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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63 views

BJT twin transistor astable multivibrator always outputs high

Noob here (again). I have made this schematic of an astable multivibrator and a Raspberry Pi. I want it to produce a square wave, but it just outputs high constantly. I use the Raspberry Pi as a ...
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73 views

PNP- NPN light flasher

I understand that R1 and R2 make a voltage divider. initially Q1 is not conductive, so is Q2? initially C1 acts as short circuit, so flashes the light L1 for a moment? Please correct me if wrong. ...
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65 views

12V motor controlled by 3.3V logic

I have a valved exhaust system in which the valve is rotated by a 12V motor, but the electronics that came with it have died. I would like to integrate it into my Raspberry Pi dash system, using the 3....
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68 views

Is it possible to make this BJT-based voltage limiter more \$\beta\$-independent and temperature stable?

The following circuit limits the rail voltage to slightly above Vref, by drawing a lot of current, when the rail exceeds Vref. The BUF Block represents a supply such as LDO, Reference or other ...
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121 views

How to choose NPN or PNP for the second stage of an amplfier?

How would you choose NPN or PNP for the second stage amplifier like the one below? What are the advantages and disadvantages of them? I see that both NPN and PNP can do the job. I understand that they'...
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75 views

BJT amplification formulas [closed]

Is it required to memorize all BJT formulas for AC amplification for all BJT configuration and biasing setups? Am I a failure of an electrical engineer if I don't memorize all of them? They are ...
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44 views

Simulation on LTSpice not matching with Calculations at Q-point

Here is a circuit- It has 2 parts, the first part generates a triangular waveform with pk-pk voltage of \$1.2V\$ approx. The second part is a CE amplifier with intended circuit gain of \$-10\$. The ...
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86 views

Voltage Controlled (w/ op amp) Wilson Current Mirror Can't Actually Mirror (Missing something)

I'm working on a current mirror circuit and I'm in a little bit of a problem. I have a current mirror circuit, which is supposed to, well, mirror the current, except that the current in the "copy&...
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4answers
956 views

Why isn't the D2 LED ON when the transistor is ON?

I attached to this question a circuit that I can't seem to understand. In the lecture I watched, we use the transistor (in saturated mode) as a switch to kind of get a positive and negative output ...
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69 views

Long tail amplifier using current mirrors

I have built this in the Multisim simulator and it works there as desired. Now I have built it on a "lame" breadboard. Is the upper PNP current mirror correct? Since the "left" ...
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139 views

Are the transistors in this 8x8x8 LED cube design doing what I think they're doing?

I'm advising a friend on building an 8x8x8 monochrome LED cube. He found this popular design, but I feel like there's a lot of room for improvement. One area in particular is this array of NPN BJTs ...
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98 views

BJT Base-Emitter Reverse Breakdown

In the following (partial) circuit, the transistor is usually in its regular forward active mode. However, when the supplies collapse abrubtly - e.g. during power-down - the base-emitter junction can ...
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95 views

Limiting the base current to BJTs in Gate Driver circuits

I want to make an MCU operated buck converter with the lowest operating current possible since most electronic shops near me only offered buck converters with an operating current of 10 mA and above. ...
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142 views

Why is it bad to drive a MOSFET asymmetrically using BJTs?

And by that title, I Mean that the drive current to the gate of the MOSFET when the BJT is on differs from the drive current for when it is off. I know that it would cause a difference between Toff ...
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480 views

Can a MOSFET be used to switch between two supply voltages?

My goal is to programmatically switch between supply voltages (3.3V and ~2.6V) for a load. The 2.6V level doesn't need to be exact, +/- 0.2V is OK. I created the following circuit, which uses my MCU's ...
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1k views

Why do PNP transistors source current and NPN transistors sink it?

