What is Bjt: Definition and 216 Discussions

A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching.
BJTs use two junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material. The junctions can be made in several different ways, such as changing the doping of the semiconductor material as it is grown, by depositing metal pellets to form alloy junctions, or by such methods as diffusion of n-type and p-type doping substances into the crystal. The superior predictability and performance of junction transistors soon displaced the original point-contact transistor. Diffused transistors, along with other components, are elements of integrated circuits for analog and digital functions. Hundreds of bipolar junction transistors can be made in one circuit at very low cost.
Bipolar transistor integrated circuits were the main active devices of a generation of mainframe and mini computers, but most computer systems now use integrated circuits relying on field effect transistors. Bipolar transistors are still used for amplification of signals, switching, and in digital circuits. Specialized types are used for high voltage switches, for radio-frequency amplifiers, or for switching heavy currents.

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  1. ZoeDale

    Engineering Basic circuit problem about BJT and buzzer

    I don’t understand what does the v_t mean?
  2. A

    Engineering Why are the voltages calculated in this way? (CE BJT Amp circuit)

    Hello! I am having trouble with this emitter circuit. I am given U0 as well as all of the resistor values and I need to calculate the following. ## I_c## ##U_{CE} ## ## U_{C} ## ## U_{a} ## ##U_{Ca}## ##U _{Re} ## Now important to note that all of these need to be calculated at the working...
  3. A

    Engineering Small Signal Input Resistance of a BJT amplifier

    The diagram of the BJT amplifier small signal model is below. This is not really a homework question as I am self-learning electronics, but I was advised it may be better placed here than in the technical portion of the forum. Thus I do not have an "attempt" at a solution (which is given as RI...
  4. Y

    Funny observation in matching BJT power transistors

    THIS IS NOT HOMEWORK I design audiophile power amps as hobby. I need to match power transistors. It is A LOT HARDER than I thought. I built a test fixture to hold up to 50 transistors at a time. I try to match sets of 13 to about 1mV which is very tight as Vbe change is about -2mV/deg C. I have...
  5. D

    Engineering Solve BJT Transistor Q Point Using Thevenin Theorem

    Hello, I need help with this bjt transistor and finding the Q point. The problem I have is that I don't know how to do the thevenin theorem on this one. If someone could help me and show me how its done that would be great. Thank you! The transistor looks like this on the picture: This is my...
  6. altruan23

    BJT emitter circuit weird equations?

    Summary:: So apparently the equations schould be correct but I am not sure about it. Isnt R1 = Ub(10V) - Base voltage / Iq + Ib --> Ub is the source voltage 10V. And schouldnt R2 be :base voltage / Iq?
  7. anubhavsingh

    Reverse saturation current and leakage current in a BJT

    I have been studying Studying Donald Neamen's Electronic Circuits 4th edition and Sedra Smith's Microelectronics 7th edition for quite some time now. I have been self studying. After searching extensively on Internet as well as trying to understand from the books, I have not been able to...
  8. I

    Input Resistance of this BJT Amplifier

    Hi i got a question ,how do i calculate the input resistance ,of the RS resistor ,so i would get half of the voltage of the voltage source (500mA) .I've created the circuit in ltspice and found out that its around 95K ohms to get 250mA,but how would i calculate it . Ive also attached my...
  9. Helena Wells

    BJT common collector Colpitts oscillator

    This is a BJT common collector colpitts oscillator.I have found how to find the feedback fraction from this site:http://fourier.eng.hmc.edu/e84/lectures/ch4/node12.html but I have searched for hours and haven't found the amplifier gain of this circuit. I have found the amplifier gain for...
  10. Helena Wells

    Base-collector current in BJT Saturation

    I can't find the formula for the current flowing from the base to the collector in saturation mode of BJT.Any help?There are many references to the emitter current in the forward active mode but none for the base-collector current in saturation mode.Help appreciated.
  11. Helena Wells

    Understanding BJT Common Collector Circuit: Voltage and Saturation Explanation

    Assume we have a common collector circuit with a emitter resistance of 1k Ohms.Vc = 10V Ve is connected to ground. Now we connect the base of the transistor to 5V . The base current will be 5-VBE/1000 Amp.Let's say a voltage drop of 0.7 in the BE junction .We will get Ib = 4.3mA. Ic = bxIb =...
  12. Helena Wells

    Why does a transistor in saturation act like a short circuit?

