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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Purpose of pullup resistor in BJT LED driver circuit

One answer to this question suggests the following circuit to drive a LED from an open collector output of a 74LS47: simulate this circuit – Schematic created using CircuitLab What is the ...
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How is possible that with same Ibase there is more than one Vce?

Ok guys, look at this picture: How is possible that with the same \$I_{\rm base}\$ we have more than one \$V_{\rm ce}\$?
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How to increase load voltage?

In my circuit diagram I measured +/- 0.7V across the motor. This barely moves the motor as it needs about 5-6V. The resistors in the circuit diagram are 1K. I've tested the circuit with a low ...
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I wanted to convert 0-1v, 2ns pulse to 0-7v, 2ns pulse

I wanted to generate a 2ns pulse width signal in particular 2ns ON time and 6ns OFF time signal. I can generate a 0-1v signal having 2ns pulse width using FPGA(by any means we cannot have more than 3....
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BJT transistors: can one see them as voltage dividers or should they be analyzed differently?

I am currently reading The Art of Electronics 3rd edition and I am struggling to understand basic transistors. On page 77 there is an example circuit with a PNP transistor driven by an NPN transistor....
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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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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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Learning transistor based circuits

simulate this circuit – Schematic created using CircuitLab In one of our electronic circuit tests, this question was asked: What is the output voltage of given circuit? This circuit looks ...
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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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479 views

Square wave to sawtooth wave using two BJT

I have two circuits that are supposed to be square wave to sawtooth wave converters: Both circuits receive a positive square wave as their input and generate a positive sawtooth as their output. Do ...
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398 views

Define input current of Op-Amp

I haven't done and studies in Op-Amp category yet but I watched a (trusted) video on Youtube and one thing is not quite logical as the next sentence: "No current flows in or out of the inputs of ...
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Effect of BJT common emitter amplifier resistor(Rc,Re) ratios? How to select it?

Above is a part of a BJT common emitter amplifier with 15V Vcc(all caps ignored for DC). I was following a tutorial which explains how to setup parameters step by step. When explaining the DC biasing ...
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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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Transistor design with beta variation

I am studying Schilling-Belove Electronic Circuits, 3rd Ed. where in its second chapter under the "Maximum Symmetrical Swing" section it gives an example where the base current is kept constant and ...
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BJT Circuit Problem [closed]

I am facing problem in solving such mixed circuits with bjt, diode, opamp and Zener diode. In the above problem 1.zener can have 3 states (ON, OFF, Reverse ON) 2.diode can have 2 (ON, OFF) 3.BJT ...
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Why to use a BJT transistor instead of MOSFET?

I read that MOSFET are more power efficient than BJT and that they are capable to handle higher current. In this case, why using a BJT ? I see most circuit use BJT to turn on / off some LED with a ...
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What limits the current in a Darlington pair?

I am unsure how the TIP120 doesn't blow up in the given equivalent circuit from the datasheet: I understand that the user will probably use some sort of current limiting resistor before the left ...
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Alternative ways to maintain temperature stability of silicon BJT circuits other than feedback resistors

So I've been messing with Bipolar Junction transistors lately and I'm trying to really nail the concepts of designing circuits with these things cause I want to start using them in some of my circuits ...
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What does this op amp/transistor circuit do? (revised circuit)

simulate this circuit – Schematic created using CircuitLab I asked a question yesterday about the circuit above, but my notes showed the op amp terminals mistakenly swapped and the circuit in ...
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Is it possible to use a NPN BJT as switch, from single power source?

I'm attempting to build the following circuit to better understand how to use a NPN BJT as a switch. What I've Tried I'm calculating the current across the LED (in order to light it up) as: ...
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Using BJT NPN to drive relay coil sometimes work sometimes not

I am just a hobbyist and I have a typical schematic I've always used to switch on/off relay coils. This has always worked the last 3-4 times I've used it (without R1 and C1) But this time, I can't ...
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PNP/NPN - Why do we not just use one of the two types?

Using PNP instead of NPN merely requires one to make circuit upside down and reverse current direction, if this is true then why don't we just do everything with NPN transistors? Are there any ...
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Semaphore using one microcontroller pin

I'm trying to make a semaphore using to LEDs green and red, controlled by one arduino pin. Idea is when pin is low red LED is on and green LED is off, when pin is up green LED is on and red LED is off....
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Transistor working with unusual biasing

What would happen to a BJT if both junctions of the transistor are forward biased? Is there any literature on this? By literature I mean characteristics/equations. Any links would help. I have also ...
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Understanding hFE values in a datasheet to choose a proper BJT base resistor

I already read the post Which resistor for NPN transistor base? But I still cannot understand the base resistor calculation. I need the transistor to be a switch to 5V 200mA relay. The base will get ...
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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 ...
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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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Audio power amplifier features identification

There is the power amplifier circuit here: How 18k/1k/.0012uF filter works in the input circuit? It's clear for me that Q1/Q2 is a transconductance stage: voltage in - current out. But where is that "...
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Confusion about a basic BJT current mirror

Below is part of a BJT current mirror: I marked the specific nodes of the reference transistor on the left as x, y and z. Many texts claim the following(I tried to summarize): Since the collector ...
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Why not use a MOSFET instead of a BJT?

