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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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679 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 ...
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3k 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 ...
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1answer
833 views

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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7k views

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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1k 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 ...
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3answers
2k views

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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308 views

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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3answers
14k 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. ...
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2answers
422 views

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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3answers
25k 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 ...
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3answers
4k 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 \$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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2answers
3k views

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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3answers
643 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}\$ \$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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2answers
1k 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 ...
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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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1answer
5k views

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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3answers
35k 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?
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2answers
1k views

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 ...
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2answers
674 views

Oscillator Driven LED - Not Oscillating

I have an IR LED which is switched at a rate of 32.768kHz from an oscillator circuit (relevant portion shown below). I have 10 prototype PCBs. 30% of them work while the others don't. The problem is ...
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1answer
10k 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. ...
3
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2answers
614 views

Where is the oscilator in the 555 chip provided here?

I have tried a while to understand how the 555 chip is built, I opened the datasheet of TI available here : http://www.ti.com/lit/ds/symlink/lm555.pdf anyway this is the scematic from the datasheet ...
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6answers
3k 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 ...
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1answer
2k views

What's the 2N3904 / 2N3906 FET equivalent?

Is there a generic low-power switching FET that's multiply sourced, generally readily available, most simulators already have models for, etc.? I'm not looking for FETs that have similar electrical ...
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1answer
340 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 ...
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2answers
7k 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 of ...
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2answers
1k views

BJT push-pull modification

I am playing with BJT push-pull circuit from a previous question (there are 3 push-pulls with various speedup tricks for comparison): I've recently realized, that's it's voltage follower, so it's not ...
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2answers
3k views

Edge detector - transistor only

I wonder, how can I make edge detecting circuit without using any logic IC's, using only transistors & passive components? I have digital signal with ~15ns fronts on my digital input (5v TTL), ...
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2answers
2k 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 ...
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2answers
5k views

Prevent high-side BJT saturation

I am building high-speed (10-20ns on BC847-class transistors) digital "buffer"/"inverter" out of BJTs. Scheme is attached. While I can prevent saturation of low-side BJT by adding Schottky diode, it'...
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1answer
2k views

Common Base Amplifier - Hybrid-Pi vs. T Model

I have a common base amplifier with a source resistance and a load resistor. I am trying to solve for the system gain including the source resistor with both the hybrid-pi and T model but seem to be ...
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6answers
53k 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 ...
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9answers
84k 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 ...
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3answers
3k 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?
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4answers
52k 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 <...
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5answers
4k views

Low-noise audio-frequency BJT? [closed]

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 ...
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2answers
443 views

BJT transistor junctions: base-collector

I've been playing with voltage swings in a basic emitter-follower circuit. If I swing the input too large in the negative cycle, the base-emitter junction is reverse-biased and the transister goes ...
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3answers
3k views

Emitter Follower Output

I cannot afford a function generator and oscope right now, so I'm trying to learn analog electronics with Spice (MacSpice, to be specific). Below is the Spice model for a simple emitter-follower ...
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4answers
8k views

Connecting piezo buzzer with BJT

I have a 4 KHz piezo buzzer and a microcontroller (arduino) that outputs that 4 KHz from one of it's pins. All is fine if I connect the buzzer to the output with one pin and to the ground with another....
5
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1answer
845 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 ...
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5answers
4k views

I don't quite understand this FET-BJT preamp circuit

I see this circuit a lot on electret microphone preamps, but I don't quite understand it. The FET is operated as a common source amplifier, so it has gain, inverts, and has relatively high output ...