# 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.

1,315 questions
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### BJT cut-off currents magnitudes and directions

I was reading the datasheet for the 2N3904 transistor (NPN BJT) recently and found out that I couldn't wrap my head around the cutoff current values. For the given conditions, both junctions are ...
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### logic gates with 2n222 transistors and leds not working

I am a complete newbie to electronics, But here i go.. I am trying to make some super simple logic gates for now an AND gate. In this logic gate i want to show which out en inputs are high by using a ...
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### FM transmitter circuit working?

I am trying to understand FM transmitter circuit working basically on how a signal is imposed on to the carrier signal. I would like to get an acknowledgement on whether my understanding is correct or ...
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### DC BJT Circuit Allowable collecter swing discepency

Im working through tutorials of BJTs from sedra smith in which the lecturer has video solutions that you can follow as you go if you get stuck. The issue is i don't agree with some assumptions that ...
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### Solving Unkonw R1 and R2 in classic descrete BJT bais arangment

im stuck solving the RB1 RB2 values is a classic discrete BJT circuit bias arrangement. Im sure its a simple thing that im just not seeing. Circuit values given are Ie=1.5mA V collector to emmiter =...
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### Are there BJTs based on metal-semiconductor junctions?

So, I know that the fundamental part of a BJT is having a very thin base region that allows minority carriers to diffuse through it before they recombine. It seems to me, then, that it should be ...
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### Problem with calculating high frequency response of a CE amp

I have this amp: The parameters of the transistor are hfe=120, Cje=40pF and Cjc=4pF. I am also told to assume Cpi = Cje = 40pF and Cmu = Cjc= 4 pF. From that I should calculate fh and compare it ...
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### Increase output power of simple class AB audio amplifier

How could one increase the output power from a class AB amplifier circuit like this one? I wish to increase the output power to around 2W. The above picture is simulated with a 200mV amplitude, 5kHz ...
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### How to keep current from draining out the bottom of this push-pull amplifier?

In an earlier question I asked about driving a speaker with a BJT transistor. @Jonk offered a push-pull amplifier design of his own, and @G36 noted its similarity to the JLH amplifier described in a ...
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### How to derive the precise gain of an NPN common emitter amplifier without emitter degeneration?

As a self imposed exercise, I tried to derive the complete expression of gain in a common emitter amplifier without emitter degeneration. I mean by "complete" that it also takes into account the ...
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### How to double the collector current of a BJT in LTSpice?

I am trying to model a multi-collector lateral PNP in LTSpice. The link here suggests to put 2 PNP transistors in parallel and split up the IS into two parts. http://pallen.ece.gatech.edu/Academic/...
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### Can someone help me with this power amplifier problem?

Wire the circuit shown:   R1 = 68KΩ R2 = 10KΩ R3 = 10KΩ R4 = 200Ω ( two 100Ω in series ) R5 = 2.2kΩ C1 = 0.1 µF C2 = 0.1 µF Q1 = 2N3904   Set the sig. Gen to 50mv RMS.  With frequency ...
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### Output Impedance of a Pre-amplifier

This is an AC equivalent circuit of a pre-amplifier. I already derived the formula for input impedance (Zi) but I'm having trouble getting the formula for output impedance (Zo). I can't find a way to ...
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### What causes amplitude clipping of single stage CE BJT amplifier with a bypass capacitor?

Where is a single stage CE BJT amplifier with a bypass capacitor supposed to clip with a normal 4-resistor biasing circuit? Like I seem to be able to get the upper peak of the output voltage all the ...
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### Figuring out the AC equivalent circuit(re model) of a Pre-Amplifier and Tone Control Circuit

I need help figuring out the AC equivalent circuit of this preamplifier circuit. I'm quite familiar with all the simple BJT configurations but this is my first configuring such a relatively ...
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### Applying hybrid-pi model to npn with connected Base and Collector

When applying hybrid pi to an npn circuit shown for small signal analysis, would the 200kΩ resistor show up across the Base and Collector or would it be excluded from the model?
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### How can there even be an input voltage on a long-tailed pair?

Assuming the following: The voltage across the base-emitter junction of a BJT is constant in forward-active and saturation mode The BJTs in a long-tailed pair are normally forward-active or saturated ...
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### Linear current driver with several BJTs in parallel

With the circuit setup shown below, if I have available a type of Q2 transistor with a certain maximum dissipated power and need that D1 withstands a large current that would lead Q2 in trouble, it is ...
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### Why can't my BJT common emitter trigger the pin interrupt of an MCU?

I'm trying to build a noise detector that gets triggered by pretty much all audible noises, provided that they are loud enough. The schematic design is as follows: simulate this circuit – ...
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### BJT 2 stage amplifier

all I'm pretty new to electronics so my question may be a little ambiguous. I have designed a two-stage amplifier using 2N3904's. the total ac gain should be over 5000. would this circuit do it? ...
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### Class A Audio Amplifier

I am trying to design a Class A Audio Amplifier with the following specs: 9V DC Power Supply, Maximum withdrawn current 15mA Used with a Low Impedance (8 Ohm) Speaker Provide a minimum variable gain ...
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### Designing a Class A Audio Amplifier

I am currently working on a project where I am required to design a Class A Audio Amplifier using BJTs. The requirement of the amplifier is as follows: - Acquire a variable gain of at least 10 (Av >=...
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### 2-stage BJT amplifier for low impedance load

I need to design a 2 stage bjt amplifier that would amplify a 0.1uV waveform at least 100 times on a 4ohm load. How do I choose the proper stages for amplification? I was thinking about the most ...
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### Why is my relay oscillating for over a half-second?

