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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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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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1answer
61 views

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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408 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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210 views

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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5answers
233 views

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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340 views

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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2answers
72 views

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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71 views

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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2answers
171 views

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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2answers
170 views

Relay not turning on when supply comes from LDO Regulator

I'm trying to turn 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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134 views

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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1answer
120 views

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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2answers
345 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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351 views

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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84 views

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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1answer
67 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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4answers
88 views

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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3answers
329 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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2answers
394 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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0answers
74 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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1answer
77 views

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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101 views

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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1answer
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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117 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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117 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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1answer
75 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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2answers
131 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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1answer
48 views

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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2answers
103 views

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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1answer
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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99 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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113 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
77 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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99 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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240 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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1answer
299 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 ...
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1answer
91 views

BJT cut-off currents magnitudes and directions

I was reading the datasheet for the 2N3904 transistor (NPN BJT) recently and found out that I couldn't wrap my head around the cutoff current values. For the given conditions, both junctions are ...
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1answer
113 views

logic gates with 2n222 transistors and leds not working

I am a complete newbie to electronics, But here i go.. I am trying to make some super simple logic gates for now an AND gate. In this logic gate i want to show which out en inputs are high by using a ...
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3answers
447 views

FM transmitter circuit working?

I am trying to understand FM transmitter circuit working basically on how a signal is imposed on to the carrier signal. I would like to get an acknowledgement on whether my understanding is correct or ...
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2answers
80 views

DC BJT Circuit Allowable collecter swing discepency

Im working through tutorials of BJTs from sedra smith in which the lecturer has video solutions that you can follow as you go if you get stuck. The issue is i don't agree with some assumptions that ...
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1answer
88 views

Solving Unkonw R1 and R2 in classic descrete BJT bais arangment

im stuck solving the RB1 RB2 values is a classic discrete BJT circuit bias arrangement. Im sure its a simple thing that im just not seeing. Circuit values given are Ie=1.5mA V collector to emmiter =...
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39 views

Are there BJTs based on metal-semiconductor junctions?

So, I know that the fundamental part of a BJT is having a very thin base region that allows minority carriers to diffuse through it before they recombine. It seems to me, then, that it should be ...
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43 views

Problem with calculating high frequency response of a CE amp

I have this amp: The parameters of the transistor are hfe=120, Cje=40pF and Cjc=4pF. I am also told to assume Cpi = Cje = 40pF and Cmu = Cjc= 4 pF. From that I should calculate fh and compare it ...
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322 views

Increase output power of simple class AB audio amplifier

How could one increase the output power from a class AB amplifier circuit like this one? I wish to increase the output power to around 2W. The above picture is simulated with a 200mV amplitude, 5kHz ...
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2answers
265 views

How to keep current from draining out the bottom of this push-pull amplifier?

In an earlier question I asked about driving a speaker with a BJT transistor. @Jonk offered a push-pull amplifier design of his own, and @G36 noted its similarity to the JLH amplifier described in a ...