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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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What are limitations of Millers theorem for amplifier?

I mean to say that are the two circuits one original and other its miller/ drawn exactly equal in all respects or their are some difference in some respect if so please mention. I know that Millers ...
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Why it is said that second transistor cannot enter saturation mode in Darlington pair?

When I made equations it came out that for both of transistors to be in saturation Vc-Ve2<.2V Vc-(Vb2-.7V)<.2V Vc-Vb2<-.5V ............{eqn a} As for first transistor to be in ...
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What will be mode of operation for BJT in figure? [on hold]

The value of Vbe active is .7V. Also give reason.
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Question about differential amplifier

why is there 0.7V instead of 1.2V on the common emitter? if you go through Q1 mesh : Ve = 0.5+Vbe = 1.2 instead of Ve = 0+Vbe = 0.7 from Q2.
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Designing 2-stage BJT CE class-A voltage pre-amplifier with specified goals and without global NFB

BJT CE Class-A Voltage Pre-Amplifier Suppose a design requirement limits the selection choices to the following class-A BJT common emitter stage topologies. No global NFB is permitted. Only local NFB ...
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What is the forward beta of the BJT KN2222A?

I don't know what is the forward beta of the bipolar junction transistor KN2222A, I'm an amateur and i don't found the value in the data sheet or i just don't understand where. Please let me know
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The influence of capacitors and resistance in a BJT circuit

I solved the circuit and found these values for each one. But I wanted to know: Which capacitor has more influence in the low frequency and in the amplification? Does RE have impact in amplification?...
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50 views

How can I prove that Vce saturation lies below .2V?

When I try to prove that Vce for saturation is below .2V and I take npn transistor I simply consider emitter to be at 0V and thus base emitter junction being forward biased base gets a voltage of (.7 ...
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Help me please to solve this question with BJ transistor [closed]

original source I found Ic=2mA and Ie=2.02ma but i can not find Vcc, Ve, Vce, Vb and R1. I am really new to this topic, so can you help me please with this question?
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BJT - How does an applied voltage at BE and BC affect the band diagram?

Let's say we have an energy band diagram of a BJT at equilibrium. Then, we apply -0.7V to BE, while 0.7V is applied to BC. How will these voltages affect the band diagram? I would appreciate it if ...
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364 views

Biasing a second stage amplifier

I built a two-stage amplifier as shown below. I have already built the first satge (R1-R5 & C1-C2) such that If I apply a signal of 1volts peak-peak, the output (at collector of Q1) would be 8 ...
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39 views

Bipolar Transistor's Input Resistance

Based on theory when you want to find the output resistance of a circuit with dependent sources you can't turn them off so you have to add another source of known value in the points of interest and ...
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61 views

Calculation of (TC) in BJT

if we have a circuit with a bjt like this one and we know that the I is 1.1mA in 25 Celsius , with Vbe=680mV and Ie=1mA , and β=100 in 25 Celsius. If we know that the TC (thermal) of Vbe for ...
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43 views

Why does the signal generator voltage change in this circuit?

In this circuit, when I measure with an oscilloscope the square wave I get this: (The switch S is open in both measurements) But when I change the R2 resistor to 15 Ohm, I get this: Why does that ...
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I'm working on a school Lab for a Constant Current source BJT and need a nudge in the right direction

I am stuck trying to determine the applicable formula for the load current. I have used \$I_b = \frac{Vin-Vbe}{(Beta+1)*Re}\$ and then \$I_L= (Beta)*I_b \$. The problem is I can't find a proper ...
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70 views

How to derive formula for emitter current flowing through Re?