I'm unable to find a definitive answer to this (trust me, I've looked, unless it's right under my snout) so please do point out the correct answer if I have. I'll feel foolish but at least then I'll ...
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4answers
104 views

How to set the initial conditions of an astable multivibrator

Consider the following oscillator circuit: I have this running and oscillating at about the frequency I want. However, when first powering it on, one of two things happen: Both LEDs momentarily ...
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117 views

BJT vs MOSFET switching speed

I've been reading various answers about the switching speed and frequency of BJTs and MOSFETs, yet I keep finding conflicting answers. Can anyone explain the discrepancy and the real answer? Sources ...
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64 views

Reproducing ST Design Gate Driver

I try to reproduce the buck-boost converter from the "STM32F334 Discovery" board. Therefore I have drawn a KiCad Schematic and assigned the appropriate spice models. But unfortunately, I'm ...
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46 views

Using an NPN Transistor as a High Side Switch

I want to use the thermistor inside of a battery pack on the BQ25887 and on a GPIO pin on a microcontroller so that both the microcontroller and the charging IC can monitor the battery voltage. For ...
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118 views

How does a Zener diode and NPN BJT control circuit provide current limit or thermal shutdown via an NPN Darlington pass transistor?

Background Application: 1995 Ford Ranger Manual Control Blend Door Actuator This 1 page pdf guide shows the block level function of a manual blend door actuator (my component is from a Ford Ranger.) ...
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1answer
53 views

Av Ltspice value for Common Emmiter amplifier is not the calculated one

I am designing a common emitter amplifier with a total gain of magnitude 5. I biased the transistor at the Q point I wanted: IcQ = 40mA (checked with LTSPICE). The expression i found for the gain of ...
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60 views

Finding the transfer function of a common base BJT transistor amplifier [closed]

I've been tasked with finding the transfer function for this common base BJT amplifier in low and high frequency but I actually have no idea how to even approach it. Any help will be hugely ...
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107 views

Why do we treat R1 and R2 as though they're parallel in this circuit?

I'm working on a homework problem and I need to solve for the base current Ib, and I know to do that I need to find the Thevenin equivalent at the transistor base. My professor solves for it as though ...
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4answers
681 views

How can the transistor be in the cut-off region?

Imagine we have an NPN transistor and the base voltage Vb = 0V. How can the transistor be in the cut-off region? I've started by assuming it to be in the active region and then calculated the emitter ...
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381 views

Switching the high side of a load - The Art of Electronics

I am going through The Art of Electronics Third Edition. There is a note 12 unter Figure 2.10. Why Q3 would not switch if R3 = 100 Ohm?
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111 views

Circuit to turn everything above 1V into logic level HIGH (5V)

I have a LED in a car and I want to hook up onto it to detect when its on. But voltage for that LED can depend on the intensity of the LED (depends if its night or day). I've measured voltages from 1....
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4answers
184 views

What is the small signal gain for this circuit?

The question looked easy at the first sight. I found the quiescent current through the BJTs. I can say that |Vbe| = 0.7 V for both BJTs. Therefore, current through 1 kΩ below Q1 = 2 mA. Similarly, ...
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150 views

How can I find the current in this BJT & op-amp circuit?

I have a background in EE but am very rusty. I was given this as a problem to help me brush up on my skills. I'm supposed to solve for I3. What I've come up with so far is that the positive terminal ...
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749 views

How can I solve this simple BJT circuit?

Looking at this circuit (first circuit): Choose a value for V2 between 0V and 10V. Then how could we know the IB, IC, IE, VB, VC, VE? Can we solve it by hand (by hand means the value IB, IC, IE, VB, ...
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94 views

How are desired specs decided upon in a common emitter amplifier circuit? (beginner)

I have been reading up on amplifier circuits recently out of curiosity and would like to attempt designing my own. However, I would first like to understand how various amplifier specifications are ...
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94 views

Why use diode connected BJT (and not just connect two BJT back-to-back) for current mirror circuit?

I have been studying about current mirror circuits using BJTs. The circuit I have is this: For the left BJT, it says: IC1 = Io(e^(Vbe1/Vt)) This is understandable ...
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77 views

Confusion regarding BJT voltage gain

I learnt from youtube the gain from this circuit is as below where it just take $$A_v=\frac{r_c}{r_e'}$$ while today I doing my revision encountered this question and the answer provided as above. ...
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1answer
78 views

How to choose the right value for a emitter cap for an RC oscillator?

