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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Converting Open-Drain to Active-Low

My question is very similar to Converting Open-Drain to Active-High, however instead of an active high, I want an active low. I am trying to interface TI BQ51050B (QI receiver + LiPo Charger) and TI ...
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1 vote
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Is there anywhere I can download BJT Transistor I-V data?

Is there anywhere I can download some current-voltage data for a bipolar junction transistor? (It doesn't matter that much which one.) To explain further, for a simple BJT circuit we have the ...
5 votes
2 answers
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What happens when Vce is more than the value in the datasheet?

I read in a datasheet about DC current gain. In the picture they say that when IC = 150 mA and VCE = 10 V, hFE is between 100-300. I want to know what happens to hFE when VCE is more than 10 V or ...
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What is the meaning of Vce in DC current gain?

In the datasheet of the 2N2222 there are several DC current gain (hFE) values. Assume that the load in my circuit is 20 mA. What hFE value should I choose? And what is the meaning of VCE for the hFE ...
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2 votes
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Finding the Q-point of a transistor: disambiguation

I find the definitions of Q-point that I have been given in class to be a bit ambiguous, so please read on and confirm or correct my understanding. As I understand from this SO answer, the Q-point or ...
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BJT circuit with totem pole output

In the circuit above, I don't understand how to close and open the BJT that I circled. To be specific, I think that when I give input 1 the circled BJT should be opened, but how? What is the voltage ...
5 votes
1 answer
239 views

What are the limits of the Ebers-Moll model for large currents?

I learned that the Ebers-Moll equation Ic = Is(T) ∗ exp(Vbe/Vt) "predicts" the collector current to rise exponentially with the base-emitter Voltage. Experimenting with LTSpice for a LED ...
3 votes
4 answers
833 views

Will a bipolar transistor have gain, even if the collector has no curent?

I understand that transistors have a gain, which in proportion to the base's current. My question is quite simple: I was wondering if, when the base had a high current (because of gain), it would ...
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Various regions of the BJT [duplicate]

I just started reading about the various regions of the BJT and I can't seem to wrap my head around how it operates in the various regions. Any help explaining how it operates will be greatly ...
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Why in the output characteristic graph of BJTs, within the saturation region, is the collector current positive?

It is known that in the saturation region of the BJTs the pn junction is reverse biased. The reason for being reversed biased could be explained as follow: Vce=Vcb+Vbe Therefore Vcb=Vce-Vbe We can see ...
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2 votes
1 answer
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Why aren't the majority charge carriers attracted by terminals of a battery when the p n junction is reverse biased?

It is clear that when the junction is reverse biased, the free electrons from the n side can not pass the depletion layer and fill the holes in the p side. But why will the free electrons not move ...
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Why use emitter current from DC response in calculations?

I am reading a textbook about BJT AC analysis. When finding Re I wondered why the value of the DC response is used for the emitter current. I assumed that when an AC signal is applied, Ib is not ...
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Cannot handle open collector output

I have a sensor powered at 24VDC which has an open collector output The output of the sensor (it is a PIR sensor), goes from 15Vdc when nothing is detected, and 0.7 Vdc when it detects some movement. ...
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4 votes
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Estimate base-spreading resistance from Gummel-Poon model

I'm attempting to calculate the base-spreading resistance of BJTs from manufacturer-provided SPICE models, which use the Gummel-Poon model. This Agilent publication (Internet Archive link here, if ...
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2 votes
1 answer
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Is there a typo in this question because I don't think the emitter node of Q2 should be grounded?

In this problem, it's asking us to draw the small signal equivalent of an amplifier but I think the answer they gave is wrong. In the second picture (b) I don't think the emitter of Q2 should be ...
1 vote
2 answers
47 views

Guitar distortion pedal gain

I am working on a custom distortion pedal. Here is the circuit: As we can see, we have the following flow: Input buffer -> gain stage with hard clipping -> buffer -> tone control using ...
1 vote
3 answers
97 views

What is the best way to control a 12 V, 5 A load with a 3.3 V microcontroller?