Ie = Ve/Re I was thinking about using that approach but i have been doubting myself if that is the right approach to go with, because then i have to express the emitter current in term of alpha and ...
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37 views

Calculate values of resistors in a given BJT amplifier circuit [closed]

I have the circuit shown below and i have limited resistors to make it work. The LED's rated current is 10mA so Ic = 10mA. I know that beta for the transistor is 100. The resistors are (I have 2 of ...
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1answer
49 views

stm32 drawing current from 40 outputs at the same time

I have an hardware question about stm32 family. I am designing a circuit with STM32F103VB. datasheet mentions that i can draw up to 25 mA current per pin(page 37 table 7). I need to drive 40 npn ...
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Split collector current mirror [duplicate]

I'm familiar with the basic current mirror, as below: simulate this circuit – Schematic created using CircuitLab However, having recently learned about split collector BJTs, I wonder if a ...
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question about bjt voltage gain

Input voltage triangular wave signal with 12.7Vmax and 9.1Vmin passing thru a NPN BJT, assuming if the forward voltage of the BJT is 0.7V then why is the output Vmax is not 12.0V but way lower like 11....
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Can I do KVL around transistors? (MOSFET and BJT)

I'm having a lot of trouble with more complex transistor DC circuits (can't wait for the AC, heh). Basically, what I want to ask is does KVL holds "around" a transistor as well? For example, if I ...
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Where does -1 in BJT’s Ies equation come from?

I don’t know whether there is already a question and answer to this but I couldn’t find one yet. Regarding the following information from a text: The equation for emitter reverse saturation current ...
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Why are BJTs common in output stages of power amplifiers?

From my understanding, the role of the output stage is to decrease the output impedance to almost 0. For that, MOSFETs seem better suited since they've got way lower \$R_{ds}\$. Yet I see quite often ...
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BJT Amplifier vs Manufactured RF Mixers

900Mhz Operation This is a BJT mixer design I found on google From my understanding in order to use a BJT amplifier as a mixer you have to be in the 1dB compression point. Question: Am I wrong ? ...
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Why does the base-emitter voltage in NPN transistor become constant?

The setup above shows the experiment that I have been working with. During the experiment, the current \$i_c\$ and the voltage \$V_{be}\$ were measured for increasing \$V_{in}\$. At some point, the ...
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Gilbert Cell Output Current

I do not understand the basic of a Gilbert cell. I see that as its output we consider the output differential current Io. But I do not understand it. Is the load connected between the points 1 an 2? ...
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Excessive Op-Amp Settling in Exponential Converter

This circuit converts a linear voltage to exponential current for controlling the frequency of a VCO. It simulates without issues, and almost works, but there is a huge amount of settling on the ...
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78 views

How do I know that this BJT is in the saturation zone without having to simulate it?

I have to find the working zone of this BJT, for different values of P, by using a simulation software like Simulink. But I would like to know how to do it manually. The way I learned is sort of ...
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70 views

Finding the negative feedback loop in the amplifier circuit

Today I was learning about the negative feedback in the amplifier circuits and some quick ways of discovering whether a negative or a positive feedback exist in the circuit by finding a feedback loop ...
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72 views

Working region of NPN BJT

I have come across the following circuit where a BJT works as switch when I apply 23 volts to base using a 100k resistor. A 3.2 mA constant current source is connected to the collector of the BJT ...
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117 views

What happens if we force a voltage on Vbe of a bjt transistor

I'm wondering what happens in a BJT transistor if we force a voltage bigger than 0.7V on Vbe. A colleague of mine said that if we put on Vbe, let's say, 5V the transistor would be saturated. I ...
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BJT Cascade Difference Amplifier Exercise

I need help with the exercise. The questions: Add or Design Components to have a current source I=2 mA at the picture. Draw equivalent small signal,low frequency model. Find Ai, Zo, Zi. If ...
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2k views

What property of a BJT transistor makes it an amplifier?

I know how we can amplify a given signal using BJT transistors by biasing them. But I would like to know what is that crux property which enables BJT transistor to act like an amplifier. Is it the ...
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BJT astable multivibrator generate triangle wave

In this circuit there is a bjt astable multivibrator. It generates a square wave. And according to the description, at the output of the opamp it generates a triangle wave from this square wave. I've ...
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is the gain for this differential bjt pair correct?

I'm referring to the diff pair consisting of Q1 and Q2, ignoring the rest of the circuit for now. I've attached a pic of my diff pair small signal model and what I calculated for the small signal gain....
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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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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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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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3answers
80 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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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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187 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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133 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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1answer
166 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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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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66 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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78 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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1answer
119 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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1answer
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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. ...