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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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PNP darlington pair calculations

I have a 20ohms discharge resistor connected to my PNP darlington pair. I have simulated the circuit in my mobile (every circuit app).please find the attachment for circuit. But here I'm unable to ...
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Reverse Saturation Current of BJT

This may be a weird question but for a BJT, we have \$I_S\$ is the reverse saturation current and \$I_C\$ is the collector current which flows through all three parts of the BJT. Now my question is,...
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Trying to plot input and output resistance for a single BJT in LTspice

I'm trying to plot a single NPN transistors's own input and output resistances in common emitter configuration looking into the base from input and emitter. I showed below with arrows that the plot I'...
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BJT Active mode - How does it work

I am finding it difficult to understand how exactly this works and would appreciate any help. From my understanding: The emitter base junction has to be in forward bias meaning that Vb is more ...
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DC Load line BJT

These are some lecture notes for my module, im pretty sure its wrong. Shouldn't the Q point be at Vcc/2? Not Vce(sat)+Vcc / 2? Even if it were to be half way between Vce(sat) and Vcc It would be Vcc-...
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(Lack of) Miller Effect in Cascode considering Common Emitter and Common Base Seperately

I'm currently working on an analysis of a standard Cascode circuit, in particular why the frequency response is so good as it reduces the miller effect. I have had a very vague explanation given to ...
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109 views

How to construct the DC equivalent circuit of a BJT amplifier

How would the DC equivalent circuit be constructed? I have a vague idea knowing that AC voltage sources are connected to the ground and capacitors are open circuited but what happens to the resistance ...
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57 views

Apparent contradiction in AC analysis of BJT amplifier

Suppose you have the circuit: For an AC analysis we can draw the following circuit: Now my job is to find \$v_{out}\$, which can be easily found by: $$ i_E=\frac{v_{in}}{R_E} \\ i_C=-\frac{v_{out}}{...
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98 views

Finding current through an LED in an amplifier using a BJT

Suppose you have the schematic: My job is to find \$I_{LED} \$ knowing that the impedance of the capacitor in negligible against \$V_s\$. Here's my attempt: I intend to do an AC analysis followed by ...
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67 views

Early effect bjt

What is the significance of the early effect? We increase the voltage at the collector, increase Ic and Ie increases. What im getting from this is that, there is a point where Ic = 0. So possibly ...
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111 views

How does this BJT affect the circuit? If it does, how should I simulate on Circuitlab to see the effect?

I am trying to understand how NPN Q23 in the picture below affects the operation of the left circuit. If it doesn't then what we get is the circuit on the right with the only transistor now being Q30....
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Confusion in setting the bias point of an amplifier

I have a textbook that details the designing process of a common emitter BJT amplifier. When designing the biasing point, the text says to set the bias collector current so that half of the supply ...
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106 views

Common collector - bias resistor?

[Original question: CCD output buffer; seemed best to make the transistor part a separate post] Taken from Wikipedia, the circuit in Fig. 1 acts as a common collector (high input impedance - low ...
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BJT configurations input

In all the configurations common collector, common base and common emitter, is the base always the input?
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70 views

BJT saturation region

What happens to the emitter electrons when a BJT is in saturation region? Do electrons move from the emitter to the collector because of the base being saturated with electrons? What are the ...
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90 views

Diode equation for BJT

For For a BJT why is the diode current Ic and not Ie? Ie = Ic + Ib The base current also passes through the B-e junction modelled diode
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Voltage Circuit Switch with BJT

I have Found One Circuit Which acts as Switch to Pass 15NV (15 Negative Voltage) to Opamps Negative (-VCC) Supply But I am unable to understand its working . Why Two NPN are used in this Circuit ? ...
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Why are both bias resistors considered parallel in a common emitter amplifier?

When calculating the input impedance of a voltage divider biased BJT common emitter amplifier, we calculate the parallel resistance of the bias resistors and the emitter resistance. But why do we ...
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BJT: Is \$ R_E || [\frac{R_s'}{\beta}+r_e] = (R_s'+ \beta r_e)|| \beta R_E\$?

I am trying to find the effect of \$ C_E \$ on voltage gain. Circuit under AC conditions: Doubt: Is \$ R_E || [\frac{R_s'}{\beta}+r_e] = (R_s'+ \beta r_e)|| \beta R_E\$ ? Reasoning: Small ...
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164 views

Driving MOSFET with 5V PWM for 12V fan speed control

I came up with this circuit to control a 12V 6A fan's speed, with a 5V PWM from another circuit. I'm not sure if it's going to work (I'm not very experienced nor knowledgeable in electronics) and I ...
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140 views

Transistor circuit driving me insane

Consider the following circuit: Now I'm asking to impose the base current as zero and $$R_C=452.35 \Omega $$ Now I have a lot of issues with this circuit because I can't find a systematic way to ...
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61 views

Alpha and Beta of a Transistor

So the parameter beta for a Common Emitter Amplifier is called the Current Gain and is defined as \$\beta =\dfrac{I_{collector}}{I_{Base}}\$ \$\alpha =\dfrac{I_{collector}}{I_{Emitter}}\$ What is ...
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How to select a diode for baker clamp?

What are the requirements when selecting a diode for baker clamp circuit? 1- for example, How the reverse leaking current of the diode affects the circuit? can a diode with relatively high reverse ...
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73 views

Driving MOSFET from MCU with BJT

1- What is the purpose of D1 in this circuit? 2- Can i connect the PWM output of MCU to Vi through a diode, to protect the MCU from 12V supply of the drive circuit? (e.g. if Vi got accidentally ...
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72 views

Where would Icbo flow through and would it cause thermal runaway?