    If I have an NPN transistor and let's say we set the base voltage higher than the collector voltage.(Emitter is connected to GND).There are 2 currents flowing in the base because we have two forward biased junctions inside the diode , 1 is the current flowing from emitter to base and 1 is the...
  13. J

    Understanding the base emitter voltage in a common emitter NPN BJT

    ##V_{BE}## is basically the difference in voltage between the base terminal and the emitter terminal. Normally when silicon is used and the transistor is biased to operate in the active region, ##V_{BE}## = 0.7 V approximately. The way I understand this is that for an npn BJT, the applied...
  14. Boltzman Oscillation

    Engineering How does this Bipolar RF Amplifier + Output Transformer circuit work?

    I am trying to create a receiver for a personal project I am working on. This is the RF amplifier that I was given by a book I am following. How does this circuit work? Usually for a BJT amplifier I find the DC bias point and then use the small signal model after I bias the circuit? I understand...
  15. Boltzman Oscillation

    Engineering Help finding this node voltage

    I am given that all the BJTs are the same. First of all, when finding node voltages like Vref should I make the node voltage a voltage source first and then do circuit analysis? If so then I would use KVL but that does not give me Vref = 2Vbe. I do know that Vref = Vb1 = Vb2. What am I doing...
  16. A

    Engineering BJT circuit with cascaded stages

    This cascade is divided into 3 sections thus: the leftmost section is an emitter follower : Ib1 = (15V-0.7V)/ ( R1 + 151R2) IE1= 151*Ib1 re1= 26mV /(IE1) Zb1= 150( re1+ R2) AV1= (-150*R1)/ (R1+Zb1) the center section is an unbiassed BJT : Ib2 = (15V-0.7V)/ ( R3 + 151(R4+Re2a) ) IE2=...
  17. F

    BJT Transistor Collector Current

    I’ve attached an image below of a section from my textbook and some circuit simulations. The textbook gives an equation for the collector current in a bjt transistor. The simulation shows two circuits (with the same bjt transistor) that hav equal values for all variables present in the textbook...
  18. Y

    Looking for a BJT with low collector leakage current

    I am looking for small signal BJT that has very low leakage current when the base is biased at about 0.2V. I have issue of transistors drawing collector current when the base is at about 0.25V that is way less than the normal turn on of 0.6V to 0.7V. I am currently using BC546 or 556, and KSA992...
  19. Peter Alexander

    Engineering Computing resistance in given circuit with BJT

    Hi, before you proceed with reading this question, I would like you to know what I do not expect anyone to solve this task for me. I have a problem with a single step in the solution and I'm only asking you to help me with this one step. 1. Homework Statement Compute ##R_B## so that Q-point of...
  20. R

    How to calculate BJT electron flow

    Hello How should i calculate number of electrons passing through it when we apply some gate voltage across it
  21. G

    Engineering Determine the input impedances of a circuit

    Homework Statement I'm studying a circuit with BJT's and I'm asked to determine the input resistances of the two amplification steps of the circuit. The circuit I'm analyzing is the following one: Homework Equations 3. The Attempt at a Solution [/B] To determine the input resistances I...
  22. J

    Solving BJT Base Current Homework Problem

    Homework Statement [/B]Homework Equations For 1st I think answer is B For second I(c) = B I(b) B is beta The Attempt at a Solution For 1st I think it's B But book says answer is D Is book wrong? For second I'm getting: I(c) = 3mA And beta = 200 So I(c) = B I(b) 3mA = 200 I(b) This gives I(b)...
  23. Jayalk97

    Difference between h-parameters and hybrid pi?

    Hey guys, I'm confused as to the difference between the h-parameters usually given by datasheets and their hybrid-pi model analogues. For instance, I know hfe = Beta in small signal analysis, but what about the other h-parameters. Thanks for any help with clarifying this.
  24. S

    Straight line plot of bipolar transistor - temperature?

    Hey everyone, I'm trying to plot a straight line for a bipolar junction transistor to find the room temperature, T, using my experimental results for the associated base-emitter voltage, ##{V}_{BE}## and collector current, ##I_C##. Here's the equation that I'm using: $$ I_C = α_F {I}_{EO}...
  25. D

    How Do You Solve a Basic BJT Problem Using KVL?