I'm building a circuit that can turn on a relay when the variable resistor cools down. Its operating principle is cooling down the variable resistor increases its resistance, causing the voltage at ...
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### BJT base connected to ground and still operates?

When in university I used to get confused how a transistor (NPN) can draw current from ground. I'm now studying power amplifiers and again, this same configuration appears. How does Ir draw current ...
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### How to reduces nonlinearities due to the saturation of one of the BJTs in a differential amplifier

Reading about the differential amplifier in The Art of Electronics, we are introduced to this circuit. It is claimed that the common mode input voltage is limited at the upper end by $Q_2$'s ...
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### Determine the input impedances of a circuit

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: To determine the ...
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### Finding node voltages of transistor

I have the following problem, it consists of BJTs connected in the following way. I am being asked to find the node voltages. Am doing the part were beta=100 and R is connected. I have done the work ...
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### Common emitter: maximize effective voltage gain

My homework is to design npn common emitter amplifier, with minimum output aplitude=5V, powered by +26V and -20V with generator resistance=40kOhm and output resistance=10kOhm. I have calculated ...
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### I need to create a switch and low pass filter

The bjt receives a square wave as an input, and at the output(from the low pass filter there should be little to no ripple. to achieve the ripple specs i used a capacitor. on it own the switch works ...
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### Buck/boost output voltage manipulation via feedback node

I'm designing an emitter follower circuit attached to the feedback node of a Buck/Boost SMPS in order to pull current out of the node and subsequently increase the output voltage by varying a DAC ...
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### Differential amplifier analysis

I've been trying to solve this circuit for the past few days, but whatever I do I seem to lack an equation or something. I've tried writing equations for two nodes, but the process is just too long, ...
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### Identifying parts of a transistor die

In books, I have studied that in a Transistor, the Base is the layer sandwiched in between Collector an Emitter. Wikipedia also mentions that "The base is physically located between the emitter and ...
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### Why can't class A amp drive 8 ohm speaker with just one BJT?

I designed this class A amplifier. It's my first attempt at designing an amplifier with transistors. My objective is to build a guitar amplifier with tubes, but I figured that building an amp with ...
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### How do I prove that the Pi and T models of a BJT are equivalent?

I want to prove the equivalence of the Pi and T-models of an npn BJT. The only way I can think of doing this is by showing that both circuits have the same Z parameters. To do so, you would need to ...
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### What does this statement mean?

In the following problem, I know how to solve the whole problem but I never understood what this statement means: " All capacitors are assumed to be large enough at the operating frequency of Vsig"
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### Switch-mode power supply output voltage manipulation via feedback node

I'm trying to design a cheap way to pull current from the feedback node of a SMPS in order to dynamically control the output voltage, as explained in this application note:http://www.ti.com/lit/an/...
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### Can we combine these voltage sources?

In the following figure, we want to find V Thevenin. I tried combining the two 3v sources (3+3=6v due to their polarities) and applied voltage divider to find the voltage across the 150kOhm resistor ...
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### What is the difference between “Collector-Emitter” and “Base-Emitter” Saturation Voltage?

As you can see on the datasheet below for the 2n2222a NPN transistor, the "Collector-Emitter Saturation Voltage" and "Base-Emitter Saturation Voltage" are defined respectively as 0.3 to 1.0 and 1.2 to ...
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### Confusion about the meaning of re and rπ

In BJT small-signal models there is both re and rπ parameters. They both represent the dynamic resistor between the base and the emitter terminals. But I read that they are different by a factor of β ...
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### Voltage divider bias (BJT) leads to a huge voltage drop across the collector resistor. Why?

I've already looked for similiar questions, but couldn't find any. Basically, I'm new in electronic circuits analysis and, after following my professor's lecture and looking on the internet I found ...
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### Common collector-emitter, feedback loop

My goal is understanding voltage and input impedance difference between calculation and spice simulation in circuit like below: Predicted: k=U_Rc/25mV=6V/25mV=240 V/V, R_input=(25mV/Ib || 1k)*B || ...
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### Common emitter, o.p. set by feedback resistor

I am analyzing common emitter amplifier where operating point is set by feedback resistor between b-c. Voltage gain showed by symulator is much lower than (-)R2/R1 and input resistance is lower than ...
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### How to tell what fraction of current is passing through which BJT

In the circuit given below we have to find output voltage. No specifications about any of the transistors are given just that all the components are "discrete". Can we find out how much current is ...
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### What's the most Important thing to look on datasheet when Replacing a Transistor with another one?

I'm trying to replace a transistor but I don't know if the transistor that I'm trying to replace is going to work. Can someone help me determine which parameters to look for before replacing?
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### Equivalent load resistance, AC and DC load lines for push-pull class A amplifier

I have a question which is about a class A push-pull amplifier as shown below, but I am not sure with my ideas: I could find Icmax and Icmin in part a), which are 240mA and 80mA respectively. As ...
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### What is the real problem with fixed point bias circuit?

In the circuit below, the texts mention the problem with this kind of biasing here is if the hfe(β) varies then the Ic will vary in linear reagion. (What varies what here also not clear, I thought all ...
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### Does Vbe remain constant when Vb changes but changes when Ve changes for this BJT biasing circuit?

I was reading about thermal runaway caused by Icbo in a BJT, and seems I understand how the temperature increases Icbo then in turn Ic and Ie. But I also read that the biasing circuit's emitter ...