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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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Interfacing with Transistors

I was reading the chapter of The Art of Electronics about bipolar transistors (chapter 2), and I came across this paragraph: But why should I use transistors, specifically, for interfacing? Why not ...
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MC34063 with external BJT. Why?

See next post in this series here adding a fet All, Here is a picture of an LTSpice simulation of a very basic MC34063 step up circuit which is behaving exactly as it should, 5V in 9V out. I'm ...
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The use of semirelative sensitivity for \$I_C\$ on \$V_{BE}\$?

When evaluating the quality of a bias network, we use the semirelative sensitivity to evaluate the sensitivity of \$I_C\$ to change with respect to \$V_{BE}\$, with the semirelative sensitivity being ...
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use bjt or mosfet in h-bridge with inductive load?

is it better to use 2 pnp bjt in high side and 2 npn bjt in low side of an inductive load drive h-bridge ? or using 2 pmos in high side and 2 nmos in low side of h-bridge? what are the advantages ...
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Units of BJT parameters

Let's say the model is the Ebers-Moll model, but this obviously extends to other models: $$ I_{\mathrm{b}} = \frac{I_{\mathrm{s}}}{\beta_{\mathrm{f}}}\left(\mathrm{e}^{\frac{V_{\mathrm{be}}}{NV_{\...
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67 views

What is the input and output impedance of a Widlar current source?

The Widlar current source is illustrated below: simulate this circuit – Schematic created using CircuitLab The small-signal AC model is shown below: simulate this circuit I know that if the ...
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In a Wilson mirror, does Q3 need to have the same beta to cancel the base current error?

I'm learning about Wilson mirrors in the Art of Electronics Third Edition (pg. 102). It shows the following circuit: simulate this circuit – Schematic created using CircuitLab It then ...
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Opamp BJT Circuit problem

Non inverting amplifier - LF351N BJT - BC337 Gain = 5.8 Opamp supply voltage is +12V & -12V Transistor collector voltage = 12Vdc Opamp i/p = 1Vpp (Sine wave) Opamp o/p = 5.8Vpp Opamp o/p is ...
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What do you think about my PSU design?

I have designed a symetrical PSU using LM317, LM337 plus power BJTs. The desired output range is at least 24V-2V @ 6A max. Altough not shown in the schematic, i plan to add power resistors and a ...
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64 views

Design of a differential digital signal buffer, 250MHz, 1V output swing

Primarily for academic interest (so please don't suggest ICs to do this, as they won't help me learn how to deal with this kind of problem) I'm attempting to design a buffer circuit to accept a ...
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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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Under what conditions can Vbe exceed 0.7

I am simulating a transistor switch in PSPICE. The goal is to switch a load (like a lamp) connected in the collector on and off. Imagine that in the place of the R2 resistor there is this ...
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277 views

BJT vs FET for VHF LC oscillator

In the ARRL (2011) section 9.3.1 about LC oscillator circuits it says that "[t]he N-channel JFET source follower shown appears to be the most popular choice nowadays", but does not explain the ...
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Astable multivibrator diode improvement

The output voltage of an astable multivibrator is not a perfect square wave; the voltage when viewed from an oscilloscope tends to resemble a curve. I'm told that this is because the voltage across ...
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BJT Constant Current, Resistor Equations

I am looking to design a simple constant current circuit to draw a precise load from a variable voltage rail. The voltage rail (V+) should be 10V-20V in normal conditions, but it is possible for V+ ...
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How to protect H-bridge from burning

I'm building an H-Bridge to drive a 5v DC motor. We made the circuit with SMD components And everything works properly. But the problem arises when the motor output gear is held manually. The motor's ...
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Getting more current through a BJT

I'm trying to alternate between two 3W LEDs (forward current of 750mA, forward voltage of 3.6V). I've got a 555 set up in astable mode, and am driving the LEDs via npn and pnp transistors. In ...
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Understanding the circuit

I am a beginner electronic student and I know this might sound a bit easy for most of you, but I would like to understand the role of the components used in this circuit - it's an LDR-Controlled DC ...
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high side MOSFET switch and BJT interface with MCU GPIO

im operating a 28V contactor using a GPIO from my MCU, i have a 28V supply readily available. im looking to surpass the gate threshold of the MOSFET by introducing a NPN BJT with a 10V supply, with ...
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Why Can't I Turned On The Relay If the Supply Come From The LDO Regulator

I'm trying to turned on a 5V SPDT Electromechanical Relay. It is planned to be used with microcontroller application. Below is the circuit that I wired on a protoboard: When connected like that and ...
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why does this approach neglect the output resistance of the bjt in calculating gain?

This is the question that I would like to ask about. My question is with regards to finding Vc for question 3. This is the memo for the question: However, I have a problem with regard to how they ...
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LED Dimmer only using NPN BJT and potentiometer

Does this circuit looks ok? I'm trying to make a RGB lamp with knob controls. The LED has a maximum current of 20mA. I've successfully created a working prototype of below circuit. I'm not from ...
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Basic Op Amp Circuit

I am reading "Intuitive IC Op Amps" by Frederiksen and getting stuck on one of the basic circuits. The book presents this image as a simple proxy for an op-amp. Can someone help me understand a few ...
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276 views

finding the output resistance of bjt folded cascode

I am being asked to find the output resistance of following BJT cascode amplifier, from "Microelectronic Circuits" by Sedra and Smith, 7th edition, problem 8.80.a on page 588. Now I decided to ...
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Some questions about the emitter resistor added to the fixed bias circuit

In many texts it is mentioned that the emitter resistor stabilizes the bias circuit. For example particularly for the below example: Which quantity is aimed to be stabilized initially? Vbe? And how ...
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Is the following equation wrong in this BJT DC bias analysis?