    Homework Statement Homework Equations KVL The Attempt at a Solution Using KVL around BE loop 0-0.6-i_e*(30*10^3)-15=0 i_e=-0.52mA However, I stuck when I tried to find i_c and use KVL around CE loop/ CB loop For CE loop: 10-i_c*(15*10^3)-Vce-i_e*(30*10^3)-15=0 For CB loop...
  26. M

    Capacitors in Hybrid-pi Model and Frequency Response

    This is what I have and I was wondering where C2 might go if I make apply the hybrid pi model on this circuit. I know that all the DC Voltages should be shorted if I were to make use of an equivalent circuit, so that in the hybrid pi model R1 should be in the input and R3 in the output. But what...
  27. Pushoam

    Input characteristics of a BJT in CB mode

    Homework Statement Does this mean that the forward bias p-n junction starts behaving as a conductor at very small input voltage i.e. of the order of 50 mV and it decreases as VCB increases? The two circuits i.e. forward bias circuit and reverse bias circuit are independent. Then why should...
  28. Alkozy

    Two-Stage BJT Amplifier Homework: Solving for Input Resistance and Voltage Gain

    Homework Statement see pic in the attachment beta = 100 Homework Equations Rinput=Rth Vinput=VTH iB= ic/beta IE=ic /alpha iB=iE/(beta+1) gm=IC/VT Avo=-gmRC Av=-gm(RC||RL) Gv=-Beta[(RC||RL)/Rsig+(beta+1)re re=alpha/gm The Attempt at a Solution solving each stage separately. First DC analysis...
  29. Faisal Moshiur

    Calculate Ohmic Resistance of Bipolar Junction Transistor

    How am I supposed to calculate ohmic resistance from a non-linear electronic device component? Say, for example, I need to calculate ohmic resistance of a pn junction of bipolar junction transistor from its forward I-V characteristics curves. (It can be either Base-Emitter junction or...
  30. V

    PNP Transistor voltage calculation

    For the B) part, I don't get why is it that for calculating voltage between collector an the emitter (Vce) I take into account diode on base (0.7V), whereas for part A) it's ignored? Why is Vce in part A and part B are calculated differently? Why is it that in part A voltage of 0.7V isn't taken...
  31. Cocoleia

    BJT and MOSFET complete analysis

    Homework Statement I have been given the world's longest transistor problem as an assignment :wink: Here is the circuit: I am asked to find: a) V1, V2 and V3 using DC analysis b) AC equivalent circuit c) AC tension gain: Ava=Vx/Vsig d) AC tension gain: Avb=Vo/Vx e) Total AC tension gain...
  32. adamaero

    Engineering Analog electronics (BJT circuit analysis)

    Homework Statement For beta = 100 and assuming ro is very large, what is the DC voltage at the collector? Homework Equations IB = IE(β+1) IC = β*IB The Attempt at a Solution Since IE = 0.5mA, IB = 0.0505 A Then, IC = 5.05 A (which doesn't feel right) VC is the same potential across the 30kΩ...
  33. E

    BJT Analysis Question: Solving for Vb with Voltage Divider Method

    Homework Statement Homework Equations KVL The Attempt at a Solution This is the solution given: VB = (VCC*30k)/(70k+30k) = 3V Why is a voltage divider being used to find Vb?
  34. A

    Engineering Help with pnp BJT amplifier circuit

    {Moderator note: moved from technical forums, so no template} Hi, My mid-term quiz is next week. I have tried to solve this question but I'm stuck on Qn 4 Part H. Any hints?? Image for Qn 4: http://imgur.com/gb9jEfW My Answers for QN 4: http://imgur.com/a/fS3GM Can you help me check my...
  35. thegreengineer

    BJT transistor analysis: common emitter confusion

    Recently I started studying semiconductors and analogue electronics. First I studied the diode and its I-V characteristics and analysis in DC circuits, as well as in AC circuits. Now I started with BJT transistors. I was explained that transistors act as electronic switches. My problem is not...
  36. 1rel

    NPN BJT transistor: base voltage / emitter current

    I don't know how to calculate the voltages at the bases of every transistor shown in the lower row of that picture (~3V (2.9..V), ~2V, ~1.8V). Any help would be appreciated! - I'm currently trying to figure that out... The other thing I don't really understand: Why do the transistors in the...
  37. Weightofananvil

    JFET, Mosfet, BJT Design Question

    Hi, In my electronics class we are learning BJT, Mosfet, JFET theory. My gripe at the moment is that when teaching this you are taught how to troubleshoot and solve for circuits already made. This means you are always solving for various voltages, currents, Gm etc etc. With no previous...
  38. T

    What Factors Determine the Performance of a BJT Differential Amplifier?

