
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4answers
1k views

Modular Analog Circuit formula solver?

I've been thinking of making a database / program to make building analog circuits easier. I wanted to make sure there isn't anything else like this that I can contribute to instead of going through ...
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2answers
13k views

Arduino/Atmega with TIP120/121/122 transistors: base current question

I am learning about transistors, and can't seem to find the answer to a few questions which are stumping me. I will be referring to the TIP120/121/122 Darlington Transistor. As I understand ...
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2answers
2k views

What's the difference between small signal bipolar junction transistors (BJTs) marketed as switches vs. amplifiers?

For instance, the MMBT3904 and MMBT3906 BJTs are listed as NPN/PNP Switching Transistors, and the datasheets mention the switching times, while the BC846 and BC856 BJTs are listed as NPN/PNP general ...
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2answers
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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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1answer
614 views

Backup battery tester circuit

I am attempting to read the voltage on a backup battery on an Arduino. To that end, I used the circuit Olin Lathrop offered in answer to this question. I am powering the Arduino with 5V, and the ...
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2answers
29k views

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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1answer
446 views

How can I use a transistor as a switch?

I'm trying to drive a DC motor using an external voltage source and control it using a transistor switch. It doesn't seem to be working during the simulation though. I know I must be doing something ...
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3answers
5k views

Transistor Replacement [closed]

I have dell laptop D600m. I have a charger for it but the charger does not work properly meaning that it can power the laptop but does not charge the battery of the laptop. I opened the charger and ...
9
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3answers
19k views

Why does the base-emitter voltage of a BJT decrease with temperature?

According to Sedra/Smith Microelectronic Circuits, \$v_{BE}\$ changes by \$-2\text{mV}/\text{°C}\$. I cannot understand how this could possibly be the case given the equations I am familiar with. ...
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1answer
557 views

Audio Signal Switching Transistor For Shutter Release

I am a complete novice when it comes to electronics but I am trying to open a transistor when there is audio coming out of my iPod, completing the circuit for my dslr and releasing the shutter. I ...
2
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1answer
2k views

Transistor With No Voltage to Base

In the newest Make magazine, they show how to connect LEDs in line to speakers to have the LEDs light up when the speaker is playing music (pulse with the music). They say you may want to hook up the ...
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4answers
657 views

Moore's Law Timeline

I find it incredible how transistors per area keeps increasing. How has it been achieved so far? My ignorance tells me that if ICs were designed properly from stage to stage they should have got this ...
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2answers
8k views

Driving Solenoids from Arduino

I was wondering what the best way to run a solenoid would be from an Arduino board. I was thinking about using a MOSFET that runs into a relay to turn on the solenoid but am not sure how to go about ...
4
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2answers
1k views

Will a TIP42 suffice for this project that calls for a BC559 (transistors)?

Im building a Nintendo 64 USB adapter as outlined in this guide. For the transistor, it calls for a BC559. At RadioShack, the most similar one I could find was a TIP42. Will this work for this ...
12
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4answers
31k views

Where does the Maximum Emitter-Base Voltage come from?

The datasheet for a BD679 transistor lists amongst the absolute maximum ratings that the "Emitter Base Voltage" has a maximum of 5v. This figure confuses me - my mental model of a (BJT) transistor ...
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2answers
4k views

PNP transistor switching time - puzzler

Updated based on feedback with better screenshots (I hope) - I am also working on getting the files onto a file sharing site so they can be accessed at full resolution I am really confused about the ...
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1answer
4k views

Driving a 12V door strike from 3V3 BeagleBone?

I'm trying to control a 12V electronic door strike lock from my 3V3 BeagleBone. Here's a diagram of my circuit: My circuit probably needs a NPN transisor of some sort, but I'm not sure which one ...
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1answer
2k views

Confused about transistor operation

I'm trying to use a BC108A (datasheet) transistor as a switch for a 12V power supply using the 5V digital out pins on an Arduino Uno board. I have connected the collector the 12V supply, the base to ...
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5answers
5k views

Why are sum-of-products implementations more popular than product-of-sums implementations?

In my book about circuit design (Fundamentals of Digital Logic with VHDL from Stephen Brown and Zvonko Vranesic), the writers always prefer a sum-of-product for representing and implementing simple ...
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1answer
11k views

Light sensor switch using LDR

What is the reason for using the transistor (2N3904) is this circuit? What good is this transistor doing in this circuit.What happens if we remove the transistor from this circuit? And what will ...
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2answers
3k views

What transistor should I use to power a motor from an xbee

I have a small motor what operates at 3.3V at 50mA (sorry no documentation) and a Xbee radio which outputs 1mW. I would like to allow the Digital I/O pins output on the Xbee to drive the motor ...
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4answers
2k views

Do PNP transistors have low noise?

I read somewhere that PNP transistors have lower noise than NPN transistors. The majority carriers in NPN are electrons where as in PNP they are holes, so I thought the noise would be much be less in ...
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3answers
2k views

How to use PNP transistor as a switch for serial communication

I am trying to interface a 3.3volt TTY bluetooth dongle to my arduino. The dongle requires 3.3volt but the arduino's Tx and Rx signals are 5 volt. Apperently the 3.3volt is enough for the arduino Rx,...
6
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3answers
5k views

Calculating Rout of a bjt while using correct RL

I'm studying the notes of my professor of electronics, and I'm noticing an inconsistency of \$R_{L}\:\$ compared to what I've seen around in books and on the web. Here's what is considered \$R_{L}\:\$...

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