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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PNP too big of a voltage drop accross Collector Emitter

I have a rather simple circuit that acts as an overvoltage protection switch with an input control signal to enable/disable the output.On the final output I am driving a motor that consumes around 2A ...
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Would either of these BJTs require a heat sink?

I am brand new to heat calculations and trying to determine if I need a heat sink for the BJTs or not. Below is the circuit with voltages and current with the data sheet of the used BJTs. The ...
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How to find common emitter current gain (beta) from graph

I am trying to find the common emitter current gain of a bjt transistor I made on ltspice, and I need to use the below graph to find it, but Im not sure how to interpret the graph to find it. Should I ...
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83 views

How does this PNP transistor circuit work?

How does this circuit work step by step and how do I get the LEDs to flash alternatively? (PNP transistors) (at what voltages do the LEDs turn on?)
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How to use Ebers-Moll equation to calculate collector current?

The Ebers-Moll equation is given by:$$I_C = I_S(T) * (e^{V_{BE}/V_T})$$ where \$V_T\$ = \$kT/q\$ = 25.3mV at room temperature. In TAOE (2.3.1), it mentioned \$I_S(T)\$ is the saturation current of ...
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159 views

Simple Audio Amplifier Problem using BJTs

Below is my constructed audio amp and graphs that corresponds to the voltage at the collector of Q1 and the emitter of Q2. Everything works great until I add the capacitor to protect the speaker (8Ω)....
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85 views

Amplifying DC signal via BJT

I am using LED as a light sensor, the range of output is from 0-700mV DC, I want to amplify/scale this signal to 0-3.3V DC. My question is, what is the best way to scale the output signal to the ...
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206 views

Why is the base resistance suggested to be one-tenth of the emitter resistance times beta?

This is a follow up question from Emitter follower biasing with voltage divider or the emitter follower design example in The Art of Electronics. Why is the recommended one-tenth ratio of base ...
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Help me understand this high voltage amplifier circuit for driving a piezo actuator

I am researching HV driver amplifiers for controlling piezoactuators. My background is in physics, so I am a bit out of my comfort zone here. I came across the following schematic and have been ...
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34 views

The meaning of constant current source in BJT

Taking a sample from TAOE Ch.2 Figure 2.9 for example. The book suggests that BJT constructs a constant current source for the LED and \$\beta\$ is subject to change from multiple parameters thus ...
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How can we do AC analysis when CE amplifier input is zero?

Hi everyone I would like to make AC analysis of an CE amplifier circuit. The circuit is some kind of LC oscillator. I would like to obtain small signal parameters but in oscillators AC input voltage ...
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79 views

Bjt Multistage Amplifier [duplicate]

I asked similarly before, but I really have to do this soon. Please help me. ı have signals that are received from sensors (seismometers) are usually noisy and of very small amplitudee. The frequency ...
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38 views

What advantages does the Darlington pair have over the Sziklai pair?

With all the free time I've had lately due to current events, I've found myself looking into analog amplifier design and analysis (if it helps provide context, my area of expertise is power ...
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Please explain the purpose of the marked resistors and diode in this transistor relay driver circuit

Please explain what the circled components are for. What is the difference between the \$V_{cc}\$ and \$V_{in}\$, can they be the same?
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Transistor Analysis of a Common Base(?) BJT

Hey. I am a biomedical engineering student focusing on the biomaterials side. However, my university makes me take a few electrical engineering classes as well. So I am not very good at this. I was ...
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Calculate output impedance of emitter follower

In AoE's chapter on BJT (specifically section 2.2.3B Input and Output Impedances of Emitter followers), it derives the input and output impedances to be: $$\mathbf{Z}_{in} = (\beta+1)\mathbf{Z}_{load}...
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maximum collector-emitter voltage in BJTs

I am reading BC817 datasheet. According to the datasheet, maximum collector-emitter voltage at open base condition is 45 V. In typical applications we don't use open base configuration. My question is ...
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42 views

