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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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Why Emitter current increases with increase in collector to base voltage in common base configuration of a BJT?

It is said from early effect that with increase in collector to base voltage there is decrease in base width due to which \$\alpha\$ (the current gain) increases due to lesser recombination of holes ...
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33 views

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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Can anyone help me with Max Swing? I'm so confused about its property

So max swing is supposed to be when ICQ=VCEQ/Rac right? I understand the fundamental part. But what is Max Swing relation to this: ICQms=min{ICQ,VCEQ/Rac}. I don't understand how it's related to max ...
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221 views

Transistor switch for VFD tube grid

I am designing a on/off switch for the grid of a VFD tube, and I would like to use a NPN transistor. The requirements are: The Grid is on when 12V are applied. At this voltage, 0.85mA flow. I want to ...
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5k views

BJT current mirror implementation

I am trying to figure out how to use a Zetex ZDS1009 IC. The chip contains 2 PNP and 2 NPN BJTs set up for current mirroring. I have learned about current mirrors, but I have not seen a 4 BJT mirror. ...
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271 views

Integrating an AND gate into another logic gate

I am attempting to make a 4 bit computer with transistors. I've made a half-adder without too much trouble, but now I'm trying to incorporate the AND gate's output (the carry) into the next adder. ...
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3k views

Thevenin Equivalent of BJT Transistor

I wonder if I can replace a BJT transistor operating in active mode, with its Thevenin Equivalent, like this: My guess is that I can, because the large signal model of the BJT operating in active ...
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Common Mode Gain BJT Differential Amp

I'm trying to determine the common mode gain and differential mode gain of the follow amplifier. Now I understand what the common mode voltage and differential mode voltages are, but I'm unclear as ...
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488 views

Discrete transistors current feedback amplifier topologies

I wand to design a headphone amplifier based on CFA topology (for some very personal aesthetic reason). There are two basic topologies of its input stage which is technically an V-I amplifier (I-V ...
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1answer
135 views

Please help me understand this bjt set up works

This is part of an automotive timer (NOT a 555!) that we have at work. Because I'm extremely bored I'm challenging myself to understand how this one operates. I've spent about 1.5 weeks with tutorials,...
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390 views

Question regarding collector leakage current with open emitter

In a transistor, we know that current amplification factor \$ \alpha \$ (DC) for CB Configuration is given by: $$ \alpha = I_{C} / I_{E} $$ Where, \$I_{C}\$ = collector current; \$I_{E}\$ = ...
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Stability factors of a voltage divider bias circuit

I am trying to derive the stability factors of a voltage divider bias circuit. I could derive S,S', however, I'm facing some problem with the derivation of S''. Please help me in this regard. How to ...
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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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819 views

How can I calculate the base current of a transistor to get a specific emitter-collector voltage?

For this circuit I am asked to calculate how much base current I need to supply to the transistor to get a Vce = 220.745 mV. The thing that has me stuck in this is that the problem does not provide me ...
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105 views

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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112 views

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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76 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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104 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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165 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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113 views

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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2answers
68 views

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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85 views

What would i change about this diagram to power and regulate this motor to make this a working model

what if the battery was 12v? would this be a working model? a previous comment suggested a BJT, and to regulate the BJT with a potentiometer.. what is a BJT? i am looking to get as small of ...
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832 views

Can the parameter \$\alpha\$ and \$\beta\$ be used for any BJT?

The reason I ask this question is because the statement in this wiki page. It says The common-emitter current gain is represented by \$\beta_F\$ or the h-parameter \$h_{fe}\$; it is approximately ...
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152 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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37 views

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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24 views

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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62 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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74 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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94 views

SPICE BJT simulation results

I'm trying to simulate simple BJT circuits to verify my hand calculations. Following is my circuit, I'm just trying to get the calculated DC collector current with the shown voltages and parameters. ...
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99 views

BJT: why does Ic increase when temperature increases? [closed]

When the temperature of an BJT increases, the thermal voltage $$ V_T = \frac{k_BT}{q} $$ where \$k_B\$ is the Boltzmann constant, \$T\$ is the absolute temperature of the junction, \$q\$ is the ...
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129 views

Convention regarding BJT collector current calculations

Using the equation Ic=Is*e^(Vbe/Vt) yields impossible values for me. I am using Vbe = .7v & Vt = .025 A. Is in circuit is 3.4mA. I am trying to determine Ic as an initial value for analyzing ...
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2k views

A confusion about the formula for transistor Vbe Ic characterisitcs

Below is NPN BJT transistor Vbe Ic characteristics and the formula: Many texts approximate this equation as: Ic = Is*e^(Vbe/Ut) and following this they say when Vbe=0, Ic becomes equal to Is. ...
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95 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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89 views

Which region is the transistor operating at?