I'm seeing the schematic of a Notebook HP Pavilion dv9000 (download). On page 17 in the corner left you can see this part: Two questions: Why they didn't use a MOSFET instead of MMBT3904? What's the ...
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Which datasheet parameters describe the switching perfomance of a BJT?

I have a Fairchild BC547 (NPN) that I'm going to use as a switch. What parameter(s) should I look at, if I want to know how long time the transistor will take to go from cut-off to fully saturated?
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Turn off n-p-n BJT on non-zero voltage

I have a controller that uses 0.2V voltage as logical zero and 0.4V as a logical one and want to turn a LED on/off depending on these two levels. I've built a cicrcuit like one below and tried to ...
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Why do we have 2 representations for load (shown below) in a power amplifier?

OR I get confused when i see two different ways to connect load resistor in a power amplifier. Why do we have two such representations (RL in series with collecor Vs RL connected across collector ...
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Current mirror improvement by adding resistors

One of the limitations of the current mirror with 2 BJTs is the Early effect. According to the Art of Electronics 2nd Ed., adding emitter resistors should make the circuit "a far better current source"...
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If two BJT bases are shorted, where does the voltage come from?

While trying to understand the circuit below in this question I got the idea that PNP base current is going into NPN base terminal. This way, you drive 2 transistors with only one base current. With ...
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BJT - absolute maximum base current?

Let's look at the datasheet for an MMBT3904, just for example. The absolute maximum section talks mostly about maximum voltage differences, and a single current limit - the collector current. I'm ...
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Why do the 2N2222 and PN2222 avalanche differently?

I've been experimenting with a 2N2222 transistor (in the TO-18 metal case) and using it as an avalanche transistor -- by not connecting the base and having the emitter and collector connected in ...
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Noticed something weird in derivation of relationship between ICBO and ICEO in BJTs

So I'm currently studying Electronic Devices and Circuit Theory by Boylestad and Nashelsky (11th ed.) and I'm confused as to how the relation between \$I_{CEO}\$ and \$I_{CBO}\$ is derived. So the ...
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Why is KCL giving me the wrong solution?

I'm trying to derive the gain of the following common base amplifier circuit: To do this, I drew the equivalent T-model circuit: . The problem I have is that I get two different transfer functions ...
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BJT input connected to another BJT's output and loading effects

Sedra & Smith Microelectronics, example 6.10, page 390. They ask us to analyze a BJT circuit. Here it is: simulate this circuit – Schematic created using CircuitLab In the next example, ...
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NPN Transistor Behavior with Base Voltage higher than Collector

Lets say I have this circuit: simulate this circuit – Schematic created using CircuitLab I think if I apply a 0 to the base of Q1, I'll develop something like 4.3V at the collector of Q1, ...
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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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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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what is the difference between these two configurations?

I can understand this circuit configuration: But i don't understand when signal is placed in Qp's base. In this configuration we don't have VBB/2 in the two base... Sorry about my english and thanks ...
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Are bipolar logic familes like ECL/CML used currently for radiation hardened computing devices?

According to this article, even though it's 30 years old, ECL is more radiation resistant than normal CMOS chips by a factor of 1000. It is also less susceptible to DPA side-channel attacks. So in ...
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Bipolar Junction Transistor problem. Where did I go wrong?

\$\beta = 100\$, Transistor Q1 is PNP and Q2 is NPN. Trying to find \$V_1, V_2, V_3, V_4\$, and \$V_5\$. I assumed active mode for both. \$\beta = 100\$ \$V_{BE} = 0.7 \mathrm{V}\$ Then $$ \alpha = \...
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NPN Circuit simulation doesn't agree with book

I'm trying to understand an exercise reported in Microelectronic Circuit Design by R. C. Jaeger & T. N. Blalock (4th ed, p.237). $$\beta_F = 50, \beta_R = 1$$. I don't agree with the discussion ...
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Parasitic oscillation in BJT booster stage of voltage controlled current source

I have constructed a voltage controlled current source for driving high power LEDs. In essence, it is a power N-channel FET in series with the LED whose gate is driven in reference to a 1 Ohm low-side ...
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Trouble analyzing transistor's circuit

simulate this circuit – Schematic created using CircuitLab \$V_{cc}=20V\\V_{BE1}=V_{BE2}=0.5V\\V_{BE3}=0.6V\\\beta_1=\beta_2=100\\\beta_3=50\$ I need to find V in this circuit. I tried to use ...
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Temperature compensation in transistor current cource

I am working with analysis of BJT current source. I didn't get the explanation of temperature compensation in NPN transistor with PNP and pull up resistor. How does it work actually?