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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Small Signal BJT Model

I want to know why there is a minus sign in the output voltage.
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Transistor saturates no matter the size of base resistor

Problem I have a circuit as depicted below, and after calculating the base resistor, I wanted to see when the base current would be too little to saturate the transistor. I kept increasing the ...
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37 views

Need help deriving cutoff frequency of output (collector) RC network

According to the book I'm studying, the cutoff frequency for this circuit is given by $$f_c=\frac{1}{2\pi(R_C+R_L)C_3}.$$ I tried to derive it by solving for the frequency at which \$V_{out}\$ is ...
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4answers
266 views

Alternating between two LEDS using BJTs

I would like to control the output of two LEDs with a single pushbutton. When the button is open, D2 should be off and D3 should be on; when the button is closed, D2 should be on and D3 should be off. ...
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2answers
79 views

Why does this circuit use a transistor buffer even though there is an high input impedance op amp afterwards?

Regarding this circuit I found on www.geofex.com detailing the famous Tube Screamer guitar pedal I don't understand the purpose of the BJT input emitter follower transistor. If it was to increase ...
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1answer
73 views

Why won't these two BJTs connected in series work?

I cannot get the second BJT amplification of Experiment 11 of Charles Platt's Make: Electronics to work, and I'm all out of clues. This question is very similar, but not identical as my circuit works ...
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1answer
73 views

What is the meaning of biasing in electrical/electronics circuits? [duplicate]

What is the meaning of biasing in electrical/electronics circuits? What is the need for biasing in BJT/MOSFET? What will happen after biasing when we apply input signal(AC/DC)? Will biasing signal and ...
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3answers
89 views

Why does my output stage rectify when I use a preceding stage?

I've been working my darnedest over the past few weeks learning more and more about amplifiers, BJTs and various topologies. As I go through Art of Electronics, I try out everything mentioned in at ...
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1answer
74 views

Current source improves linearity on collector BJT

I'm learning about analog electronics. I'm looking into amplification stages at the moment. Question 1: Single BJT C-E Amplifier. Usually you put a resistor between collector and V++, so as \$I_b\$ ...
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106 views

Transistor working with unusual biasing

What would happen to a BJT if both junctions of the transistor are forward biased? Is there any literature on this? By literature I mean characteristics/equations. Any links would help. I have also ...
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469 views

Schottky diode like BJT symbol

In this schematic, what do the Schottky diode like "barbs" mean on some of the BJTs where the base is connected? I haven't encountered this symbol before.
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1answer
63 views

Analyzing Opamp circuits

I was asked to analyze this Op-amp circuit and I need a little bit of advice about it. I have presented my analysis about the circuit, kindly correct me wherever I am going wrong. simulate this ...
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4answers
343 views

Purpose of pullup resistor in BJT LED driver circuit

One answer to this question suggests the following circuit to drive a LED from an open collector output of a 74LS47: simulate this circuit – Schematic created using CircuitLab What is ...
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1answer
71 views

Where to find derivation of s'' (stability factor for beta) for BJTs?

I'm looking for the derivation of the formula for the stability factor (\$S''\$ or \$S(\beta)\$) of BJT bias circuits, particularly the ones with emitter resistance. The book I'm using says that the ...
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1answer
126 views

Noticed something weird in derivation of relationship between ICBO and ICEO in BJTs

So I'm currently studying Electronic Devices and Circuit Theory by Boylestad and Nashelsky (11th ed.) and I'm confused as to how the relation between \$I_{CEO}\$ and \$I_{CBO}\$ is derived. So the ...
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3answers
146 views

BJT switch vs amplifier mode

I'm getting really confused about a specific BJT circuit. I want to control LCD brightness and contrast as suggested here. Apprently a capacitor is used to filter PWM output to the contrast pin, ...
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2answers
73 views

