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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Base resistor too low problematic?

I have recently designed and ordered some PCBs which can combine a horizontal and a vertical video sync signal into one composite sync signal (all are TTL signals). This is done using a BJT circuit ...
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Designing a common emitter audio pre-amplifier

I am trying to design and simulate a common emitter audio pre-amplifier using 2N2219 with characteristics based on industry standards (audio frequency response, etc. Corner frequency must be less than ...
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Transistor Switch Output Current Increasing

I'm using a BJT switch to send a 48 V, 2.5 mA signal (thanks for all the help in the past!). I've been tuning the resistance value I need to output 2.5 mA and noticed that the output current is ...
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NPN transistor on optocoupler ground catching on fire

I was experimenting with ways to cut power to my board and so I added an NPN transistor on the low side of an optocoupler (Q202.) I noticed my voltage regulator was oscillating from 3.3V to GND every ...
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How can I find my operating point in datasheet with i-v-characteristic of bjt?

I have this datasheet of the BC546 BJT: I need to find the exit-conductivity at the operating point of 60mA and 4V, which I marked as a blue point in the i-v-characteristic. But no curve crosses ...
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Graphing modes of BJT

Suppose we have this Ic vs Vce graph of a BJT transistor I have been taught this graph is applicable for this BJT transistor configuration. simulate this circuit – Schematic created using ...
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Output reflection coefficient of a power amplifier

I'm struggling to understand the implication of a poor large signal reflection coefficient at the output of a power amplifier. I designed a power amplifier and checked the large signal S22 (for the ...
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Where are the transistor number 18 and 21 in op amp 741 according to Wikipedia?

I was trying to implement this op amp from Wikipedia in Proteus : Like this: But I could not find Q number 18 and 21. What is the reason that there are not Q number 18 and 21? Or if this image is ...
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In this formula, where does T come from to the power of 3 divided by 2?

I read the tutorial that says Also, as T → 0, so do T^3/2 but i did not understand this argument. In this formula, where does T come from to the power of 3 divided ...
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Why is this transistor in my bootstrap driver failing?

I'm trying to implement a bootstrap switch circuit for an N-channel high side switch. I implemented the following circuit on a breadboard: Testing it with a 24V power supply and a grounded input ...
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2 votes
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How does the common emitter configuration offer higher output impedance?

I know that the base-collector region is reverse biased and because of that it offers high resistance but it is the resistance due to biasing. In a video, the person was explaining the hybrid-pi model ...
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Are emitter and collector of a transistor fixed, or can I use either pin as either emitter or collector? [duplicate]

The question pretty much sums up everything.
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Can the connection of collector and base in these circuits be considered as a diode connected transistor?

I was learning how to design an op amp and I saw something thatis called "diode connected transistor." Like this image: Later I saw a person that was teaching about these circuits, and he ...
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If there are multiple holes and one electron wants to release then how can i know which hole will trap that electron? [closed]

I saw a tutorial about concept of holes that says : This trap and release is slower than the moving of electrons on its own. According to that image there is just one hole that replaces its position ...
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How to simplify the resistor with base current in BJT?

Attached below is the circuit of BJT for the sake of practice. In this BJT circuit, any ideas to simplify the circuit in the blue rectangle (base part) in order to calculate base current, IB? (...
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How can KCL be satisfied here?

In the transistor biasing using diode below assume that at time t1 the circuit is stable and apply KCL at the junction of the three devices you have: ...
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Using a square wave function to select output

Im new here! I been having some trouble thinking about a circuit wich selects one of two outputs based in the value of a square wave function like this To put some numbers Freqs Vin=1Hz Vout1=1kHz ...
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BJT: definition of "edge of saturation"

The book Sedra/Smith (Microelectronic circuits) tells in chapter 5 the following: My question: I found no statement on why the EOS is defined by the point where vc < (vb - 0.4V). Seems like other ...
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Which part of this transistor model op amp represents pin 5?

In this op amp oscillator pin 5 is connected to an RC circuit: I have a transistor model of that op amp and I want to connect an RC circuit to it like the above circuit: Since pin 4 and 8 represents ...
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Why in this circuit the collector of Q2 is receiving negative voltage?

According to when we say the collector in a transistor is reverse bias and the base and emitter are forward bias, why in this circuit (since Q3 or Q4 are NPN, so the emitter should be connected to ...
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Why is the output voltage in my second op amp not the same as for the first op amp?

I saw this tutorial on a website. From that, I designed these circuits: In my circuits the voltage source is 12 volts. The tutorial says the output gain should be 2. In my second circuit the gain is ...
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In this image where is the base of Q3 connected to?

In this image the base of Q3 shows an arrow that points to the 10k resistor, but I do not know where is it connected to. Image: According to that image I do not know what should I do with the base of ...
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Collector resistor value affects the delay in this simple turn-off delay circuit. Why?

Here's my simulated circuit: As you can see, when plotting the output voltage, the smaller the collector resistor value is, the shorter is the delay. But why? What BJT graph predicts this behavior? ...
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BJT "Ground emitter propagation characteristics" graph

I'm reading a datasheet (https://www.mouser.com/datasheet/2/348/qs6z5-774191.pdf) of a BJT and noticed that there is a chart titled "Ground emitter propagation characteristics". I can see ...
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LTspice JFET Voltage Divider Bias Configuration

I want to simulate this circuit in LTspice program and find Idss,Vp and Idsq,Vdsq but cannot. How can i simulate transfer characteristic?
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BJT operation in saturation

I am currently learning the fundamentals of BJTs from "Microelectronic cicuits" by Sedra and Smith. More specifically I am reading about how BJTs can be used as voltage amplifiers. From my ...
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Question about the BJT emitter resistance

I'm trying to teach myself a little bit about BJTs, particularly through the Art of Electronics. I came across something I've been trying for days to wrap my head around. (page 92, third edition) I ...
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Are the Q4 and Q5 transistors in reverse action mode?

