BJT stands for Bipolar Junction Transistor. It is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications.
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MOSFET is not turning off when the gate terminal is grounded
This figure shows a small portion of my circuit:
The maximum Vgs of the MOSFET is 20V. The MOSFET is initially turned on and now I want to turn it off. For that I am grounding the terminal A to ...
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1answer
67 views
How to determine Frequency response in BJT
What do I want to look for on a BJT datasheet to find out the maximum operating frequency before distortion occurs? Or perhaps the question should be: How do I know how fast a transistor can cause ...
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0answers
52 views
Small-signal input and output resistances of the circuit
I am given this circuit:
And I am trying to determine the small-signal input and output resistances.
I was either given or found all the following info for the circuit:
VA = 110V
β = 115
T = ...
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5answers
166 views
Is BJT a Voltage controlled device or Current controlled device?
I have searched already for this question, but everywhere I see debate instead of clear answer. Though it is not that important of a matter whether we call it a voltage controlled device or current ...
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1answer
69 views
How can you assure JFET Q-point?
As Beta changes from BJT to another, Vp and Idss also change from a JFET to another, but how can you assure that when the JFET changes, the Q-point does no change greatly?
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2answers
134 views
Raspberry Pi GPIO
I need to make a connection between the "Device"'s 2 wires using a Raspberry Pi, but I am unsure how to go about doing this (and I don't want to just play about with it and get it wrong as I might ...
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3answers
91 views
Use bipolar transistor to power LED from a certain power on?
I'm trying to do the following: I have a LED that is supposed to be on, whenever a control voltage (CTRL) exceeds a certain voltage (0.7V). To do this, I created the following circuit:
I know that ...
2
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2answers
59 views
Concept of “resistance” and “power dissipation” in a transistor - Explanation?
Ok, so I've got way more experience with mosfets than BJTs. I'd become very accustomed to the idea of mosfets having a specific 'on' resistance, which would allow me to calculate actual heat ...
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2answers
96 views
> 20 Minimum Hfe on a BJT Power Transistor?
I'm having trouble finding what I want due to the way mouser.com limits searching on BJTs. I'm trying to find out if anyone manufacturers a medium to high power BJT with a gain of at least 30. The ...
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3answers
139 views
Switching 12V with an active low 5V signal
I’m trying to switch 12V (max 30mA) with a signal from a MCU that I’d prefer to be active low (so the output voltage is 12V when the control signal is 0V, and 0V when the control signal is 5V).
Since ...
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5answers
123 views
NPN BJT in saturation, base voltage irrelevant?
I'm trying to build as simple a circuit as possible that will pull a logic line low every time an automotive ignition coil 'fires'. My first inclination is to simply attach the 'negative' terminal of ...
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2answers
84 views
Why aren't coupling capacitors used in a basic BJT diff amp? [closed]
In a basic BJT differential amp it is mentioned that since both positive and negative biased voltage sources are used the need for coupling capacitors is eliminated. I don't see the relation and why?
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1answer
65 views
Low voltage/current power supply for wide range of inputs
I was thinking of simple low voltage/current power supply that can accept wide range of input voltages, up to the hundreds. Let's say, the input is 20 to 160V DC and output has to be 10V with 20ma ...
2
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1answer
66 views
BJT Amplifier I/P O/P
In a CB Amplifier, base is common to both I/P and O/P. Why is I/P applied at Collector and Output at Emitter?
Could it have been the opposite?
Similarly for CC and CE amplifiers why the chosen I/P and ...
16
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4answers
888 views
Do engineers still design for volume using discrete through-hole bipolar transistors?
As a hobbyist who likes plugging BJTs and discrete components into solderless breadboard for entertainment and education, I recognise that people like me are an insufficient market to make it ...
2
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1answer
245 views
Voltage regulator with op amp(negative feedback) and MOSFET
The schematic above is a very good voltage regulator, excepting the high power dissipation for Q1 (also I'd use an NPN instead of a PNP). Can I replace Q1 and Q2 with a high current n-mosfet and ...
2
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1answer
146 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 = ...
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5answers
320 views
MOSFET or Transistor for using as switch for audio signals
I was wondering which component is better for using as voltage controlled switch for audio signals (e.g. music signal from mp3 player). Should I use MOSFETs or transistors? The max rail voltage is ...
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2answers
307 views
BJTs versus (MOS)FET to switch a load from a microcontroller
I would like to know what are the reasons for choosing bipolar junction transistor (BJT) versus a field-effect transistor (FET) (MOSFET or a JFET) to switch a load from a microcontroller. In our ...
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1answer
104 views
Why does my VU Meter work?
Good Day,
I was trying to build a VU meter for my audio circuit. Basically, when I have a louder sound, more leds turn on. My design looks something like this: And yes it works, but I just want to ...