Bipolar Junction Transistor Base-Emitter voltage

I've seen several datasheets for Bipolar Junction Transistors providing the value of the Base-Emitter voltage at saturation with values way above 0.6~0.7V (Example). What exactly does this entail? I ...
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2answers
213 views

Odd BJT circuit

This circuit came in my today's exam. It asked to find every base, emitter and source current.Base emitter voltage .8 and \$\beta\$ for both BJT is 49. first I tried normally where \$I_B\$ found ...
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1answer
102 views

Cascade BJT [ Exercise solve ]

How can I solve this circuit for \$\beta = 100 \$ , \$V_D = 0.7V \$ and \$V_{BE}= 0.7V\$ and arrows go to power \$V_{DD}=9V\$ simulate this circuit – Schematic created using CircuitLab ...
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5answers
187 views

Which parameters in datasheet give information about a BJT's step response quality?

I did a test with randomly chosen 12 NPN BJTs. I applied a clock pulse to each of them and observed their collector voltages. Some of them showed very low rise time, while some responded terribly ...
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1answer
58 views

Calculating Quiescent base current

I am given a question relating to a bipolar junction transistor. The information I am given in the question is shown below. I am first asked to select a quiescent point from the graph to find ...
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1answer
105 views

Something better than an H-bridge to drive motor?

Is there something better than an H-bridge to drive two motors using the ATmega8 with a 7V power supply? My teacher said the H-bridge overheats and is bad. Is there a better solution? For instance: ...
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3answers
177 views

Why is KCL giving me the wrong solution?

I'm trying to derive the gain of the following common base amplifier circuit: To do this, I drew the equivalent T-model circuit: . The problem I have is that I get two different transfer functions ...
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3answers
88 views

BJT amplifier gain rolloff

I'm playing around with BJT amplifiers and have some questions regarding the gain rolloff you see at higher frequencies, I'm using the 2N2222 for my tests and my simulations and measured results match ...
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2answers
110 views

How to calculate the required base resistor for this BJT-relay circuit?

I'm trying to design a relay circuit for my first PCB and have come up with the following: The 10K value for R7 is just a placeholder. R7 is what I'm trying to determine. The transistor should ...
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6answers
114 views

BJT Four resistor bias circuit analysis

I am currently studying BJTs but am having trouble analysing the four resistor bias circuit. My textbook gives the following circuit and says we can use voltage divider across R1 and R2 to find VB. ...
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4answers
612 views

FET vs bipolar junction transistor

I know this question might sound obvious, but still I cannot find an answer. What is the purpose and best-fit applications for field effect transistors? I mean, we have bipolar junction NPN ...
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36 views

Grading/Doping of pn junctions

I was studying about pn junction grading and junction capacitance and I came to look at step graded and linear graded junctions. I read that step grading in done for the base emitter junctions of a ...
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1answer
102 views

Driving a load with 1.8V input

I have a Gumstix expansion board and want to make it communicate with some 24V PLCs. I've read these topics for reference: 1.8V to 5V, still not comfortable with transistors Transistors: why are ...
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1answer
105 views

BJT Base Collector short circuit

When BJT's collector and base are shorted, then what happens for the circuit or may I say the change of BJT's performance in the circuit as there Vbc is zero. simulate this circuit – ...
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2answers
101 views

Need solution for this CB BJT circuit

How can I solve this circuit? simulate this circuit – Schematic created using CircuitLab This capacitor confuses me when I tried to do DC biasing. Also when I draw small signal ...
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2answers
177 views

When do which assumption for BJT

In my book I've seen many assumption done for to analyse BJT circuit. Like; Thumbs rule (when we assume Vce~ Vcc/3, Ic*Rc ~ Vcc/3) or assumed Ve is one tenth of supply voltage in another analysis ...
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2answers
134 views

How to suppress current spikes in transistor feedback loop

I am trying to pulse a load with a small constant current pulse (2mA @ 500us). The load does unfortunately have an unknown capacitance in the range 10nF - 1uF, parallel with an unknown resistance ...
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2answers
114 views

BJT cascading amplifier

Im trying to analyze this piece of circuit and the feedback between the second transistors emitter to the base of the first transistor is confusing me. Does anyone have any ideas how to analyze the ...
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115 views

BJT biasing in amplifier circuit

While reading the book I got stuck in a line, which says: For a given value of supply voltage \$V_{CC}\$, the higher the value we use for \$V_{BB}\$, the lower will be the sum of voltage across ...
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78 views

Can BJTs operate in the nano-ampere range?