I have a circuit like this: Given a low or a high impedance feedback network, how do I choose the right emitter cap value Ce? I know that the emitter cap depends on the impedance of the feedback ...
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94 views

BJT As a Switch. Efficency?

I needed a 2 digit display for a project and simply ordered a device 1, without the proper thought. To be fair I have learned a few things about 7 segment LED displays. Unfortunately I got one which ...
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63 views

How do you derive the output impedance of this BJT stage?

I am given the following circuit (neglect Early effect): And, I am asked to find the output impedance looking into where V_out is taken By inspection, I can tell ...
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89 views

BJT(NPN) has a VBE higher than 0.7v

I am confused about this datasheet for a transistor I am using. From my understanding, most transistors have a VBE of about 0.7v but the max on this one is 1.2v which I thought would destroy most ...
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85 views

I need to design 182 RGB LED array circuit

I looked for designs for PWM dimming RGB LED arrays on the internet, and I found this circuit with a current mirror: As the title says, I am designing an LED panel with 182 RGB LEDs. I think the ...
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124 views

Input impedance of emitter follower

I decided to calculate the input impedance of the following emitter follower circuit as a practice. I used this well-known small signal model for the BJT: For voltage gain, I obtained $$A_\text{v}=\...
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2answers
63 views

AC analysis of circuit: capacitors in parallel with BJTs

It's been a while since I've done AC analysis for circuits, but the sources I've looked at say to short DC sources to GND, and to treat capacitors as shorts. Given the placement of C7, I think Q3 can ...
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1answer
56 views

Oscillator amplitude dependent on specific transistor in circuit puzzle (carrier oscillator board repair)

Dear people of the internet, I seek your wisdom in figuring out a specific (analog) situation that puzzles me. Recently I got my hands on a HF transceiver, the Kenwood TS-120V, which was not working. ...
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46 views

Not sure about the way to compute the voltage across a DC motor driven by a NPN BJT

Long story short, I wanted to make my own H-bridge but I was out of p-channel MOSFETS so I tried to design one with BJTs instead, but before doing just that I simply wanted to try to run an average 3-...
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146 views

What is the purpose of the D4 diode between base and emitter of PNP transistor?

I am having problem with understanding the purpose of the Q7 transistor. . The anode of the D4 diode is connected to the base and cathode - to the emitter which makes Veb = -0.7V. This will always ...
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118 views

Negative feedback in collecter-to-base bias BJT circuit

I'd like to calculate the 'feedback factor', \$\gamma\$, for the following simple amplifier. By definition of \$\gamma\$, the base voltage is \$V_\text{i}-\gamma V_\text{o}\$, where \$V_\text{i}\$ is ...
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53 views

Improving grounded emitter amplifier with collector-to-base bias

According to books, this simple grounded-emitter amplifier has some shortcomings. For example, 1) the d.c. operating point is unstable. 2) since the intrinsic resistance of emitter, \$r_\text{e}\$, ...
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110 views

Have I understood transistors correctly?

These are what I know and what I've concluded about transistors. Correct me if I'm wrong: It's all about current. When we have a determined voltage differential source, BJTs are used to boost the ...
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114 views

In the PNP transistor, base current is the sum of emitter and collector current. How is this possible?

The PNP transistor at the right side doesn't follow any characteristics of the 4 modes of operation of BJT. How is this possible? PSPICE simulation has shown quite similar results. Here, beta=100.
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71 views

The small-signal impedance looking into the emitter when base voltage is not fixed

In section 2.3.2 of the 3rd edition of The Art of Electronics, the small-signal impedance looking into the emitter, \$r_\text{e}\$, for the base held at a fixed voltage is defined as "the ...
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145 views

What is the "small-signal impedance looking into the emitter"?

In section 2.3.2 of The Art of Electronics (3rd edition), the authors define \$r_\mathrm{e}=dV_{\mathrm{BE}}/dI_\mathrm{E}\$ (with fixed base voltage) for BJT transistors, and state that: This ...
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169 views

Applying Kirchhoff's voltage law to a bipolar junction transistor [closed]

I am new to the field of electrical engineering. As a part of my curriculum, I am required to study about bipolar junction transistors and their configurations. Question: In the active common emitter ...

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