I came across a section of a tutorial regarding BJT DC analysis as follows: The first two equations for Ic and KVL for the BE loop seems correct. But I'm stuck at the last equation inside the box ...
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66 views

How does the voltage divider model a voltage source in this transistor bias circuit?

I encountered a text which says that the leakage current Ico (or called Icbo) adds up to the signal current hence causes thermal runaway because it causes temperature to increase and it also increases ...
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Help with Voltage Divider driven NPN emitter follower

I am trying to solve the Exercise 2.5 in Art of Electronics. Use a voltage divider and emitter follower with 15V supply to make a 5V output, and within 5% of 5V at a 25mA load. This has been solved ...
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273 views

Common cathode RGB LED from 3.3v

I have an RGB LED (common cathode) that I would like to trigger from the GPIO pins of an ESP8266 (3.3v). The device I am using (Wemos pro mini D1) has a 5V supply which I was planning on controlling ...
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297 views

BJT schmitt trigger threshold voltage calculation

Here is the classic Schmitt trigger: simulate this circuit – Schematic created using CircuitLab And I would like to find the equation for the threshold voltage (\$V_L, V_H\$). First I ...
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67 views

Better polarization for push-pull

I have a circuit that used to drive a lighter load. I need to increase the current it manages to drive (and mainly the turn off time). In the following schematic I show what I want to do: the old ...
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Opamp-bjt circuit question

I am solving the operating point of this circuit to find \$V_{OUT}\$ and \$I_{C}\$, assuming ideal opamp and BJT. Due to the opamp the node \$B\$ is a virtual ground so \$V_{OUT}\$ is from the ...
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BJT Veb matter & use of the Ic-Veb Exponential Relationship

In the following circuit Ic must be found. I begun by open-circuiting the 3 capacitors, and finding the voltage at the base which is 2.5V (from the voltage divider). By assuming that Vbe=0.7V it's ...
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36 views

Transconductance of devices

I wanted to ask what are the cases when it's compulosory to know the transconductance (gm) of a device, whether it is BJT, JFET, MOSFET... As far as I know, when you want fast switching with very ...
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93 views

Transconductance of a device

I would like to know what does it mean when someone says "that the transconductance of a FET or MOSFET is less compared to that of a bipolar junction transistor (BJT)". I even heard someone saying ...
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95 views

Differential gain on BJT differential pair: not symmetric?

I'm having the following problem. I got to analyze a BJT differential pair with emitter degeneration of both transistors. I calculated the theoretical expressions of my gain and obtained (since the ...
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73 views

Transistor operating region

How to decide the operating region of given transistor as shown in the figure? I mean is it possible to decide the operating region of bjt without knowing the Load(Assuming pure resistor and supply ...
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112 views

Which current is largest?

I have this circuit and the question is what is the relationship between the 3 currents(meaning if I1 is > < or = to the I2 and so on). The way i see it I2 and I3 are zero since they are npn bjts ...
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Bipolar Transistor (BJts) [duplicate]

I want to know that since in bjt's saturation state base and collector junction is forward bias why we take saturation voltage (Vce) as 0.2 Volts? Base and collecor junction is PN so voltage must be 0....
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Negative sign of collector current (Ic-Veb characteristic curve) of pnp BJT on LTspice

In the exact same circuitry with a npn transistor i get the same curve but with positive sign. I didnt expect the Ic to turn out negative since on Sedra-Smiths book its also positive. Shouldn't it be ...
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85 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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91 views

BJT theory and practice - please help me understand

I've never built a circuit using BJTs before but I'm more inclined to go this route over FETs for this task. I have 3x 12V 200mA (approx.) loads in a vehicle which I want to switch on with only a 12V ...
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104 views

Transitor keeps base turned On even at zero volts in dual power supply system!

Circuit Description: I am powering transistor with two power supplies, One supply is providing 5-VDC to its collector load through Ammeter (+Ve - Ammeter - load - collector - emitter - Common/Ground) ...
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1answer
73 views

What values of components to use in this 30V-2V, 3A(max) current limiting circuit?

I'm looking for a current limiting circuit for my lab bench PSU, and found this one. What are the values for the components? Is R_sense the pot? Thanks in advance!
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Emittors and Collectors in a transistor, what exactly makes them different? [duplicate]

For my high-school physics class, we are studying the theoretical basics of transistors. I have not been able to understand which side of the transistor is the collector and which side is the emitter ...
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94 views

In op-amp, how do they remove Quiescent votage (bias voltage of a bjt) from the output?

We use a coupling capacitor to remove the dc component from the output in simple ac amplifier circuits. In below base biased CE amplifier, the voltage at collector node is not ac, it is pulsating dc ...
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BJT-Large Signal Model circuit analysis

In the following circuit we assume that all 6 BJT transistors (Q1,Q2,...Q6) are the same and that \$ V_T \$, β, Is, IEE, V1 and V2 are given. By using the Large Signal Model of BJT, I want to find the ...
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157 views

What's the name of this circuit?

As i said in the title what's the name of that circuit and any characteristic for it?
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171 views

Why collector current is small in value in saturation mode?

Why is the collector current smaller in value when a transistor is in saturation mode of operation? My guess is that although both the junctions are forward biased, the (forward) currents are '...
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234 views

Parallel current limiters, ballast resistors and transistor matching

Are ballast resistors (R17, R8 and R5) required when using parallel current limiters as show below? I found it hard to find working examples on the internet with values already present to tweek ...