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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66 views

What is the purpose of this resistor between BJT Base and Emitter?

Can someone please explain to me what the purpose of this resistor is?
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51 views

Which characteristic of a BJT determines the timing in the following given application?

I currently face a question about the BJT circuit given here: We see a 2N3904 transistor with constant base current of 2 mA. A switch seperates a 15 V/20 ohm source from the collector of said ...
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93 views

CE power amplifier design

I am trying to further my knowledge in discrete transistor design so I am designing a CE power amplifier to drive a speaker. What I have trouble with is determining where to bias my BJT at. I can't ...
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47 views

PNP folded cascode circuit question

I have a homework question but no idea what this circuit does and how to make calculations. In the simulation input and output voltages are same why? Thanks for any help
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68 views

Connection between two pre-amplifier stages

In the circuit shown above, I am unclear how Q1 connects to Q2+Q3. I understand Q1 is part of a common base amplifier and Q2 and Q3 form a direct coupled amplifier. The circuit is described at ...
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58 views

Finding AC equivalent seen between two nodes of a BJT Amplifier

In the textbook, Electronic Devices and Circuit Theory, 11th Edition by Boylestead and Nashelsky, on page 566, the lower cutoff frequency is derived for a voltage ...
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64 views

Relaxation oscillator

I was wondering how inside the relaxation oscillator heating is not produced inside the BJT junction causing destruction of the transistor. simulate this circuit – Schematic created using ...
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106 views

Why does the positive wave of my class AB amplifier output get clipped far below rail voltage?

For the past 6 months I've been designing and building a 30W 3MHz audio modulated Tesla Coil driver. For this, I've created a +-15V linear regulated power supply, an AM modulator using an MPY634, and ...
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74 views

How to calculate Ic for a very simple NPN transistor circuit

I built a very simple circuit with an NPN (2N3904) transistor. Then, I swept Vce from 0 to 5V and plotted a Vce vs Ic graph (as shown in the picture). I heard Ic = Is*e^(Vbe/VT), where VT=0.025V, and ...
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MOS and BJT, why the difference

It takes longer for a BJT device to come out of saturation than a MOS device to come out of triode region. It is explained by the base charge storage time, the charge has to recombine and go back to ...
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138 views

LEDs have a low glow “noise”

Interim conclusion edit at bottom. I'm driving a stack of 5mm LEDs off 24volts as in the diagram below. I have four stacks of these, two have 5 white LEDs and share their base drive, one has 7 red, ...
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Design an emitter-follower amplifier with bjt

I'm studying the emitter-follower configuration for bjt on the book "Practical Electronics for Inventors". On pag 439 there is an example related to the design of this system. In this problem it knows ...
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Output resistance of CE amplifier small signal model

I was studying input/output impedance of CE amplifier small signal model and I wanted to know why the output impedance (looking from the collector) is just Rc in parallel with dynamic output ...
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31 views

Reconciling power dissipation and output characteristic of BJT

From the datasheet of 2SC1384 I got the following characteristics. I plan to use this transistor in an RF amplifier. The power dissipation characteristic sets a limit on the dissipated power of the ...
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Need Help debugging Colpitts oscillator

I've built the circuit shown in the picture on the breadboard. The simulation works as expected but when I build it i don't get any oscillations. I've measured all the DC voltages and they are as ...
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BJT - finding Ic with two opposite voltages connected to base

Here's a question from my homework: Find \$ I_C \$ and \$ V_{CE} \$ for the circuits below, assuming \$ |V_{BE}| = 0.7V \$ and \$ \beta = 100 \$. I understand circuit (a), but for (b)...
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Hand Calculations of Emitter Coupled Logic circuits

I am having trouble approaching doing hand calculations for the MECL shown below. I know since Qa, Qb, Q1 all have common emitter voltages so they cannot all operate in the same mode if there base ...
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39 views

How to calculate the gain of a BJT Differential Amplifier (Long Tailed Pair)

I'm currently reading Designing Audio Power Amplifiers by Bob Cordell. After seeing the "Basic 50W Amplifier" that he shows in the beginning of chapter 2, I now am trying to use the same building ...
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133 views

What could be the purpose of the diode at emitter for this transistor buffer?

I have been using the following buffer(inside the dashed rectangle) as a driver where the 3.3V uC pulse is converted to 12V pulse which is then coupled to a MOSFET gate in this case. I have seen this ...
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107 views

Unity Gain Buffer as a Current Source

I keep having issues with creating a unity gain buffer amplifier to isolate a voltage source. My voltage source is very limited in current ~1-2 mA and I need that signal to drive a transformer at 10 ...
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32 views

Is this hybrid pi model of a BJT unileteral or bileteral?

In introductory texts, the BJT is said to be a unilateral device. I think it means that the base current is only a function of Vbe; and Vbe is not function of Vce and Ic(?) Is the following BJT model ...
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48 views

Problem in H-bridge by stm32f103c8t6

I have a 5 volts DC motor and want to make H-bridge. This is my schematic: I doubt whether the transistors I used are correct. IRF9630 on top and IRF630 on bottom. Could I use BJT instead of MOSFET? ...
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I can't reach the correct current with a BJT in LTSpice

The diagram above is what was given to me. I have to make: Vce = 1V Ib = 30uA I have two variable components, namely, Vcc (Voltage source from the collector), and Rin(supposed to be a ...
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49 views

What mechanism is responsible for the β/hFE variations in manufactured bipolar transistors?

It's common sense that the current gain β (hFE) in a bipolar junction transistor is somewhat unpredictable, and one must design a discrete transistor circuit to be insensitive to the variation of β. ...
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47 views

In npn transistor, why doesn't some base current flow in conduction band?