    Homework Statement Analyse the circuit to provide predictions for (a) the differential input resistance, (b) differential voltage gain, (c) single-ended voltage gain, (d) common-mode voltage gain, (e) common-mode rejection ratio (CMRR) in dB, (f) common-mode input resistance and (g) the...
  39. dwn

    BJT Switch - Circuit Simulation

    I am reading The Art Of Electronics, and I realize this is more of an encyclopedia rather than a textbook. I have referenced other sources as this has mainly served as a compass for me to learn about each subject area. My question is fairly simple and straightforward. I'm using multisim to...
  40. U

    Audio Amplifier Using TIP 147, 142

    << Obnoxious all caps post fixed by Moderators >> I have made an audio amplifier using an LM386 IC, but it does not amplify any sound from the microphone. Please help me to improve this using a Darlington pair or any other power transistor like the TIP147/142...
  41. I

    What are common issues when using 2N2222 in a BJT NPN amplifier?

    Hello people, I am working on a BJT NPN amplifier the model that I've choosen is 2N2222 : http://www.e-voron.dp.ua/files/pdf/tranzistor/2N2222.pdf I am having 3 problems here: 1) am not having any AC voltage at output Vce seems like I have set wrong operating point? 2) I am trying to figure...
  42. U

    Need help in making my Electronics Project using BJT....

    I need help in making my project using Bipolar junction transistor. I'm Engineering student (Ist year). Can someone here suggest me a Funtastic project that i can make easily and impress my teachers as well using BJT transistor. I have rea the Cascade, cascode and Darlington technique to amplify...
  43. ToonBlue

    Engineering How does BJT work in this circuit?

    Homework Statement This is not a homework question. I am trying to learn how BJT work as a switch so I come out with circuit to understand how BJT functions as a switch. I use mulitsim by the way to try out these circuit 2. Relevant diagram The Attempt at a Solution [/B] I understand that...
  44. K

    Understanding KVL Loop in BJT Biasing and Calculating Ve: A Guide

    Please help how Ve is calculated what i think is that it should be: -Ve=-Vee+IeRe; How Ve is taken positive please someone draw equivalent circuit for same
  45. mooncrater

    Collector Current: Majority & Minority Carriers

    Homework Statement In my book ,I found this line which says ,"The collector current comprises two components-the majority and the minority carriers .The minority-current component is called the leakage current and is given the symbol ##I_{CO}## .The collector current, therefore is determined in...
  46. STEMucator

    BJT Current Mirror Homework: Derive Eqn & Simplify w/ Assumptions

    Homework Statement 1. Given the above circuit, derive an exact equation for ##I_{out}## in terms of ##I_{REF}, V_{BE1}, V_{BE2}, \beta_1, \beta_2, R_{E1}, R_{E2}## and ##V_{CC}##. Do not make any assumptions. 2. Simplify the equation from part 1 by assuming ##V_{BE1} = V_{BE2}## and that...
  47. STEMucator

    Multiple BJT Analysis: Homework Statement

    Homework Statement Hi everyone, this is not homework, but I can't seem to see something. I have a multiple BJT circuit I've drawn, and I want to analyze it: Homework EquationsThe Attempt at a Solution Assume the transistor ##\beta##s are equal for both transistors and are known. Assume...
  48. A

    BJT hybrid pi model,why "always" ro resistor is shown?

    In BJT hybrid pi small signal model, we have a resistor "ro" between Collector and Emitter. This resistor is to include the change in collector current when there is a small signal voltage change between collector and emitter. (due to early effect) I am finding that all BJT hybrid small...
  49. P

    Why would cascode amp oscillate?

    I'm having the worst time ever with a two stage cascode bjt amp. See attached circuit image. In the circuit photo the "In R: 168ohm" is the amps input resistance according to LtSpice. I usually have luck with LtSpice. The circuit shows real measured resistor values. I built a LtSpice version...
  50. GGGboat

    Engineering BJT analog circuit design

    Homework Statement hello, i need to design analog circuit that has a transfer curve as seen in the following image using resistors, capacitors,voltage, and BJT (with beta value of 100). the parameters are: A=75 f1= 21Hz f2=21 KHz M1=40 M2=-20 the circuit also need to have input resistance of...