PNP and NPN transistors in the circuit have identical Is values. Based on the PNP transistor is connected to a DC voltage source. k = 1.38064852×10-23 J/K qe = 1.60217662 ×10-19 C Is = 0.69 fA at 17°...
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72 views

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. ...
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BJT Clapp Oscilator for 3-6MHz

I designed a Clapp VFO to aid in my Ham Radio projects (Non-superhet SSB and CW recievers for 80M and hopefuly 40M). I chose the clapp topology for it's frequency stability. I want it to tune at least ...
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74 views

BJT differential amplifier: What causes common mode voltage gain?

In any normal op amp, if it was ideal it would have a gain such that the output voltage would be However in reality this is not the case as there is a common mode voltage gain and hence in reality ...
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132 views

Emitter follower and input impedance

I'm designing an active bandpass filter (using only BJTs, voltage sources, resistors and capacitors) with the following properties: Corner frequencies \$8 ~ \text{kHz}\$ and \$800 ~ \text{kHz}\$ (...
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92 views

High Input BJT Differential Amplifier

This is a basic differential amplifier. I want to modify it in order to have high input impedance (a few \$M\Omega\$ from DC to at least \$200~kHz\$) I thought about replacing the CE pair input with ...
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27 views

Where to find details of dimensions and doping of a typical bjt

I have been searching for dimensions and doping levels of base, emitter and collector for a typical bipolar junction transistor presently in use. I have tried searching on internet and seen datasheets ...
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What is the theorem behind the basic BJT biasing strategy (divide Vcc into three parts)?

I have learnt that there are strategies say, $$\frac{1}{3}\cdot V_{CC}=R_C\cdot I_C$$ and $$\frac{1}{3}\cdot V_{CC}=V_{R2},\frac{1}{3}\cdot V_{CC}=V_{CE}$$ finally, $$10\%\cdot I_E=I_{R1}$$ Ignore $$...
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27 views

Non ideal behaviour of a current mirror

Voltage across emitter and collector of QA is 0.7V. In a current mirror, why is it such that when the voltage across the collector and emitter is different for Qb than QA the value of Iout increases ...
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734 views

How does this circuit given with diode works different from the standard current mirror circuit or will it work in the same way? [duplicate]

This question had already been asked here by someone else few years ago but I could not see any satisfactory answer so I am asking it again. All I can see relevant in that post was the image attached ...
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89 views

Are bipolar logic familes like ECL/CML used currently for radiation hardened computing devices?

According to this article, even though it's 30 years old, ECL is more radiation resistant than normal CMOS chips by a factor of 1000. It is also less susceptible to DPA side-channel attacks. So in ...
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A question about understanding a BJT differential amplifier?

I'm trying to understand how a differential amplifier rejects common mode noise and amplifies the differential/normal mode noise. For that I use the following circuit topology and simulation: When ...
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2answers
43 views

Spike in RTL Inverter

I do not understand the cause of this spike which can be observed in an RTL inverter (but also in a CMOS inverter). In the following image it is the peak at 10 mS (I have considered as input a signal ...
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24 views

Frequency Modulator with sinusoidal output

I am creating a frequency modulator, all the circuits that I have looked so far on the internet are generating a Pulse wave or a Triangular wave output, so if you guys can help me to create a circuit ...
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61 views

Help with making sure a correct use of a BJT

I'm designing a PCB that receives 3 power inputs 3.3v, 5v and 12v. I need simple way to switch all of them of in one go. Here is a schematic of my PCB My 12V line is for running a ws2815 led strip ...
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84 views

CE/CB BJT Amplifier(s) + Load Lines!

This diagram from a lab assignment is not making sense to me...any help would be greatly appreciated. I don't understand what \$I_{C_\max}\cdot R_e\$ is and why it appears on this diagram. Even if it ...
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1answer
117 views

Common collector - bias resistor?

[Original question: CCD output buffer; seemed best to make the transistor part a separate post] Taken from Wikipedia, the circuit in Fig. 1 acts as a common collector (high input impedance - low ...