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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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BJT differential amplifier: What causes common mode voltage gain?

In any normal op amp, if it was ideal it would have a gain such that the output voltage would be However in reality this is not the case as there is a common mode voltage gain and hence in reality ...
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BJT Clapp Oscilator for 3-6MHz

I designed a Clapp VFO to aid in my Ham Radio projects (Non-superhet SSB and CW recievers for 80M and hopefuly 40M). I chose the clapp topology for it's frequency stability. I want it to tune at least ...
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Where to find details of dimensions and doping of a typical bjt

I have been searching for dimensions and doping levels of base, emitter and collector for a typical bipolar junction transistor presently in use. I have tried searching on internet and seen datasheets ...
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Non ideal behaviour of a current mirror

Voltage across emitter and collector of QA is 0.7V. In a current mirror, why is it such that when the voltage across the collector and emitter is different for Qb than QA the value of Iout increases ...
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What is the theorem behind the basic BJT biasing strategy (divide Vcc into three parts)?

I have learnt that there are strategies say, $$\frac{1}{3}\cdot V_{CC}=R_C\cdot I_C$$ and $$\frac{1}{3}\cdot V_{CC}=V_{R2},\frac{1}{3}\cdot V_{CC}=V_{CE}$$ finally, $$10\%\cdot I_E=I_{R1}$$ Ignore $$...
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SPICE BJT simulation results

I'm trying to simulate simple BJT circuits to verify my hand calculations. Following is my circuit, I'm just trying to get the calculated DC collector current with the shown voltages and parameters. ...
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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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Spike in RTL Inverter

I do not understand the cause of this spike which can be observed in an RTL inverter (but also in a CMOS inverter). In the following image it is the peak at 10 mS (I have considered as input a signal ...
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Frequency Modulator with sinusoidal output

I am creating a frequency modulator, all the circuits that I have looked so far on the internet are generating a Pulse wave or a Triangular wave output, so if you guys can help me to create a circuit ...
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Help with making sure a correct use of a BJT

I'm designing a PCB that receives 3 power inputs 3.3v, 5v and 12v. I need simple way to switch all of them of in one go. Here is a schematic of my PCB My 12V line is for running a ws2815 led strip ...
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CE/CB BJT Amplifier(s) + Load Lines!

This diagram from a lab assignment is not making sense to me...any help would be greatly appreciated. I don't understand what \$I_{C_\max}\cdot R_e\$ is and why it appears on this diagram. Even if it ...
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60 views

Why should the bias servo transistor not be placed on the main heat sink for a Sziklai pair?

The Wikipedia page on Sziklai pairs contains the following statement (emphasis mine) Sziklai pairs can also have the benefit of superior thermal stability. [...]. [Q]uiescent current is much more ...
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How can I measure a TIP120's beta?

I was promised a current gain of roughly a thousand. We had an experiment involving this circuit to measure the input base current and output emitter current: (That base resistor is a 50k ohm ...
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91 views

NPN Transistor common emitter calculation

I built the following circuit in LTSpice using the default NPN BJT model (beta = 100). I calculated the Ib to be 115.754uA. This is how I did the calculation: Equivalent voltage at base is: ...
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push-pull explanation

I have this circuit on a board: That circuit is supposed to act as a push pull amplifier working as the following: In case it is not really visible the voltage connected to the collector of Q3 is 3....
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IGBT transistor with auxiliary emitter

Recently I discovered, that some IGBT transistors in IGBT modules have the second pin for an emitter, called auxiliary emitter. In the picture above 2nd and 4th pins are auxiliary emitters. This is ...
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How to tell if a BJT is PNP or NPN by looking at the circuit? [closed]

Is it possible to tell if a BJT circuit is PNP or NPN simply by looking at the circuit? For example, for the circuits shown, how can we tell if they're PNP or NPN?
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Increase voltage drop required to saturate PNP BJT to avoid noise

I have this circuit which must signal +3.3V or 0V depending on the switch status. The the switch is about 20 meters away and is getting EMI from mains. I realized that it only takes -0.7V of ...
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BJT problem. Are my calculations correct?

\$\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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How to choose resistors for a multiple resistor voltage divider circuit?

I am trying to create a voltage divider circuit to off offsset two BJT input voltage is for a push pull amplifier configuration, and I am having trouble choosing the proper resistor values for my ...
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Common Emitter Stage BJT Amplifier with Early Effect Inclusion

How can I find Rin, Rout and Gm parameters of a BJT amplifier in common emitter stage without neglecting Early Effect? We have such homework and I couldn't find the exact same example in Behzad ...
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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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voltage traces across pwm driven dc-motor: is that ok?

I'm driving a dc-motor with a BJT turned on and off with a 555 generated PWM signal. Here's the circuit (V1, Rl and L1 simulate the motor): There actual circuit - same parts - is on a breadboard and ...
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BJT amplifier design with input resistance, output resistance, and open-loop gain requirements

so currently i'm designing a BJT Amplifier with these requirements: Open-loop gain = 35 dB Input resistance = 600 Ohm Output resistance = 200 Ohm Another thing is the only BJTs I can use have hFE of ...
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BJT: why does Ic increase when temperature increases? [closed]

When the temperature of an BJT increases, the thermal voltage $$ V_T = \frac{k_BT}{q} $$ where \$k_B\$ is the Boltzmann constant, \$T\$ is the absolute temperature of the junction, \$q\$ is the ...
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72 views

Why doesn't the emitter resistance of a BJT depend on the AC current?

