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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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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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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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Convention regarding BJT collector current calculations

Using the equation Ic=Is*e^(Vbe/Vt) yields impossible values for me. I am using Vbe = .7v & Vt = .025 A. Is in circuit is 3.4mA. I am trying to determine Ic as an initial value for analyzing ...
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A confusion about the formula for transistor Vbe Ic characterisitcs

Below is NPN BJT transistor Vbe Ic characteristics and the formula: Many texts approximate this equation as: Ic = Is*e^(Vbe/Ut) and following this they say when Vbe=0, Ic becomes equal to Is. ...
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Can a BJT base-collector junction be safely used as a diode?

In a (hobby) project I'm doing, I noticed that the diodes I needed were more expensive than the NPN transistors, which left me wondering if I could substitute the transistors for the diodes. I'm ...
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32 views

To add a reasonable parasitic in a BJT circuit, how much resistance should be added in series with an inductor?

I am working on a tunable image reject notch filter using Agilent_HBT SiGe BJT model in ADS software of Keysight. To consider parasitics in the circuit performance I need to put a value of series ...
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87 views

Which region is the transistor operating at?

PNP and NPN transistors in the circuit have identical Is values. Based on the PNP transistor is connected to a DC voltage source. k = 1.38064852×10-23 J/K qe = 1.60217662 ×10-19 C Is = 0.69 fA at 17°...
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67 views

Drive Nixie tube with BJT

I'm trying to replicate the Nixie clock designed by Kevin Lee (https://0x7d.com/2017/nixie-tube-clock/). It uses a NXP PCA9685 PWM controller and a MSD42WT1G BJT to drive each tube cathode separately. ...
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89 views

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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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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113 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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92 views

High Input BJT Differential Amplifier

This is a basic differential amplifier. I want to modify it in order to have high input impedance (a few \$M\Omega\$ from DC to at least \$200~kHz\$) I thought about replacing the CE pair input with ...
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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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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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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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How does this circuit given with diode works different from the standard current mirror circuit or will it work in the same way? [duplicate]

This question had already been asked here by someone else few years ago but I could not see any satisfactory answer so I am asking it again. All I can see relevant in that post was the image attached ...
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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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1answer
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A question about understanding a BJT differential amplifier?

I'm trying to understand how a differential amplifier rejects common mode noise and amplifies the differential/normal mode noise. For that I use the following circuit topology and simulation: When ...
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2answers
43 views

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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1answer
115 views

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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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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1answer
106 views

Common collector - bias resistor?

[Original question: CCD output buffer; seemed best to make the transistor part a separate post] Taken from Wikipedia, the circuit in Fig. 1 acts as a common collector (high input impedance - low ...
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53 views

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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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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1answer
226 views

Tuning BJT avalanche noise

I'm playing with the idea of building an audio noise maker. I found several nice sources. Most seem to be based on PN junction avalanche noise. As a starting point, I put this on a breadboard: It ...
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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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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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109 views

circuit analysis for an amplifier

so I had my original circuit looking like below: simulate this circuit – Schematic created using CircuitLab I was able to simplify it to look like as follows: simulate this circuit but now ...
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617 views

Help understanding 2N3904 VCE breakdown voltage?

While I was designing a constant current source using a 2N3904 I noticed a behavior on the bjt that prompted me to look closer into it. The issue is that although the datasheet specifies a minimum ...
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113 views

Strange BJT configuration, working as a diode?

I was looking at an old schematic and I came across this strange circuit for low voltage detection section of a larger circuit (not shown). The operation of circuit is straightforward, Q1 is normally ...
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2 cascaded transistors- where is Vout of a transistor when there is a shorted collector resistor?

I have circuit drawn in AC, they were 2 cascaded BJT transistors, and I have changed them to the hybrid pi model. I wish to remove the source and load in order to write a general gain equation for ...
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705 views

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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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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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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132 views

The BJT characteristic curve

Lets say you want to operate at 20uA base current. To move along that 20uA graph, you need to vary Vce. But to vary Vce, you either vary Vb (voltage at base) or change the values of the resistors at ...
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112 views

How can I have an ECL logic input for a CMOS logic gate

I'm working on a triggering system that uses a comparator to check if the signal is below a threshold value. I found a great comparator for the job, except its output is ECL logic. I wanted to use the ...
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34 views

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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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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93 views

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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104 views

Changes when \$V_{be}\$ is increased in npn transistor

For the given circuit, a statement has been given in my book, saying It is obvious that if Vbe is increased by a small amount, both hole current from emitter region and the electron current from ...
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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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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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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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1answer
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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Designing a Class A Audio Amplifier

I am currently working on a project where I am required to design a Class A Audio Amplifier using BJTs. The requirement of the amplifier is as follows: - Acquire a variable gain of at least 10 (Av >=...