Questions tagged [bjt]

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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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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Flow of holes in transistor?

Bipolar transistor are said to have both electron flow and hole flow. Movement of electrons can be understood, but holes are fixed part of the atomic/crystal structure. How can we characterize their ...
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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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Oscillation won't pass voltage buffer

This is the full schematic of the circuit theremin http://img443.imageshack.us/img443/9292/fulltheremin.png Full size: http://img443.imageshack.us/img443/9292/fulltheremin.png It didn't work on the ...
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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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563 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 the ...
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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 it ...
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1answer
902 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. I'...
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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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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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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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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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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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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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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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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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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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158 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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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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current amplification of a circuit containing a transistor

I'm having trouble with a BJT circuit. What we are given: \$U_{CC} = 10\text{V}\$ \$R_C = 972 \Omega\$ \$R_B = 14\text{k} \Omega\$ \$U_{\text{BE}} = 0.7\text{V}\$ \$I_C = 12\text{mA}\$ We need to ...
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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 divider, ...
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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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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 ...
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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 ...
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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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PNP transistor to command LED

Usually I use NPN to drive LEDs but in my project I have to use PNP because the cathode of the LED is tied to GND. Then I have a question, is it possible to have a command of 3.3V and the PNP tied to ...
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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 ...
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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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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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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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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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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 ...
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Bias analysis in a dual BJT circuit with feedback

I am building an Wooly Mammoth audio circuit, and I am really interested in finding how and why it works. You can check out and simulate the whole thing here. I must say I haven't been able to get ...
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BJT, resistor, and diode work but ULN2803 does not

I am trying to control some 5V relays with a mC. At first I went with using a BJT, a 470 ohm resistor, and a 1N4004 diode to control each one of the relays. Each mC output pin is 5V, 40ma. This ...
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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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Do two stage common emitter current amplifiers add or multiply?

So I've seen alot of explanations on two stage voltage amplifiers and how how their gains can be multiplied together. My question is when you are using a two stage common emitter amplifier for current ...
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Do Fixed Bias BJT 's have a cut in voltage?

I am doing a DC analysis of a Fixed bias BJT and am trying to calculate its collector current. So far I have these following values. I am using a MMUN2211LT1 bias resistor transistor network. My ...
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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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How to sharpen blue LED voltage-to-current knee at 3.0 V?

I want to use blue LED as simple 3.0V nominal vs brownout indicator. It will be mounted on tiny proto board and will be plugged into rails or other various parts of breadboard. I want to make same ...
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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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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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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 ...
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What must be the minimum input voltage of a series regulator?

I'm designing a series regulator. I need 3.3V in my circuit, the circuit has 5V and 9V available in it. What should be the minimum input DC voltage in a DC series regulator in general case? In this ...
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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}\$ \$V_{BE}=0....
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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 ...
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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 ...
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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?
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BJT current mirror implementation

I am trying to figure out how to use a Zetex ZDS1009 IC. The chip contains 2 PNP and 2 NPN BJTs set up for current mirroring. I have learned about current mirrors, but I have not seen a 4 BJT mirror. ...
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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?
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Short circuit protection in simple audio current amplifier

In my class, we are to design a current amplifier with short circuit protection. In our design, we utilized 2 emitter follower amplifiers (Q3, and Q1). We then added Q4 and Q7 to lighten the current ...