I have a 12 V heating element which draws up to 5 A that I want to control from an ESP-12 microcontroller. I have tried to find a MOSFET for driving it, but all seem to need at least 5 V on the gate ...
2 votes
4 answers
116 views

Why do some transistor packages have multiple leads for the emitter?

I want to use a BJT transistor (edit: BFU768F, https://www.nxp.com/docs/en/data-sheet/BFU768F.pdf) that looks like this: I think I get why lead 1 is bigger than the rest. It's because both collector ...
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7 votes
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How to avoid or compensate for reverse biased base-emitter breakdown in this BJT active clamp design?

This active clamp design is elegant and simple, and works well to prevent OUT from exceeding the supply voltage (+15V), but fails when IN is low enough to reverse bias Q1's base-emitter junction ...
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2 votes
4 answers
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On a BJT, is there a relationship between the base voltage and the Vcc voltage?

I just built a circuit that utilizes a BJT as a switch. It consisted of a voltage source connected to the base through a 100k ohm resistor. I grounded the emitter and connected a diode and a resistor ...
0 votes
2 answers
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What is the mode of MOSFET or BJT in buck converter?

I want to implement buck converter with conventional design (not synchronous etc) like below. And I want to implement the converter for two cases. One is to use BJT as switching device and another is ...
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2 votes
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Common-collector gain for guitar pedal

I am currently working on a distortion pedal. I have just made the initial input buffer to prevent loading the source signal. It is simply an NPN transistor in an emitter follower (common collector) ...
3 votes
1 answer
81 views

Wilson BJT current mirror output impedance

I am trying to have a small signal analysis of a Wilson BJT current mirror to find out what is the output impedance. Already I know that the answer is $$R_o=\beta r_o/2$$ In the first step, I need to ...
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Why is my BJT current control circuit not controlling current?

I have built this circuit (except the Re I bought came out to Re = 0.21 ohms after measuring it, so I used Rin = 680 ohms and R1 = 18k) to control current through eight of these LEDs using this BJT. ...
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Calculating lower cut-off frequency of a three-stage CE-CE-BJT amplifier

I have been trying to calculate the lower cut-off frequency of this three-stage BJT amplifier. For some reason the calculated result is 41% off frim the simulated result from LTspice. Also, is there a ...
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1 answer
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Transistor parameter in Proteus

I am just learning the electronics and by the help of proteus I am learning the behaviors of different components. Could someone please tell me how do I change the Emitter current values of a ...
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AC analysis of three-stage BJT amplifier [closed]

I have designed a three-stage BJT amplifier in LTspice, CE-CE-CC. I have calculated the gain on every stage using the small-signal model. The problem is that the overall gain I get by multiplying the ...
1 vote
1 answer
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How to stop sharp transitions of square wave from leaking through transistor

I've simulated a circuit in spice depicted below. The Vin voltage source (on the right) outputs a 100mV pulse every 2 microseconds, and the other voltage source (on the left) is a square wave that ...
2 votes
2 answers
112 views

Heater controller circuit using MOSFET for PID control

I want to implement some PID control for a 48 W heater cartridge. I have a microcontroller which can only putout a PWM output at 3.3 V and 8.1 mA. I current am trying to use a IRLZ44N MOSFET. Is the ...
0 votes
3 answers
123 views

P MOSFET without gate resistor. Why is this not burning up?

I have used this circuit several times and it appears to function as intended, but looking at it I am not sure why it doesn't fail. The intention was for MCU GPIO on the NPN transistor's base to make ...
4 votes
1 answer
331 views

Differences between low-power and high-power bipolar transistors beyond power handling

My intuitive understanding is that the BJT designed for higher currents will always have higher transconductance simply because of the physically larger junction, almost like a number of smaller ...
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1 answer
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RC delay only on negative input voltage transition

I would like to construct an RC delay circuit that has no filtering/delay when my gpio transitions to voltage logic high (Vlh) but does filter the voltage as the pin transition to voltage logic low (...
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2 answers
47 views

Turn on load BJT, MOSFET or Relay

I want to turn on a load of around 1.5A with a reed switch. I tried to just use a reed switch but found out pretty quickly that it will not take the current. I was considering a BJT but there is a ...
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Can you (theoretically) reverse bias a BJT?

