A type of bipolar transistor comprised of n-type semiconductor in contact with p-type semiconductor which in turn is in contact with a n-type semiconductor -> thus NPN. The transistor has three terminals, the Emitter, Base and the Collector. Current injected into the base controls the flow of current from Emitter to Collector enabling amplification and switching.
Questions tagged [npn]
824 questions
20
votes
2 answers
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 communicate with a 5V microcontroller.
I understand…
Nate
- 4,496
- 11
- 56
- 75
18
votes
3 answers
Transistors in parallel
I want to use several transistors in parallel to control current through a load. This is to distribute the current through the load across the transistors so that individual transistors with a rated collector current less than that going through the…
Jodes
- 5,044
- 9
- 48
- 75
17
votes
3 answers
Why NPN and PNP in parallel in this distortion pedal circuit?
I've come across a guitar pedal circuit called a "Tube Sound Overdrive". The schematic is shown below. It looks like a typical common emitter transistor circuit with an adjustable bias (RV1), except there are two transistors, NPN and PNP in…
Dave H.
- 510
- 3
- 12
17
votes
6 answers
Diode connected BJT instead of a diode
I have read wikipedia and other websites regarding the purpose of a diode connected BJT, but I am not clear on where should we use a diode connected BJT.
Where is a diode connected BJT used? Like, in what type of requirements do we need to use a…
user220456
14
votes
1 answer
Protecting NPN transistor from negative base-emitter voltage?
I have a circuit that converts 5V RS-232 polarity signals (logical 0 = +5V, logical 1 = -5V) to 3.3V TTL polarity (logical 1 = 3.3V, logical 0 = 0V) using a BC548 transistor.
It forms a NOT gate so that when the RS-232 output is high, it pulls the…
heypete
- 551
- 2
- 6
- 14
13
votes
2 answers
Which resistor for NPN transistor base?
Which is the way to choose a resistor for a NPN transistor base?
I want to use P2N2222A as a switch, in a design as shown below. When I have voltage in base (1.8 V), I want a connection to be created between NODE1 and ground.
simulate this circuit…
dempap
- 649
- 1
- 11
- 22
13
votes
1 answer
Can the Common Emmiter Amplifier use a PNP Transistor instead?
This is a simple Class A amplifier using an NPN transistor:
Is there a way this can use a PNP transistor, instead (with only 0V and +Vcc, not 0V and -Vcc)? Why or why not?
Ehryk
- 1,141
- 5
- 16
- 28
12
votes
8 answers
Sinking and sourcing current
I have been reading that NPN transistors are sinking and PNP are sourcing devices. I do not really understand this concept. It says current source device connects load to Vcc and current sinking device connects to ground (low voltage).
So does…
Abhishek
- 153
- 1
- 2
- 4
12
votes
1 answer
Understanding a tachometer signal conditioning circuit I inherited
I have inherited a tachometer signal conditioning circuit.
It has an NPN transistor (2N3904) with a Schottky diode (BAT54) between the emitter and the base. It is preceded in the circuit by an LT1017 comparator - so input should be a 5 V square…
Michael Nunan
- 121
- 4
12
votes
3 answers
How do I get +5v for loud noise, 0v for silence from Electret Microphone (or other components)?
I have been asking a few questions here to get to a proper one, the initial questions I asked are linked to at the end. I used Fritzing to make up some schematics of my initial thoughts, but at the very least I need help with the values on the…
Ehryk
- 1,141
- 5
- 16
- 28
11
votes
3 answers
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 switch the power to the relay on and off. Here's…
Mason
- 543
- 2
- 8
- 15
10
votes
7 answers
With a transistor (as a switch), does the order of the parts matter?
I've wired up my project(s) similar to how they're laid out in A to control whether or not an LED is on or off. It works fine (as far as I know), but I've seen lots of examples online of circuits laid out in a B formation (including this SparkFun…
Dan Forever
- 243
- 2
- 8
10
votes
2 answers
NPN transistor slow to turn off even when operated outside of saturation region
I am currently studying how transistors react and behave to simple signals. I have the following circuit:
A square wave of approximately 83kHz is applied. The transistor used is a 2N5551. The resistor values were carefully chosen to try to avoid…
Kevin
- 103
- 5
10
votes
3 answers
Why would I use 2N3904 instead of 2N4401?
2N3904 and 2N4401 appear to be very comparable parts in all specs. 2N4401 has a higher current rating, but otherwise they look to be about the same in price and everything else. Obviously it's difficult to give a 100% universal answer since both…
Stephen Collings
- 17,373
- 17
- 92
- 180
10
votes
1 answer
How protective diode protects transistor from breakdown?
Please, explain the process of breakdown; how exactly this protective diode protects the transistor?
In a Horowitz & Hill book, "The Art of Electronics", 2nd edition, in "Chapter 2 - Transistors" (page 68), i read the following:
Always remember…
yulian
- 203
- 2
- 6