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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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Why base emitter voltage (Vbe) of BJT is not showing as 0.7v in proteus simulation?

I was just trying to simulate a simple transistor circuit as common emitter in Proteus 8. However the I observe that the base-emitter voltage (Vbe) of the transistor 2N2222 is showing as 1.36v instead ...
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Can anyone help me with Max Swing? I'm so confused about its property

So max swing is supposed to be when ICQ=VCEQ/Rac right? I understand the fundamental part. But what is Max Swing relation to this: ICQms=min{ICQ,VCEQ/Rac}. I don't understand how it's related to max ...
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What kind of transistor turns on with 0.2 volts?

I need a circuit to turn on a LED with 0.2 volts. I want to do it with a transistor as a switch. What kind of transistor will turn on at this voltage?
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High pass filter for use in amplifier circuit

I've designed an amplifier for a small ac signal (amplitude of around 5mV, frequency 252kHz) with a BJT (2N2222.) The BJT is set up in a common emitter amplifier configuration, with the biasing ...
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112 views

What's happening to this 3906?

I'm creating a soft-latching circuit for my project, using a 3906PNP transistor and a N-Channel IRLZ44N MOSFET. The circuit is fairly simple; Once switch one is pressed, it brings the gate of the ...
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76 views

Bipolar Junction Transistor [closed]

I am struggling with a question(ideal npn) where there is only one resistor Re. Where and how do I start with this question? Vbb=10V Vcc=20V Re=10kOhm The Question states : determine the terminal ...
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68 views

Using an analogue multiplexer to control a motor controller?

I am designing a control circuit for a Sabertooth 2x60 motor controller. When this controller is in analog mode, it runs in full reverse (0v control signal) until my software starts and applies 2.5v ...
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165 views

Amplifier Design Project

I need help with starting on the design of amplifier design project, given the following listed below specifications. To be more precise, having learned about the different BJT amplifiers, such as ...
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104 views

Understanding the basic common emitter amplifier

I'm struggling to understand intuitively this basic circuit: simulate this circuit – Schematic created using CircuitLab I know that out is \$k\cdot\mathrm{in}\$. What I'm struggling with is to ...
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152 views

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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37 views

How do I calculate the collector current of this circuit with a bipolar junction transistor?

I need to calculate the collector current of this circuit, but I don't know how to approach it. I think the circuit is in the active zone but since my teacher isn't responding to my emails I'm not ...
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common collector design with MMBT3904, will cutoff happen?

My circuit is as below. It's a typical common collector design with MMBT3094. Currently I have a requirement to increase the value of base resistor to 100kohm. To make the emitter output voltage as ...
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62 views

Arriving at a wrong output impedance for a BJT Emitter Follower Configuration Circuit

We've to calculate output impedance of the circuit (figure 1). Although i understood the textbook solution (figure 2, figure 3), I took a different approach and arrived at a wrong answer. step 1: I ...
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74 views

12 V pulses with 100 MHz repetition rate

I have to control an high impedance load (> 25 MΩ) with 12 V pulses with maximum 5 ns of width and a repetition rate of 100 MHz. I can generate the pulses with the required shape using an FPGA ...
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95 views

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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72 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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74 views

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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27 views

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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61 views

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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84 views

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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67 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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3answers
65 views

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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1answer
94 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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64 views

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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708 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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930 views

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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1answer
79 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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88 views

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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99 views

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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74 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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40 views

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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130 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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75 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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48 views

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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79 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 ...