Driving a small water pump with a microcontroller and a transistor

I have bought some water pumps for a project I am currently working on, the thing is that I have never used water pumps before, I am sure that pumps are inductive loads, but I am unable to know the ...
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37 views

Determine if a heatsink is required (Darlington TIP120)

I'm planning to switch 1A load at 5V with a TIP120. Looking at it's power derating graph I see that at 5W case can be around 140C, but how can I tell if it will get hotter than that? In the ...
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67 views

Design multistage amplifier

ı want to make a Multistage Amplifier with the following features.But ı dont know how to arrange dynamic range ,multistage amplifier and noise?How can ı desgin it? Maximum output voltage is 10 V p-p....
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44 views

Current Notation in BJT

We are currently studying BJTs in electronics I and I am a bit confused on the notation of the current, specifically the collector current here. Throughout this chapter they will swap \$I\$ and \$i\$ ...
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543 views

Will a BJT blow up if there isn't any load/resistance either in collector or emitter?

Taking NPN BJT as example. When an NPN BJT is operating in the active mode, \$I_C = \beta I_B\$. That means we can fully control the current pass through the BJT by controlling \$I_B\$ only. In such ...
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What is the best transistor model for bjt transistor ac analysis(middle response)?

Nowadays I am working on bjt transistor ac analysis. I use r(e) model for bjt transistors ac analysis in my university. I know there are separate methods for low frequency medium frequency high ...
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Where does the input resistance formula of a BJT amplifier come from?

I just solved the (a) question of this problem from one of my past exams. And I am a bit confused on to why the solution is as follows: Rin = 100kΩ || Rib Rib = (rπ + (1 + β)Re) And the way Rib is ...
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Modified Gummel-Poon BJT Modelling Issue

First of all, sorry for my bad english. Output for Vb: From the schematic above, I redrew the 2N3904 BJT into the modified Gummel-Poon model. Output for Vb: The equation for B1, B2 and B3 were ...
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Why does the Collector-Base Junction need a lower magnitude voltage to be forward biased compared to the the Emitter-Base Junction?

We are learning about the different modes of BJT transistors currently and something that is a bit odd to me is that our textbook states that "... an npn transistor whose EBJ is forward biased (...
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BJT Base-Emitter Voltage

I have a question about Bandgap Reference circuits. How did he get Vt*ln(n)?? This is how I did it:
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Initially on Power Control with BJTs

I've removed a voltage regulator from my circuit and replaced it with a series of BJTs. The regulator was just switching voltage. But I need the circuit (which happens to power/wake the micro) to be ...
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62 views

How to find Base, Emitter, and Collector Voltage of a Circuit Demonstrating Miller Effect

Given \$R_S=100\ k\Omega\$, \$R_1=150\ k\Omega\$, \$R_2=20\ k\Omega\$, \$R_C=7.5\ k\Omega\$, \$R_E=1\ k\Omega\$, \$C_1=C_2=C_E=0.1\ \mu F\$ and \$C_{BE}=10\ pF\$ I need to solve for \$I_C\$, \$V_B\$, \...
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BJT voltage divider bias problem

Hi I am having trouble with how to find Req of the resistor network of the bjt circuit on the schematic. How can I calculate it to find Ib Vb values? thanks.
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Finding the necessary currents for AC analysis for CE NPN BJT amplifier

I have this circuit with a CE NPN-BJT amplifier and I am struggling to find the necessary values for calculating r(pi) for AC analysis, i.e. the hybrid-pi model: $$r_{pi} = \frac{V_{thermal}}{I_b}$$ ...
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Help with BJT Transistor analysis. The amplified waveform is more like a square wave any needs to be more sinusoidal

I have been exploring BJT transistors and implemented a common base amplifier. I connected a signal generator to the voltage source labelled V2 in the below schematic. The oscilloscope image shows ...
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Voltage divider bjt biasing with thevenin theorem

Say I have this circuit: I am confused as to how using Thevenin's theorem yields correct results. Say I choose a bias point on an I-V characteristic curve at the IC = 16mA. If this transistor has a ...
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21 views

Increasing in slope result in increasing in impedance of BJT Early voltage?