When designing BJT transistor amplifiers, one calculates the emitter resistance of BJT: \$r_e=\frac{26mV}{I_c}\$ where the voltage is the thermal voltage at room temperature and \$I_c\$ is the bias ...
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RC integrator+ BJT emitter follower

i am designing a circuit able to draw IV curves of LEDS and diodes through Arduino. Basically i use the PWM function and an RC integrator to have an approximately constant voltage. Then i would ...
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128 views

Sound distorted from amp configuration, and one transistor variety getting too hot

The sound is distorted from my configuration. The transistor I'm using is an MPF4393. I need to add resistors and capacitors and how do I get more power (to make it louder) What should I add? Also ...
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74 views

Can I apply Kirchoff's law in the open contour in the picture

So is it right if I apply Kirchoff's law : Vcc=I1R1+I2R2+I3Re if I1 is the current that flows in R1 resistor, I2 in R2 and I3 in Re
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Level shifter using BJT schematic

I was reading the book Analogue electronic circuits and systems, by Amitava Basak, when I stumbled in this level shifter circuit that uses BJTs instead of the usual mosfets. See the below picture. ...
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Find output impedance of BJT amplifier

I need to find the output impedance of the BJT amplifier as shown below: The equivalent small-signal circuit looks like the following: I know that the output impedance is $$Z_o=R_{E1}\ ||\ (R_{E2}+...
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Logic gates with Mosfets and Transistors

Does anyone have a link for the list of all the configurations of BJT and MOSFET which create the logic gates? Like for NAND gate below:
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76 views

BJT common collector amplifier with a voltage gain sligthly higher than one

How could I explain why a common collector amplifier has a voltage gain higher than one ? In a lab session, I built this circuit : With Vg=30 mVpp and Vcc=15V, students were asked to give the voltage ...
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How can an inductor/coil turn a bjt on and off?

I've seen a boost converter circuit on YouTube, and the guy uses a second coil, wound on top of the first coil, and in the same direction, it is connected to the base of a BJT. The BJT acts as a ...
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I'm having trouble understanding the properties of this circuit

The objective of this question is find beta, which is ( Ic/Ib) The 3V is supposed to be 2.3V The 27 k ohm is supposed to be 20k The 230 ohm is supposed to be 230 ohm I first found Ib = 4.3 v- 2.3v /...
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Please help me understand this bjt set up works

This is part of an automotive timer (NOT a 555!) that we have at work. Because I'm extremely bored I'm challenging myself to understand how this one operates. I've spent about 1.5 weeks with tutorials,...
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What is the point of 2 inverters in series? [duplicate]

In the datasheet for the Nexperia HEF4543B, in the logic diagram, there are 2 inverters in series: What is the point of these inverters in series?
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Do BJT (Darlington) transistors have a minimum collector current?

I am using a TIP122 Darlington transistor. From the graph provided in the datasheet, I can see that the saturation voltage decreases as the collector current decreases. Since the load I am driving ...
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105 views

Emitter follower and input impedance

I'm designing an active bandpass filter (using only BJTs, voltage sources, resistors and capacitors) with the following properties: Corner frequencies \$8 ~ \text{kHz}\$ and \$800 ~ \text{kHz}\$ (...
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Effect of base doping on early voltage,base transport factor and emitter injection efficiency?

According to me as base doping increases recombination will be less at base region so base transport factor and emitter injection efficiency will increase and as effective base width will decrease due ...
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60 views

BJT Astable Multivibrator - What is the max. oscillation frequency?

What is the max. oscillation frequency of this Astable Multivibrator circuit? Is it directly related to transition frequency of the transistor? I plan on using it with BC547B transistor. The hardwired ...
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53 views

The collector current of BJT

On what parameters does the Ic collector current of a transistor depend upon, base emitter voltage or base current? I know that BJT is a current controlled device. But still we have graphs that show ...
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39 views

Necessity of a current mirror circuit - MOSFET/BJT?

I am studying the current mirror circuit. I already went through similar questions and answers on this forum. But I was unable to get clarity from the answers. I am just a beginner. I request if you ...
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Understanding class A amplifier waveforms

I am working on an audio amplifier but got stuck on to a basic of npn transistor circuit. Attaching a class-A amplifier circuit. Take a look here or here. I tried to simulate the circuit but as ...
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BJT: Finding Ie and Ib [duplicate]

I am struggling to find out the Value of Ie and Ib here. Ic I have taken to be Vcc/10k. If this is correct I could use Ic/alpha = Ie . I would like to know if this is the case but seems to easy to be ...
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Does PNP collector current influence base current?

I'm trying to understand the classic "ideal power diode" circuit. An example (taken from the Raspberry Pi) was in this question. My question concerns the left-hand PNP transistor. The function of it'...
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What are the differences between the NTE102/103 and NTE102a/103a Germanium transistors?

SO the NTE102/103(a) complementary transistors are a bit more compact(both are cans but the a ones are smaller and thinner). But I've noticed that in particular, the NTE102 is much more prone to ...
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Finding Width of MOSFET

We are struggling with part iv of the question above where we have to find the width of both MOSFETS. As you can see from our working out, we have found a width of 2 meters for the nMOS, which we know ...
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84 views

PNP Transistor with low base current

I designed a PCB with the following schematics. simulate this circuit – Schematic created using CircuitLab The circuit uses a MAX6369 external watchdog timer which has an open-drain output(...
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PNP darlington pair calculations

I have a 20ohms discharge resistor connected to my PNP darlington pair. I have simulated the circuit in my mobile (every circuit app).please find the attachment for circuit. But here I'm unable to ...