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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Why aren't coupling capacitors used in a basic BJT diff amp?
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
50 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 ...
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1answer
63 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
609 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
137 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
99 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
215 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 ...
3
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2answers
214 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
95 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
132 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 ...
2
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1answer
115 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 ...
3
votes
3answers
170 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
56 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
175 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
86 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
244 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
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
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4answers
324 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
136 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
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2answers
152 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
159 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
156 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
106 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
146 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
221 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 ...
7
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1answer
230 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
824 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 ...
4
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1answer
164 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
328 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
210 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
342 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 ...
3
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4answers
322 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
308 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
213 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
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2answers
154 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 ...
3
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2answers
248 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
182 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 ...
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0answers
148 views
Z(in) for small signal analysis with BJT for unbypassed emitter and r0 in place
I have been studying the small signal analysis of BJT with re model with the emitter terminal not bypassed. I am able to derive those Zi, Zo,Av, Ai long as ro is assumed to be infinite. But I am ...
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2answers
526 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 ...
7
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4answers
408 views
Why would you attach a Diode to the base of a BJT?
I was looking at a DC BJT setup for sourcing current and came accross this
I have never seen a diode attached to the base of BJTs before and was wondering what it might be used for? I believe it ...
2
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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?
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2answers
400 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 ...
0
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1answer
135 views
Exam question: measure amplifier \$h_{oe}\$ using a signal generator and oscilloscope
I'm running through some old exam papers at the moment and one of the questions are:
The parameter \$h_{oe}\$ is needed to calculate the output impedance of an amplifier. Explain in detail how to ...
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0answers
42 views
Hybird Model Equations for a BJT
I want to get the hybird parameters for the common-base configuration in terms of the parameters for the common emitter configuration. If I have (hie, hfe, hoe, hre) I want to find (hib, hfb, hob, ...
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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 ...
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3answers
418 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 ...
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1answer
294 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 ...
2
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2answers
268 views
What is the ON resistance of a BJT like a 2N3904?
I wanted to use a BJT as a switch.e I want to drive the base from a uC pin at 3.3V. I have been told a FET needs a higher turn on voltage so I thought a BJT would be better. I am switching on a ...