Can BJTs can operate predictably with very low currents, in the order of nano-amps or less? I am involved in microelectronics design and have made a voltage reference chip that is MOSFET-only and ...
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3answers
71 views

bjt biasing and applying input signal

At the first fig. there is a basic common emitter amplifier circuit where signal source v_i is "superimposed" on the dc bias voltage V_BE. At the second fig. a biasing scheme (poor, nevertheless ...
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2answers
85 views

Which is better as a switch? [duplicate]

The offset voltage of a saturated transistor, makes bjt less attractive as a switch than mosfet. Why? Is there any other logics aside this?
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3answers
115 views

High-Gain Common Emitter Circuit

I am trying to design a high-gain common emitter circuit. I know that I need to use a bypass capacitor with emitter resistor to increase the gain. My problem is the formula A = Rc / re makes things ...
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1answer
150 views

Simple, Cheap, and Poor Quality Audio Amp [closed]

So I am attempting to build/design a audio amp. Basically around a 1 watt amp for a walkie-talkie. I'm planning to run it off a 9V battery. When thinking about a power amp several ideas came to mind. ...
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1answer
137 views

BJT base resistor

I read this post: Which resistor for NPN transistor base? But I still cannot understand the base resistor calculation. I need the transistor to be a switch to 5V 200mA relay. The base will get its ...
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2answers
178 views

Transistor to run relay

In addition to my last post I have more information and details. I want to run 240V Relay (5V Coil) from my AVR chip (ATmega328P). My EMR relay takes up to 200mA on it peak. I have 2N2222 ...
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2answers
82 views

Reduce effect of supply voltage on pulse stretcher

Considering the following simple pulse stretcher, how can I reduce the influence of supply voltage on the pulse length? simulate this circuit – Schematic created using CircuitLab I ...
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2answers
100 views

Transistor rating with capacitive load

Considering the following circuit, how do I reliably protect the transistor Q1 from exceeding the collector current rating: simulate this circuit – Schematic created using CircuitLab ...
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1answer
96 views

What does multiple emitters/collectors imply in a BJT schematics symbol?

I sometimes come across with BJT symbols from which multiple lines come out of emitter or collector. Are such BJTs different kind of devices? What does it mean in solid-state level?
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1answer
79 views

Improve recovery time of discrete bjt monostable multivibrator

How can you improve the recovery time of a monostable multivibrator build of discrete pnp BJT transistors? I can't use integrated circuits like 555 timer since I need to tolerate voltages up to 36V.
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2answers
54 views

VCE in pnp transistor

If VCE is negative in a PNP transistor does that mean that it is in saturation? Note: not given the saturation value. This is related to a homework question. Thanks
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3answers
195 views

BJT Class A Amplifier - Help calculating resistor values \$R_C\$ and \$R_E\$

I'm having difficulty working out required values of resistors \$R_C\$ and \$R_E\$ in the circuit below. Can't seem to find any formulas on the web. I know how to find \$R_1\$ and \$R_2\$: $$R_1 = ...
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1answer
107 views

Real-life discrete monoflop

I my current project I need a high-side PMOS to disconnect a load once a over-current or (downstream) over-voltage condition is detected. Obviously without a permanent latch, once the PMOS opens, a ...
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226 views

How to drive optocoupler with BJT properly [schematics]

Recently I asked a question about optocoupler: Shematics: optocoupler controls a transistor and doesn't work as expected I have one more question related to the topic. PWM input comes from the ...