I was reading this page about thermal run away. And it mentions: Due to increase in the temperature of collector junction, Ico will increase, Vbe will decrease. But for Ico to increase Ic and ...
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91 views

BJT as a switch: what am I doing wrong?

I'm studying the following circuit The image is explicit but as you can see the input measured in the upper terminal is a square wave with \$E_c\$ and \$-E_c\$ as levels. Now I also had no trouble ...
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319 views

Temperature coefficient MOSFET vs. temperature coefficient of BJT

If you use transistors for an amplifier scheme, you want to make sure that when you parallel devices, you don't start to conduct more current if your transistors heat up. I know that when using ...
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206 views

Voltage buffer in power supply

The output voltage from the NPN transistor (TIP142) is 4.7 volts with no load attached and only decreases as the load increases. However, voltage at its base (from an LM7805) remains a stable 5V. How ...
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60 views

Biasing Two BJTs with One Signal

simulate this circuit – Schematic created using CircuitLab I currently have a circuit with a 2N3904 BJT that acts as an on/off switch. I want to add a second BJT in parallel that toggles an LED ...
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How base and collector has nearly the same majority charge carrier concentration

Size: Collector > Emitter > Base Doping: Emitter > Base > Collector In the Function section of this Wikipedia page, they have said that Collector & Base have the same concentration of majority ...
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89 views

Output current in collector of BC107

I am working on following circuit involving BC107 NPN Philips TO-92 transistor and voltage source of 4.2 V. simulate this circuit – Schematic created using CircuitLab I got some confusion when ...
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120 views

Why is the square wave signal so distorted at the output of a push-pull pair?

I have this example circuit on my workbench: simulate this circuit – Schematic created using CircuitLab If I remove the RLOAD=1K resistor, then the output signal gets distorted. Yellow is ...
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Series BJT transistors with bases connected through a resistor

For an assignment I was given a problem which consists of two BJT transistors, a PNP and an NPN, in series with their bases tied together. simulate this circuit – Schematic created using ...
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Input Resistance and Mid-band Gain of a CE-CE Stage Cascade Amplifier

I am trying to calculate the Input Resistance and Mid-band Gain of a CE-CE stage. Here's what I got so far: I am having some doubts regarding my calculations though. Mainly with the input ...
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47 views

BJT saturation question

For the following question, how do we find \$I_B\$ and \$I_C\$? Would it be $$I_C = \frac{V_{in}-V_{BE}}{80k}$$ and $$I_B = \frac{12-0.12}{5k}$$
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Darlington pair. Purpose of components

Giving the following schematic: What is purpose of R1 and the diodes ?
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52 views

Bipolar transistor. Unknown graphs

I read a course about power transistors and I encountered these graphs The chapter is talking about safe operation of BJT. Can anyone tell me what those graphs are ? (And how to read them).
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How to amplify current and buffer voltage

I have a battery supplying 5V@500mAh and an LED (the load) which draws 1000mA@5V (twice the current the battery supplies per hour). I found some circuits that have NPN transistors only and other ...
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71 views

Questions About BJTs in My Circuit

I have an LED multiplexer circuit (inspired by https://www.instructables.com/id/Multiplexing-with-Arduino-Transistors-I-made/) in which I have set the resistor values to maximize the current through ...
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Determining the output resistance of a BJT amplifier

I'm given the following amplifier: (all the transistors have the same characteristics; Early effect is neglected) In order to calculate \$R_{out}\$, we replace the circuit with its small-signal ...
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53 views

2-stage BJT CC-EC amplifier [closed]

Using proteus I designed this amplifier: But I got this result: I don't know what is the problem in my design. I've been changing the values of the linking capacitors but didn't get any result.
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My mobile detector circuit won't work

I'm trying to get C3 to detect RF signals from a mobile phone. Amplify this signal to detect it using an LED. My main issue is simulating the antenna on multisim since multisim doesnt have an antenna!...
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56 views

BJT LT spice DC transfer characteristic

Why is it that the DC transfer characteristic isn't flattening out at -5 v, so it looks like a Z-shape? I did a DC sweep from -10 V to +10 V. The input wave is a 2V sine wave for the graph below:
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43 views

Plateau in collector-emitter voltage during BJT turn-off

I have the following circuit that uses a NPN transistor as a switch: I've built the circuit on a PCB, and while testing I noticed the following plateau in Vce (blue trace) when Vb transitions to zero ...
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33 views

How could i calculate the voltage induced in a detector circuit by picking up RF signals from a Mobile phone?

Is there any formula/equation i could use to estimate the voltage induced at the input from the RF signals?
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81 views

Which region is the transistor operating at?

PNP and NPN transistors in the circuit have identical Is values. Based on the PNP transistor is connected to a DC voltage source. k = 1.38064852×10-23 J/K qe = 1.60217662 ×10-19 C Is = 0.69 fA at 17°...
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74 views

Current sense amplifier

I came across this circuit for a high-side current sense using BJT current mirror, and I am trying to derive a formula for how I could calculate the sense voltage output (going to the FB pin) in terms ...
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137 views

Transistors why do they amplify current?

Ok guys I can't understand a thing of the Transistor here is the scheme: My teacher said that when I put 0,7V on the base the PN junction between Base and Collector is direct polarized so the whole ...
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154 views

Is the circuit in this answer wrong?

In the Q/A link below, the circuit in the answer seems to be wrong to me. Answer to "How to gradually light up an LED?" Circuit: simulate this circuit – Schematic created using CircuitLab In ...
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72 views

Dual power supply with BJT. How to do tracking without opamp?

I'm making a 0-±12V 20mA supply for breadboard. Circuit simulates fine with ltspice. How to implement tracking (or tracking ratio) WITHOUT opamp? I found a schematic here https://groenholdt.net/...