4
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3answers
189 views
Help understanding BJT voltage comparator
I'm trying to understand how a transistor voltage comparator works. For example I've been studying this LM339:
My questions are:
What are the current sources for and how would those actually be ...
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1answer
129 views
Q-Point of a circuit
In a BJT or MOSFET circuit we have this curve:
What is that q-point? From my research I have the following information:
The operating point of a device, also known as bias point or quiescent ...
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3answers
184 views
BJT Bias Design and Operating
I'm to the task to design a BJT Common-Emitter Amplifier with resistance at the emitter as in the figure, and for that I need to:
get the values for \$R_{1}\$, \$R_{2}\$, \$R_{C}\$ and \$R_{E}\$
...
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1answer
60 views
Saturating a BJT with an open load
Is there some drawback in having a BJT transistor saturating without a load?
The scenario is this: BJT is driving a relay, but there is a switch to have the relay always on or off leaving the BJT ...
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3answers
201 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 ...
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2answers
91 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 ...
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2answers
276 views
Input and Output Impedance of a BJT
Hi I'm new to electronics and I would like to know how to measure experimentally the input and output resistance of the following common emitter configuration. I have access to a voltmeter, signal ...
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1answer
95 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 ...
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4answers
532 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 ...
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1answer
189 views
Learning Multiplexing with LEDs, transistor switching speeds
I am working with some friends trying to multiplex. The issue we run into is we aren't sure if we are hitting a limit on transistor switching speeds.
I am unsure if this is a hardware or software ...
4
votes
2answers
163 views
Signal volume LED Array
I am done a google search on this and have come up blank. What I have is a signal/sound being fed through a wire that ranges from (0-9V). When the volume is low, and i need 1 or 2 leds to light up, ...
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1answer
170 views
Emitter Resistance in BJT Amplifier
What is the importance of resistance at emitter and collector of BJT amplifier? What will be effect on output if they are removed?
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2answers
164 views
Oscillation won't pass voltage buffer
This is the full schematic of the circuit
Full size: http://img443.imageshack.us/img443/9292/fulltheremin.png
It didn't work on the first try.
A theremin is an electric instrument which emit a ...
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2answers
128 views
in saturation of BJT, the current gain is reduced
I studied in sedra and smith 'microelectronic circuits' that in saturation, beta gets changed and becomes less than the original beta (which is constant for a transistor).
How is it possible?
Because ...
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3answers
152 views
How to improve symmetry of +/-1 gain circuit?
In an attempt to have a square wave with a voltage-controlled amplitude, I've made this little circuit:
Unfortunately, when negating, it has a slight offset from -Vin.
I assume this is because ...
3
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3answers
260 views
BJT Pre-Regulator
I have a circuit that has an input of 35V peak and I would like to drop the voltage down to 5V 200mA. I was recommend to use a pre-regulator in the following configuration:
The good thing is that ...
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1answer
246 views
Nmos-bjt darlington pair dc bias. [Homework Help]
I have a homework assignment I need some help with.
I wanted to know if someone could tell me if I'm doing the dc bias calculations correctly, if not, could someone nudge me in the right direction.
...
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5answers
986 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 ...
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1answer
180 views
Current source with IGBT
A simple current source using a bipolar transistor uses the voltage over an emitter resistor as feedback of the current. The current is given by the base voltage, the \$U_{eb}\$ of the transistor, ...
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3answers
355 views
Why pull base of BJT switch?
I often notice usage pull resistor for base of bipolar transistor, e.g. \$R2\$ here:
Why it's used? I understand pull resistors for FETs, because of gate's high impedance an EMI can easy switch it. ...
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1answer
219 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, ...
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3answers
387 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 ...
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4answers
371 views
Quiescent power of BJT vs MOS transistors
This is a question about the power necessary to control a transistor circuit, i.e. the base current in BJTs vs. the gate current in MOSFETs.
Why is there quiescent power dissipated by base currents ...
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2answers
413 views
How much voltage/amperage can a NPN transistor take?
I got the MakerShed Ultimate Microcontroller Pack and it came with a "Transistor NPN". I want to use it (or probably buy something else) as an electronically controlled switch so that on arduino I set ...
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1answer
234 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 = ...
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2answers
158 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 ...
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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 ...
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2answers
284 views
Current through the reverse biased junction in transistor
A transistor is a three terminal device. One terminal is called emitter, one collector and in between them is base. Now, during biasing the junction between emitter and base is made forward biased and ...
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6answers
1k views
Speaker BJT amplifier function
I'm trying to understand the below circuit:
I have to analyze an amplifier circuit from a microphone to a speaker and don't understand how it works. I know that the first stage is a voltage ...
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1answer
189 views
BJT Amplifier Design [closed]
I am struck between the right method of biasing for BJT.I want to design a BJT power amplifier, i need maximum current at the output because of the low impedance of the load as well as adequate ...
