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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Designing a common-emitter amplifier circuit

We are asked to design values for RC, RE, R1, and R2 for the following common-emitter BJT amplifier: It's easy to calculate RE and RC: But I couldn't find a ...
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Class AB amplifier becomes hotter, using more current and then becoming even hotter

In the circuit above a Class AB amplifier (this is one also called Class A+AB) can be seen with Q1 and Q3 used to get the current biasing voltage for Q2 and Q4. transistors Q2 and Q4 become hotter ...
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Alternating between two LEDS using BJTs and SPST Switch

I made a circuit based on this post, Alternating between two LEDS using BJTs. It seems to work in the simulation just fine but when I'm connecting it with real components, I get funny behavior. ...
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Identify this 2xNPN AC-coupled preamplifier circuit

My workmate has this nifty little EMI-detecting wand he built back in 2000. It senses EMI fields using an approx. 200 turn air-core coil, and converts them to audio signal. He uses it to detect e.g. ...
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66 views

Why hFE(DC gain) shown on multimeter isn't equal to the datasheed hFE?

I have a tip122 and discovered my multimeter uses Vce=2,8V DC and Ib=10uA for its hFE test. When I insert the transistor on the terminals for hFE test, the gain on the multimeter alternates between 1 ...
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46 views

Negative base current in Joule Thief

simulate this circuit – Schematic created using CircuitLab My question is regarding the Joule Tief shown below... The graph is from an LTSpice simulation...It seems from the graph that ...
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40 views

Calculating bias resistor with two BJTs

I'm trying to solve for R1 given the following circuit, given Vin=3.3V: Both Q1 and Q2 are ON MMBT3904L, but the datasheet specs are confusing to me. Do I assume the current through R2 is 200mA ...
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Alpha/Beta Parameters of BJT

The BJT Current equations are given as :- IE = IC + IB IC = \$\alpha\$ IE IC = \$\beta\$ IB Does the relations with \$\alpha\$ and \$\beta\$ parameters include the reverse saturation current ...
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What could be the target applications for these BJTs?

NXP recently introduced a new family of BJTs with switching capabilities of up too 100V, 10A , see "The toughest bipolar transistor - 100 V in LFPAK/Power-SO8". I cannot think of a switching ...
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34 views

What are the benefits of this Class A followed by Class AB instead of just a Class AB amplifier?

In figure 1, there's are two BJTs at the left side in Class A amplifier configuration followed by two BJTs in Class AB amplifier configuration. The two transistors that are added at the left side ...
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23 views

Achieving higher current at the lowside MOSFET to pulse LED

I have attached the following circuits which i have been developing overtime. The OBJECTIVE is to get as high as possible a current at the gate of the MOSFET (VF1) to achieve faster pulse times of ...
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Intuitive explanation for this hysteresis circuit?

simulate this circuit – Schematic created using CircuitLab I am trying to get an intuitive understanding of this small circuit. I have both breadboarded it to look at behavior on a scope ...
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A question about Transistor in forward-active biasing?

I have question about transistor biasing. Between picture 1 and 2, what's difference, why we add the resistor RE in pic 2 ? what's it effect to circuit ? Similarity for pics 3 and 4, instead of ...
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83 views

Why are BJT operating modes named the way they are?

I know that a Bipolar Junction Transistor has four operating modes dictated by the base-collector junction and the base emitter junction's biasing: What I can't seem to find is the reasoning behind ...
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20 views

How to use the “Switching characteristics” parameters in the BJT's datasheet

In Fairchild's MMBT4401's datasheet (Note: This is not the newest one from official site, but it seems it contain more information than the newest one, so I use it as an alternative), there are some ...
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Current mirrors as active loads in BJT differential amplifier, what does that mean?

I have seen current mirrors being used as current source in differential stage of an amplifier design. The reason given is that the current mirror has a very high output impedance and behaves like an ...
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32 views

H Bridge Voltage Drop w/ Motor Installed

first post after searching the net/ site for quite some time. I have this H-Bridge circuit http://www.circuitstoday.com/h-bridge-motor-driver-circuit I'm using 3 AA batteries to test this BEFORE ...
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BJT - absolute maximum base current?

Let's look at the datasheet for an MMBT3904, just for example. The absolute maximum section talks mostly about maximum voltage differences, and a single current limit - the collector current. I'm ...
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Why are MOSFETs better than BJTs in application of Voltage Controlled Resistor?

Why are MOSFETs better than BJTs in application of Voltage Controlled Resistor ? Thanks for your responses.
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BJT VCVS with high gain

In a lecture I learnt a BJT VCVS which has gain = 1, i.e a voltage buffer as shown in the figure. The speaker said that it was the ideal VCVS model of a BJT. My question is that is it possible for ...
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87 views

what does it mean for a to BJT operating at the edge between the active and saturation modes?

ive recently been fascinated with diodes and was wondering how i'd design a circuit that works at the edge of the breakpoint between the active and saturation mode. How would the circuit be analysed?
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BJT Amplifier in AC mode

In the lesson on BJT Amplifiers, we were told the we can find the AC equivalent circuit by replacing all capacitors by short circuits, inductors (if any) by open circuits, dc voltage sources by ground ...
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63 views

FET and BJT transistor question

We have the circuit in the figure BJT transistor has the parameters β=80 and the JFET transistor has the parameters Vp=-4 Volt and Idss=8 mA; Vgs=-2.6 Volt and Ids=1 mA We have R1=470 Kohm ,Rc=3.6 ...
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70 views

What does this symbol represent?

I am trying to understand the internal circuit schematic of the LM565 phase locked loop IC, on page 6 of this datasheet: PDF Datasheet for LM565 from TI website In the upper middle, there is a ...
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47 views

How to remove degradation in voltage gain when replacing the original \$R_L\$ with a small value resistor?

