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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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Does PNP collector current influence base current?

I'm trying to understand the classic "ideal power diode" circuit. An example (taken from the Raspberry Pi) was in this question. My question concerns the left-hand PNP transistor. The function of it'...
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Finding Width of MOSFET

We are struggling with part iv of the question above where we have to find the width of both MOSFETS. As you can see from our working out, we have found a width of 2 meters for the nMOS, which we know ...
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What are the differences between the NTE102/103 and NTE102a/103a Germanium transistors?

SO the NTE102/103(a) complementary transistors are a bit more compact(both are cans but the a ones are smaller and thinner). But I've noticed that in particular, the NTE102 is much more prone to ...
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73 views

PNP Transistor with low base current

I designed a PCB with the following schematics. simulate this circuit – Schematic created using CircuitLab The circuit uses a MAX6369 external watchdog timer which has an open-drain output(...
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157 views

high side MOSFET switch and BJT interface with MCU GPIO

im operating a 28V contactor using a GPIO from my MCU, i have a 28V supply readily available. im looking to surpass the gate threshold of the MOSFET by introducing a NPN BJT with a 10V supply, with ...
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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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27 views

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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81 views

(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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55 views

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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420 views

MOSFET Amplifier: Maximum Signal Swing without Clipping

I have a degenerated common source amplifier. I want to know what's the maximum input signal I can apply before the output signal begins to clip. What I currently do is to increase the input signal ...
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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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54 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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91 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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53 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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162 views

How to find collector resistor value in bjt differential amplifier

What value R2 may have in that schema? Anyway it will be loaded by pn-junction of Q5 and will have fixed voltage drop near 0.65V. What value R2 must have if the current source is used instead R1? The ...
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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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169 views

Rig a BJT to drive a fan

I wanted to see if i could use a BJT to power a small pc fan. I have a potentiometer into the base but how would I wire the fan so that depending on the base current it would adjust the current to the ...
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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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BJT Voltage Divider Bias Circuit problem

Let's say we have the following circuit: The known values are: \$\beta=100\$, \$Vcc=2.5V\$, \$V_A=\infty\$ (meaning the Early effect is not taken into account), \$V_{BE}=0.7V\$, \$I_S=8\cdot10^{-16}A\...
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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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69 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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58 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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275 views

Why is the common emitter input insensitive to base width modulation?

According to Robert F. Pierret (1996) in page 411 of the 2nd edition of the textbook Semiconductor Device Fundamentals, common emitter input of PNP BJTs is insensitive to base width modulation. Isn't ...
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69 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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34 views

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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Depletion region BJT

For the above image, why do the holes move towards the negative terminal of the battery when the diode is in reverse bias? And why dont the free electrons just move through the wire to the negative ...
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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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87 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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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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110 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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37 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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62 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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51 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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480 views

BJT or Mosfet as Cascode

I was going through this lecture pdf. If I want to have maximum output resistance which cascode configuration I should use. BJT cascode or Mosfet cascode?
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88 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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339 views

Output Impedance of a Wilson Current Mirror

Is the ideal output impedance of a Wilson Current Mirror 0? Also, in the formula for the actual output impedance of Wilson Current Formula (i.e. R_out = [B/2]* r_o), what does r_o stand for? Also, ...
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179 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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249 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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59 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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345 views

TTL gate function

Can you explain the logic function of this TTL gate? I think it is $$ Y = \overline{AB+CD} \ $$ but I am not sure. If $$ V_1=5V \\ V_2=0V $$ does the transistor Q1 work in reverse-active mode?
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264 views

NPN Transistor Parameter Determination Gain Question

In the interest of making an oscillator, I made the above circuit after reading about gain, current flow, active region, and how an NPN transistor works. I then put the circuit in LTSpice and note ...
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1answer
72 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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99 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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267 views

LC Oscillator Gain Compression

I am designing an LC Colpits common base Oscillator for the FM broadcast band. The project is going to be extremely low radiated power as I will only be using it in my car. I am currently using this ...
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32k views

ICEO, ICBO physical interpretation in BJT

I am stuck with the equation: ICEO = (1+β) ICBO My questions are: What is the physical interpretation of this equation? Will both the currents exist simultaneously? Are any of the reverse currents ...
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453 views

Increasing the compliance voltage range for a variable, two-quadrant constant current pin-driver circuit

The following is for hobbyist work and I have no commercial intentions at all. Only a handful (two?) will be built. (I use these for part testing and curve generation, though with the higher voltage ...