I'm studying about the AC equivalent model of BJT and in the re model, it said that: "As the base current increases the slope of the line increases, resulting in an increase in output impedance with ...
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110 views

What is the purpose of this resistor between BJT Base and Emitter?

Can someone please explain to me what the purpose of this resistor is?
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58 views

Which characteristic of a BJT determines the timing in the following given application?

I currently face a question about the BJT circuit given here: We see a 2N3904 transistor with constant base current of 2 mA. A switch seperates a 15 V/20 ohm source from the collector of said ...
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102 views

CE power amplifier design

I am trying to further my knowledge in discrete transistor design so I am designing a CE power amplifier to drive a speaker. What I have trouble with is determining where to bias my BJT at. I can't ...
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PNP folded cascode circuit question

I have a homework question but no idea what this circuit does and how to make calculations. In the simulation input and output voltages are same why? Thanks for any help
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72 views

Connection between two pre-amplifier stages

In the circuit shown above, I am unclear how Q1 connects to Q2+Q3. I understand Q1 is part of a common base amplifier and Q2 and Q3 form a direct coupled amplifier. The circuit is described at ...
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81 views

Finding AC equivalent seen between two nodes of a BJT Amplifier

In the textbook, Electronic Devices and Circuit Theory, 11th Edition by Boylestead and Nashelsky, on page 566, the lower cutoff frequency is derived for a voltage ...
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76 views

Relaxation oscillator

I was wondering how inside the relaxation oscillator heating is not produced inside the BJT junction causing destruction of the transistor. simulate this circuit – Schematic created using ...
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Why does the positive wave of my class AB amplifier output get clipped far below rail voltage?

For the past 6 months I've been designing and building a 30W 3MHz audio modulated Tesla Coil driver. For this, I've created a +-15V linear regulated power supply, an AM modulator using an MPY634, and ...
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How to calculate Ic for a very simple NPN transistor circuit

I built a very simple circuit with an NPN (2N3904) transistor. Then, I swept Vce from 0 to 5V and plotted a Vce vs Ic graph (as shown in the picture). I heard Ic = Is*e^(Vbe/VT), where VT=0.025V, and ...
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MOS and BJT, why the difference

It takes longer for a BJT device to come out of saturation than a MOS device to come out of triode region. It is explained by the base charge storage time, the charge has to recombine and go back to ...
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LEDs have a low glow “noise”

Interim conclusion edit at bottom. I'm driving a stack of 5mm LEDs off 24volts as in the diagram below. I have four stacks of these, two have 5 white LEDs and share their base drive, one has 7 red, ...
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Design an emitter-follower amplifier with bjt

I'm studying the emitter-follower configuration for bjt on the book "Practical Electronics for Inventors". On pag 439 there is an example related to the design of this system. In this problem it knows ...
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Output resistance of CE amplifier small signal model

I was studying input/output impedance of CE amplifier small signal model and I wanted to know why the output impedance (looking from the collector) is just Rc in parallel with dynamic output ...
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32 views

Reconciling power dissipation and output characteristic of BJT

From the datasheet of 2SC1384 I got the following characteristics. I plan to use this transistor in an RF amplifier. The power dissipation characteristic sets a limit on the dissipated power of the ...
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Need Help debugging Colpitts oscillator

I've built the circuit shown in the picture on the breadboard. The simulation works as expected but when I build it i don't get any oscillations. I've measured all the DC voltages and they are as ...
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BJT - finding Ic with two opposite voltages connected to base

Here's a question from my homework: Find \$ I_C \$ and \$ V_{CE} \$ for the circuits below, assuming \$ |V_{BE}| = 0.7V \$ and \$ \beta = 100 \$. I understand circuit (a), but for (b)...

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