My understanding of how a BJT(bipolar transistor) works looks like this: From https://www.allaboutcircuits.com/textbook/semiconductors/chpt-2/bipolar-junction-transistors/ Picture link: https://www....
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On-delay circuit not working

I'm trying to make On-delay circuit with passive components, I use 3 identical AAA alkaline batteries (4.5 v) and use PNP 2N2907 Bjt with 600mA Collector Current to heat(glow) up a small resistor ...
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4 answers
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How do I (can I) replace the relay in this circuit with a transistor?

Background I have the following circuit and it has been working for years. Please note that the circuit drawing may not match the real circuit exactly, but it is just a diagram to attempt to get my ...
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Self biased transistor current limiter

I am analysing the below self biased transistor circuit and have some questions: Why does the transistor in self biased arrangement never go into saturation for a light load? When the emitter ...
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Bipolar junction transistor circuit added after common source amplifier

I am studying for a class at university and our teacher gave us this circuit diagram and asked us to calculate the amplification, the DC version of the circuit and the small signal model for it. I ...
0 votes
1 answer
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How to choose a BJT used as a diode-connected BJT

I am choosing components for the following circuit, which will be used as a class AB amplifier: The resistors will be chosen to bias the transistors. I chose Q1 such that the quiescent current Iq = 1....
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2 answers
112 views

Is there a voltage drop across collector and emitter on a BJT?

Is there usually a voltage drop across the collector and emitter of a transistor? If so, where is it indicated in the datasheet? From what I read, the saturation voltage is only a voltage above which ...
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Can I use multiple BJT transistors in parallel if a single one can't carry enough current?

The maximum current my transistors are designed for is 1.5 A (from collector to emitter). I am looking to use a Raspberry Pi GPIO pin to switch a white LED strip on/off, by placing a transistor in the ...
3 votes
0 answers
40 views

Common base amplifier's input impedance

simulate this circuit – Schematic created using CircuitLab In a BJT common base amplifier configuration the input impedance is ~\$r'\$ (emitter resistance), right? If I understand correctly, ...
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2 votes
3 answers
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How does a BJT differential amplifier amplify?

I am reading The Art of Electronics. I found a related question here. I understand that differential gain of the amplifier is $$G_{\text{diff}}=\frac{R_c}{2(r'+R_e)}$$ but how? OK, using hybrid-pi ...
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Series voltage regulator with Zener diode and BJT - equations to explain the working principles

I'm studying the following series voltage regulator, and I'd like to know how to "prove" the last two sentences by using equations. For example, in the case of the 2nd sentence, the first ...
1 vote
0 answers
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How does the feedback circuit of this mini USB adapter work?

Summary I came across an old Apple mini USB adapter CLONE. It even had the green dot from the 2008 recall, so it's about a decade old. I'm actually glad I took it apart because one of the prongs fell ...
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Why don't inverters use BJTs to easily produce a pure sine wave?

This has been in my mind for a while now, I'm curious. Why not use a BJT H-bridge on the high voltage side, to convert the high voltage DC to AC. I saw a simple circuit that converts square waves to ...
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Why do you put the load of a transistor on the collector side and not the emitter side? [duplicate]

Everything that I have read has said to put the load on the collector side and connect the emitter to ground directly, but I am having a hard time understanding why this is. Why do both PNP and NPN ...
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2 votes
2 answers
57 views

How do I interface a low power controller with a higher power load?

I am not an electrical engineer (rather a photonics engineer), so I am looking for some direction on how to attack an issue I have. Basically, I need to drive a 150 Ω load with a computerized ...
2 votes
1 answer
52 views

Controlling external device by tapping into existing transistor

I'm working on one project where I cannot figure out on how I can control my external device by tapping into some home appliance. Just to note, that I have very limited knowledge of electronics and I ...
2 votes
1 answer
67 views

How to calculate the current through the LEDs/the collector current in the BJT circuit

For this exercise I have to calculate the voltage Vce and the current through the LEDs. The LEDs have a knee voltage of 2 V, the transistor has one of 0.7 V. Beta has been given as very big. To start ...
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