Tagged Questions
2
votes
1answer
100 views
Why transistor is non-linear in Cutoff & Saturation mode?
I am imagining Linearity in the sense that
F(ax+by)=aF(x) + bF(y)
Now suppose I have BJT characteristic curve :
In Active region, The response is : Ic = ...
0
votes
3answers
176 views
Input impedance is higher in Common-Emitter than Common-Base?
Why is input impedance is higher in Common-Emitter configuration as compared to Common-Base configuration as signal is applied on Base-Emitter junction in both Amplifiers. Not in both configurations ...
1
vote
2answers
87 views
DC Voltage Grounded with AC Input
This is an image from a book showing us how to solve BJT circuits via AC analysis:
So I understand that when doing AC analysis to ground all the DC voltage sources but I don't understand why. Can ...
1
vote
1answer
94 views
Fet is named field effect transistor,why?
Fet is named field effect transistor transistor while field effect is also producing in bjt. What are the factors/reasons that Fet is called field effect while bjt is not? I have read textbook but it ...
4
votes
4answers
358 views
Why JFET is called voltage controlled device while BJT is called current controlled device?
My question is why JFET/FET is called voltage controlled device while BJT is called current controlled as both required voltage to operate properly.In both electric field is produced so why is the ...
20
votes
5answers
835 views
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 ...
-1
votes
1answer
213 views
Design rules for power AB amplifier stage [closed]
could you please point me to a good guide for a simple class AB power amplifier design based on BJTs (referring to the "classic" schema with complementary push pull, diodes to remove cross over, ...
3
votes
3answers
350 views
Problems with Transistor TIP122
I was testing some things on my protoboard to learn a little about transistors.
I made this:
The batteries are two AA 1.5V in series.
The resistor is a 1Kohm.
The transistor in a TIP122 npn ...
-4
votes
1answer
218 views
Typical BJT npn transistor - Change 0-5V sensitivity [closed]
What is it in an npn transistor that determines the voltage "maximum" for the base?
because in a typical npn transistor it is 5V, meaning if you feed 5 volt into the base it will flow 100%, 1 volt = ...
2
votes
2answers
155 views
BJT Saturation Design Point?
I was reading an answer to this other question which asks about saturation of a BJT transistor.
In that answer it states to be saturated you should shoot for a 1:10 ratio or more between base current ...
1
vote
1answer
94 views
Will a MMBT5087 saturate with a 1K resistor in the base?
I am using the configuration in picture and the MMBT5087 PNP transistor. I am not using the top base resistor, just the lower one, so there will be 5V on the base.
My question is: Using a 1K ...
1
vote
2answers
536 views
Could I replace a diode with a BJT in a pinch?
I'm trying to learn the device physics of these transistors. If I were out of diodes and really needed one to complete my circuit, could I just replace the diode with the emitter and base of a BJT? Or ...
2
votes
2answers
105 views
finding the state of a transistor
If I am using the 'beta' (dc gain) value of a transistor to calculate Ic and Ib in a bjt transistor , does that mean I have already assumed that the transistor is in active region?
2
votes
2answers
404 views
Latching power switch question
I have the following circuit (source: EDN):
I understand the basic working principle of the circuit and it works fine when it is powered from a 1.5V source.
But when I power the circuit from a 9V ...
1
vote
1answer
170 views
cutoff state of transistor
A transistor has an npn junction. It has collector, base and emitter region. For it to be in cutoff, we can make the base lead in open mode. Now, what will be the correct schematic diagram, (showing ...
2
votes
3answers
422 views
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 ...
8
votes
1answer
296 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 ...
6
votes
1answer
213 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 ...
2
votes
1answer
285 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.
...
4
votes
3answers
2k 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 ...
5
votes
3answers
381 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 ...
6
votes
5answers
510 views
Why is my transistor accepting power from a base?
I have a transistor (NPN, to be exact it's a SS9014). Whenever I run 4.5 V through the base, connect a resistor and LED to the emitter and connect the LED to ground, the LED has a faint glow. Why is ...
2
votes
3answers
276 views
bjt transistor circuit - finding the resistances
I have been given the followign circuit diagram:
and have been asked to compute \$R_2\$ and \$R_C\$ such that, at the Q-point, the circuit will behave as follows:
\$V_{CE}=5\:\mathrm{V}\$
...
3
votes
2answers
278 views
BJT in Active/Linear mode — How can it still conduct current?
So I've read quite a few books on how BJT's work and all of them seem to gloss over how a BJT in the linear mode with the base/emitter junction forward biased and ...
3
votes
2answers
196 views
When is a lower gain transistor preferable? - Letter suffix on bipolar transistors
Some transistors are available with a letter suffix indicating hFE.
Eg. BC547 is available as (typ. @Ic = 2mA):
- A => 170
- B => 300
- C => 520
I understand why you would choose C, hFE being high ...
5
votes
1answer
1k views
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?
3
votes
2answers
2k views
What is the formal definition of Quiescent Point?
What is Quiescent Point? Is it necessary for it to be related with any V vs I , I vs I, or any such characteristic curves intersection point?
4
votes
1answer
1k views
How does this diode compensate bias current change on a BJT due to temperature variation?
'Im working on some exercises related to BJTs and found this circuit which is said to compensate (in theory) changes caused to bias current by temperature.
...
4
votes
5answers
519 views
The 'gap' between understanding of transistors and their application in real circuits
How do they even work? I'm in senior high school year and have electronics as a subject. I'm really interested in getting to understand this and take up electronics in college too. But now this seems ...
1
vote
1answer
222 views
Features values of typical BJT/MOS transistors
Is there a page that lists the values of all the features of typical BJT/CMOS transistors? e.g. ON-resistance, gate/base capacitance, amount of power the D-S/C-E junction is able to hold etc.
Of ...
20
votes
2answers
737 views
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 ...
3
votes
2answers
400 views
limiting base current to BJT
I'm actually working with a darlington pair of BJTs.
My question is if I need to put a 1kohm resistor between my MCU pin and the BASE to use it as a switch. Since the Hfe ratio for my transistor is ...
13
votes
4answers
5k views
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 ...
12
votes
4answers
7k views
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 ...
9
votes
2answers
813 views
What's the difference between NPN and PNP transistors?
Suppose that I know how an NPN transistor works.
How different is a PNP transistor? What are the operational differences between a PNP and a NPN?
4
votes
3answers
2k views
How do BJT transistors work in a saturated state?
This is what I know about NPN BJTs (Bipolar Junction Transistors):
The Base-Emitter current is amplified HFE times at Collector-Emitter, so that Ice = Ibe * HFE
...
2
votes
2answers
853 views
Low-noise audio-frequency BJT?
I found myself looking for a low-noise PNP transistor the other day.
I couldn't find any parametric searches that listed noise performance. Even from the transistor manufacturers. You can search by ...
3
votes
1answer
418 views
How should I understand the amount of base current in BJT transistors?
I'm having trouble understanding how much current flows through the base of a BJT. With a MOSFET the answer is easy: 0. How do I calculate the amount of base current in a BJT and am I doing it wrong ...