Free electrons in emitter are in conduction band. Free holes in base are in valence band. Since base is thin, most electrons pass the base while staying in the conduction band and enter the ...
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Switching Devices [closed]

I am currently working on a project looking at research materials on low voltage but high current switching devices. Is there any technologies/research out there that are rated for high Amp low ...
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40 views

Advice for P-Channel Mosfet in Depletion Mode

i want to be able turn off a led with a mosfet and a pushbutton. In order to do this i want the led to be in off state when voltage is applied to the gate of the mosfet. After researching for a while ...
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49 views

How is the emitter current kept constant in common base BJT?

While studying the output characteristics of a common base BJT I had the following question: How is the emitter current kept constant? Do we ignore the early effect, if so then why?
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A fundamental question about crossover distortion

Following shows the input output waveforms to illustrate crossover distortion: The supply rails are indicated as if it is a dual supply(+V & -V). I then tried to simulate this in LTspice as in ...
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90 views

Amplify slow changing dc signal

A slow-changing DC signal goes from 0-30mV and I need to shift it to 700mV-900mV. I cannot use op-amps and need a transistor-based solution. Please suggest.
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On this problem, why are they disregarding the collector for the KVL?

I've tried this problem 3 times before looking at how the professor solved part (a), and every time I keep coming up with the same KVL equation. Why is my KVL wrong?
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29 views

Omitting ΔVbe in emitter follower KVL equations

In the forward active region, if we draw an emitter follower voltage variations as follows: It seems by using KVL we can say that: ΔVin = ΔVbe + ΔVe Now in many texts they omit ΔVbe and equate ...
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70 views

Why currents in the circuit of BJT change like that?

Why don't the currents in those two circuits change the same way when I change voltage of the voltage sources?
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BJT working in saturation mode

I have the following circuit. The transistor is working in saturation mode. It means both p-n-junctions are forward-biased. Thus, there are two diffusion currents in transistor (electrons move from ...
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Can't find Fast switching BJTs

I'm looking for a replacement for a FET switch that switches at 40 kHz. When the FET is on it is connecting an RFID antenna to ground. When off, the antenna is open. It does this in order to ...
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57 views

Why does this BJT work as a switch in this logical-NOT circuit, no matter how I orient the emitter and collector?

I have built the following circuit (obtained from Electronics All-in-One For Dummies (For Dummies 2nd Edition)) It works quite well. When the the momentary button (normally open) is not pushed the ...
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Why is the metal tab of a BJT power transistor is always connected to its collector pin?

Until now I thought the metal tab of a BJT power transistor is connected to ground but it seems like it is connected to its own collector pin´as shown in the below illustration: Is there a particular ...
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Understanding Touch Switch with BJTs and Relay

We worked on this circuit today in one of my labs and it had my groupmates and I puzzled. We had a 1k resistor in series with an LED on the NO side of the relay. I searched for the circuit online and ...
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59 views

Using Thevenin Theorem to solve for unknown values in BJT voltage divider circuit

Assuming this is a DC bias of a voltage-divider type BJT circuit: V_CC = 10V Beta value = Not Known V_BE = 0.73V (Base-emitter voltage) And the Thevenin circuit ...
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81 views

How to design BJT active filters

In High school and University courses, I have only seen active filters made around OpAmps. I'd like to know something about active filters built with BJT amplifiers. Figure: Example of BJT filter ...
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51 views

Impedance matching in common collector

The last stage of a power amplifier to an 8 ohm speaker is usually a common collector. Now many textbooks mention that it is used as a buffer as it has a low output impedance compared to the speaker (...
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214 views

Analyzing a simple circuit with transistor

I've been trying to understand how to analyse circuits with transistors, but I'm having a hard time finding out where to start when confronted with the following two types of circuits. (Gain = ...
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69 views

Current Sensing with Op-Amp, do I need a shunt resistor in series with the load on the output?

Hi I am building a circuit to measure current draw from a load, I opted for a voltage follower with a transistor and a shunt in between, The transistor is needed to allow higher currents to flow ...
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292 views

How to set output resistance of an amplifier to be 50k

I need to build an amplifier using BJTs / resistors / capacitors. It should have specific parameters (up to 5% error), and I’ve reached them all except for the output resistance. The parameters are ...
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How I can solve this BJT?

I edited circuit like this: I tried representing currents like: $$ \begin{aligned} &I_{R1}=100_{B1}+I_{B2}\\ &I_{R2}=101 I_{B2}-I_{B1}\\ &I_{i}=101 I_{B2}+100 I_{B1} \end{aligned} $$ ...
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179 views

negative miller capacitance

I'm learning about the Miller capacitance effect when a capacitor is tied to both the output and input of an amplifier. This is specifically about learning to deal with the parasitic capacitances of a ...
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124 views

How to measure amplifier circuit loop gain

In the last few days I've tried to understand how to get the loop gain (Aol*ß) of my amplifier circuit. I've received several suggestion. I would like to hear your opinion on this matter. Are these 2 ...
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Wrong result for the emitter current

I've been given the following simple pnp transistor circuit where β=20 and Veb=0.7V and I'm trying to find the emitter current: I have the solutions but they don't agree with my results. My attempt:...
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108 views

Proving that the transistor in this simple schematic is off

I'm trying to find the collector, base and emitter currents of the transistor in the following simple topology: In order for the transistor to be on, the emitter needs to be at a higher potential ...
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BJT low frequency analysis

I have a little bit of confusion on how the textbook author is determining the low frequency cutoffs for the 3 high pass filter components of this circuit: For the capacitor Cs, he finds Zi which is ...

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