Are the Q4 and Q5 transistors in reverse action mode ?
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Why BJT thermal voltage can be not equal to kT/q in SPICE3

In SPICE, apart from effects such as basewidth modulation and base-collector leakage the collector current is given by the expression (according to the original SPICE2 PhD work) But actually, ...
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Small signal output impedance for a circuit with bipolar transistor?

For the following circuit, I need to find the small-signal impedance looking into the collector of the transistor at terminal X, not taking the load resistance Z into account: My attempt at a SSM is: ...
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NPN BJT emitter current only from base

I'm working with a BC637 transistor. I want to deliver a 48 V, 2.5 mA signal to my load (diagram below.) I have the load connected through the emitter pin (19.2k Ω resistor.) I was under the ...
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BJT Is Saturation current temperature dependence singularity

So, I am currently looking at the ways transistors change their parameters with temperature according to SPICE for a thorough investigation of synthesizer VCO exponential converter. Particularly, the ...
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How to calculate the output resistance of the CE amplifier, which has only a coil in its collector?

In order to be able to match the impedance, it is necessary to know the output resistance of the amplifier. In radio frequency amplifier circuits, there is usually a coil in the collector instead of a ...
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Collector current increases with Vcc in common emitter mode

I was analysing a simple common emitter bjt circuit when I found that the collector current was changing with changing Vcc even though collector current is independant of Vcc in theory .Why is this? ...
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9 votes
2 answers
778 views

Why does this BJT leak so much collector current at cutoff with high V_CE?

I have some 2N4401 NPN transistors that I'm trying to use with a 12V supply. I saw some strange behavior at cutoff and reproduced it in this simplified circuit. (The base is not connected, in case the ...
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3 votes
2 answers
241 views

BJT collector power limit in pulsed usage?

Assuming that my circuit respects the voltage, current, and thermal limits of a BJT, can the device be electrically damaged by exceeding an instantaneous collector power limit? In other words, are BJT ...
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Output Resistance BJT regarding Early Effect

I have a doubt respecting how to calculate the output resistance on this question below. I've searched at 4 different books and find the same way to compute this value but seems its not be right in ...
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1 answer
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How does the current mirror stabilize tail current in an op amp?

Op amps often use the "slave" side of a current mirror to provide the tail current for the input transistor pair, such as in this simple op amp voltage follower schematic: The "gates&...
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Simulated 741 op amp has unexpectedly small output swing

While trying to understand the internals of op amps, I tried to implement a 741 circuit from Wikipedia in LTspice like this: (hooked up as 4x non-inverting amplifier, 0 and 24V supplies, some light ...
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3 votes
1 answer
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Active load vs active diode

It makes sense that when VGS = VDS, the channel resistance (lower slope) is large compared to the resistance in ohmic region (steeper slope,) so a MOSFET can be used as a resistor. From a previous ...
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Common-Emitter Analysis

I am making an error in the analysis of the above circuit. $$i_i=i_b \implies v_1=i_b\cdot r_{\pi}$$ So the value of $$g_m v_1 = g_m\cdot r_{\pi}i _b= \beta i_b$$ But, this is the same current (\$i_c =...
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Why does my transistor explode in a simulator?

I'm new to electronics and I can't identify the reason why my transistor explodes when I simulate it. I am simulating a simple common emitter amplifier with a power supply of 12 volts.
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Why this 4-bjt current miror got better precicion?

I just wonder how precise of 2-bjt current mirror circuit then I found this circuit from link form circuitlab.com My question is How much we can rely on 2-bjt how to determine how precision there ...
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Why is the base current of a BJT in PSpice higher than expected?

I have made a common emitter amplifier in PSpice with an NPN transistor. I have set the Beta value to 200 in the model settings, however, as you can see from the picture, the collector current is not ...
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1 vote
2 answers
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Why is this voltage "negative" in this simple BJT circuit polarization?

A question about this circuit was posted and immediately deleted. It is a simple circuit, but I found this when I simulated it. First, DC analysis, nothing strange: Then analyzing more: parameters DC ...
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Why is the Base-emitter ON voltage as high as 2V in the BJT datasheet?

Below is a snippet from a BJT datasheet. Why is the Base-emitter ON volatge mentioned as 2V? The Base emitter voltage should be 0.7V when the BJT conducts.
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7 votes
5 answers
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BJT To Prevent Debug UART Backfeeding PCB?

Question Can a simple NPN BJT transistor prevent a debug UART from backfeeding the MCU on the target PCB? Problem UARTs and associated USB-to-UART dongles (such as an FTDI branded one) are commonly ...
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2 votes
1 answer
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Changing Vbe_on in Pspice - Student capture

I want to change the V_BE_on in my BJT model but I couldn't find any explanation on how to achieve this. I need VBE on of 0.7[V] and the default I think is 0.8[V] I tried editing the .model -- ...
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1 vote
2 answers
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Using two op-amps to measure the current in a predefined window

I've been trying to use an op-amp to measure the current and then use two op-amps as a comparator to detect the whether the measured current is within a window. The circuit defines the upper and lower ...
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Re model for Collector feedback bias with emitter resistance

For the given NPN transistor with \$\beta=99\$ and \$r_e=0.02 k\Omega\$, I need to find the voltage gain using Small signal Analysis. I drew the equivalent \$r_e\$ model as below. To find the Voltage ...
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