For the following common-emitter amplifier, the original \$R_L = 4.7\space kΩ\$. When I replace it with a small value resistor (\$100\spaceΩ\$), the voltage gain will drop. Is there any way to drive ...
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Bridging Discrete (BJT) Amp Channels

For my Electronics II class, I designed a discrete BJT amplifier circuit including a VGain, Buffer, and Power stage. I succeeded in meeting the specs, and the next step is to improve our design in ...
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BJT in reverse active mode [duplicate]

I was wondering, what are current directions in reverse active mode? I know that in this mode gain Beta is much smaller than Beta in forward active mode and for BJT to work in thise mode it must be ...
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84 views

BJT in Saturation

Is it possible for the base current to exceed the collector current for a BJT in saturation mode? (If the base current was increased without the collector current increasing at all)
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94 views

why do 2N2222 and PN2222 avalanche differently

i've been experimenting with a 2N2222 transistor (in the TO-18 metal case) and using it as an avalanche transistor -- by not connecting the base and having the emitter and collector connected in ...
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84 views

BJT Switch with a Diode from Base to Emitter

In this circuit, the diode is supposed to force a higher input voltage to turn the LED on, however when I created it on a breadboard I measured about the same input voltage (without the diode) to turn ...
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2k views

Basic transistor questions

I've created the circuit shown. I'm using a 9V battery (actually throwing out 9.53V) and 5V coming from an Arduino to test with both 9 and 5 volts. The transistor is a BC 548B (datasheet I'm using is ...
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67 views

Complex circuit analysis

I need help solving the circuit above. I don't understand how to analyse the op amp using the small signal model in order to calculate the Vout/V3, the input resistance (Rin) and output resistance ...
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92 views

Output Impedance of BJT/Mosfet amplifiers

I have heard in lectures that the desired output impedance for the BJT/Mosfet Amplifiers is LOW. But these transistors are Transconductance amplifiers 'by birth'. I mean they give output current ...
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80 views

Square wave into emitter follower

I put a square wave of relatively low rise and fall time (of about 10ns), and wanted to see the response from an emitter follower output. I put together the circuit and here's what I see on scope. ...
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62 views

What are the limitations of the Ebers-Moll model of the BJT

The Ebers-Moll model is considered simple yet effective model of the BJT. What makes it special and when does it break down i.e is not applicable?
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180 views

Solar controller with BJT transistor and PIC24 microcontroller to charge 3.6V NiMH batteries

I'm designing a solar controller circuit and am having some confusion about how to switch the BJT on and off. I have a single solar panel charging a battery. The BJT is there to prevent over-charging ...
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79 views

In logical progression, what things does a person need to know to design a robust opamp from BJTs

Opamps contain things like current mirrors, differential stage, push-pull pair and much more. Once a person starts from learning that BJT has 3 terminals for which Ic≈Ie≈βIb, what topics must a person ...
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48 views

Difference Between The Transisor Denotations V(th) and V(tn)

I have seen these values V(th) and V(tn) used interchangeably as the transistor 'threshold voltage' but have also seen them used together on the same diagram seemingly representing differnt things. (I ...
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169 views

BJT Driving an LED - Above or Below?

I will freely admit that I am mainly digital when it comes to electronics, so may help explain my confusion (that or just general incompetence). I am getting review feedback on a design, and one ...
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199 views

How does the negative feedback in this oscillator work?

EDIT: I don't think I'm using the term "negative feedback" correctly. What I meant is, how does the feedback that makes the closed loop gain equal to 1 work? I'm trying to understand the very basics ...
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42 views

Selectively outputting 2 inputs based on a third input

I currently have 2 inputs that can be on or off, they could be 00, 01, 10, or 11. I want to output them only if a third input is active. I figured BJT transistors would do the job, but they seem ...
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115 views

Bipolar transistor amplification biased

This exercise is about a BJT transistor common emitter circuit, with a biasing resistor between the base and the collector. I am supposed to compute the signal amplification of the circuit \$V_o/V_s\$ ...
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115 views

Why does the colllector current decrese with decreasing collector-emitter voltage in this circuit?

I'm trying to understand how the circuit below works so I ran a simulation with TINA and I got the curves shown on the right (C3 is already charged). Here is a bigger one Considering the ...
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54 views

BJT driven LED lengthy fall time

I am designing a circuit for a strobe amplifier and am having some difficulty with the LED emitting light for longer than expected. I am measuring the light out of the LED with a photo receptor ...
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107 views

Analyzing a BJT circuit

Here is the example with its solution: The problem is that when I apply KVL to the circuit (the second circuit) from the 10V source to the ground at the bottom, I get wrong values. Here is what I ...
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106 views

How to avoid voltage drop on undervoltage protection circuit?

I'm having trouble with a circuit I designed and I'm hoping someone here may help me fix it. I've successfully used the circuit below a few times. It was proposed by Russel McMahon in this answer. It ...
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Figuring/setting BJT mode of operation

I am trying to make an IR transmitter, and have the following set up: simulate this circuit – Schematic created using CircuitLab I was thinking of this circuit, because I want to run ...
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86 views

How collector-base junction in BJT works

I have read some answers about how BJTs work, but I'm still not convinced. To show my doubts better, suppose we extract base and collector from BJT NPN transistor with all its construction details ...
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Instantly Hot Transistor

I have a TIP42G PNP transistor, and when I used it, it instantly smoked. Would it be because I shorted the base directly to ground? (through a button for the initial turn on, and through a 2n2222 for ...
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Parallel Transistors in Constant Current Load

I'm designing a constant-current dummy load for testing my power supplies. This load should be able to handle a maximum of \$2\mathrm{A}\$ at \$24\mathrm{V}\$. My current design, so far, is loosely ...