# 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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### Fundamentals of Output Characteristics of a BJT, what is the difference between load line and output characteristics?

In any transistor, we plot the output characteristics, i.e. effect of change in Vce on Ic. Next, we write the KVL equation on the transistor, i.e. Vcc = IcRc + Vce and plot the load line from this ...
1answer
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### How to invert an open drain output?

I am trying to use a output signal to reset a counter, but the signal I have is "low when counting" and I need "low during reset". I want to "invert" an open drain output to a new signal so that when ...
3answers
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### How calculate and chose the right BJT that comands a relay

I was projecting a simple circuit that, from a digital OUTPUT from an Arduino zero (3.3 V) switch on a BJT that is connected to a relay. The base of the BJT must have a current of about 5mA or less. ...
0answers
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### BJT Latching Circuit Help

I have build a latching circuit using both a NPN & PNP BJT to turn on and off an LED, as shown below: simulate this circuit – Schematic created using CircuitLab When I built the circuit ...
1answer
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### Cascaded common emitter amplifier

Im trying to design a cascaded amplifier with 3 common emitter stages. As long as the input AC voltage is below about 20mV the amplification is fine, as you can see on AMP1. But the output of AMP2 is ...
0answers
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### Constant current source with high Vce voltage

At the moment I implemented a current source with two FMMT560 for about 1 mA. These types allow -500V Vce. I expect startup peaks of 520 V which is too much. Are there any current sources which use ...
1answer
59 views

### Current Source Biasing with Diode Load

I am trying to replicate a circuit I found in the Art of Electronics (3rd Ed, page 229) but I am having issues. My load is a LED (Vf=~2.5V) and using a BC237 as the PNP (measured Vbe=~.675V). Using R=...
2answers
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### NPN LED variable resistor

I am still beginner to design and so I would to confirm that the variable resistor setup is correct? I have an LED that operates at 2.7V with 100mA so I added a 23 Ohm resistor. Vcc provides 5V and ...
2answers
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### BJT to aid switching a MOSFET

I am trying out the following circuit, I want to use the Arduino so that I can more easily program pulses to drive my load. I was previously using a function generator (FG), but for some reason it won'...
1answer
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### How does BJT act as a switch and as an inverted switch?

These were the graphs shown for the functioning of BJT as a switch . I understand it intuitively but I don't know how to actually explain these graphs. Could some one, in the form of equations ...
1answer
63 views

### BJT transistor working and construction

Why is electron concentration zero at collector side of the base in BJT transistor? And if electron concentration decreases with distance and becomes zero at the end base region how can it produce ...
0answers
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### Why does SOT89-ECB footprint exist?

Because of an oversight a part that has a footprint SOT89-ECB (PNP-BJT), ended up on my board, but when I tried to find a transistor that actually has compatible pin configuration I came out with ...
1answer
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### Trouble with FSK VCO modulation circuit

Circuit shown is a VCO that reliably creates a 200MHz sine wave at 3V pk-pk. I would like to use this VCO to create an FSK modulated signal. My bit 1 frequency = 200Mhz and my bit 0 frequency = 205Mhz ...
1answer
59 views

### Biasing in this FM transmitter

Here is a simple FM transmitter that I've been studying recently. What I'm wondering about currently is how the BJT amplifier in this circuit is biased. I believe that from the audio signal standpoint ...
4answers
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### Small Signal quantities

Behzad Razavi in his book mentions It is important to bear in mind that small signal analysis deals with only (small) changes in voltages and currents around their quiescent values.... If the ...
2answers
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### Bipolar Transistor

why can't two discrete BJT have the same beta $h_{fe}$ value? In case of integrated circuit the beta value of transistor is almost same, but in case of discrete transistors it is not same. On which ...
1answer
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### Collector-Base capacitance in simple FM transmitter

Here is a schematic of a simple FM transmitter. I understand that this work in the following way: the sound picked up by the mic is translated into a voltage at the base of the Q1 transistor. As the ...
1answer
60 views

### How BJT Collector current behave in common base saturation mode?

I'm trying to understand the characteristics of bjt in different modes of operation . .when it comes to the saturation mode of operation in common base configuration ,some books says that the ...
1answer
57 views

### Determine total current in a darlington transistor

I'm trying to figure out what will be the total current consumption of a TIP120. Looking at the internal schematic I see that the second BJT's base is following the emitter of the first one so it ...
2answers
148 views

### Transistor design with beta variation

I am studying Schilling-Belove Electronic Circuits, 3rd Ed. where in its second chapter under the "Maximum Symmetrical Swing" section it gives an example where the base current is kept constant and ...
3answers
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### Why have both: BJT and FET transistors on IC output?

This is the structure of FAN3100 gate driver IC: (taken from its datasheet) As you can see - there are two ouput switches: CMOS and BJT. Why they put them both?
1answer
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### In your experience, how well do NPN SPICE models replicate real DC current gain

Title, I'm currently running some simulations and noticed that my DC current is in the very rough ballpark of datasheet parameters. Is this normal or does it indicate there may be something wrong with ...
1answer
67 views

### How to adjust this BJT design to increase brightness of LED without changing PCB?

I am having issues with this previously designed circuit from a former co-worker. What we want it to do is turn on the LED when IN1 of TPS2121 (Power mux) is chosen as the output. According to the ...
1answer
107 views

### Why does collector current increase with base current in BJT?

The base current in the BJT is due to the recombination of holes and electrons. To neutralize the p region an electron leaves the p region. If we take it the other way round, the larger the base ...
0answers
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### Opamp output on loaded and unloaded cases

I have a design project to rotate a motor. In order to do that, I have designed a very simple but smart circuit. However, in signal comparison stage, if it is unloaded, i.e. not connected with the ...
3answers
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### Examples of Common Emitter Circuits [duplicate]

I want to practice more bipolar transistor, more specific Common Emitters. Can someone give me any source where there are exercises with solutions of Common Emitter Circuits . i just want to go ...
0answers
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### Switching speed of MOSFET and BJT

I've read on the internet that MOSFET is faster switching device than bipolar transistor, however, i don't understand why. Considering the fact that transconductance of bjt is grater than ...
1answer
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### what is the difference between these two configurations?

I can understand this circuit configuration: But i don't understand when signal is placed in Qp's base. In this configuration we don't have VBB/2 in the two base... Sorry about my english and thanks ...
1answer
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### How can U_CE be smaller than U_BE in this circuit?

Recently I was wondering why UCE can be smaller than UBE. I've drawn a small diagram with the typical values of the BC547 NPN transistor. This amazes me all the more as UBE shares its PN-regions with ...
4answers
209 views

### What are these transistors doing and how are they working in this VU Meter?

I was thinking of making KITT's voice box from Knight Rider, which is essentially a VU meter. I did some researching and found an appropriate IC, LM3916, http://www.mouser.com/ds/2/405/lm3915-443929....
1answer
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### What does this value mean in a BJT datasheet?

In the switching characteristics section of a BJT datasheet I often see test conditions like "Ib1=Ib2=1mA". I understand this is base current but what exactly do they mean by Ib1 and Ib2? Example: ...
2answers
495 views

### Square wave to sawtooth wave using two BJT

I have two circuits that are supposed to be square wave to sawtooth wave converters: Both circuits receive a positive square wave as their input and generate a positive sawtooth as their output. Do ...
1answer
55 views

### Need help with 2 Transistor Current Limiter Circuit in LTSpice

I am new to LTSpice. I have attached a picture of my circuit below: The goal of the circuit is to provide current to the R1 until It is too high. Then the BJT will activate, shutting the FET off. ...
0answers
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### Stable current source with discrete components

The purpose of this is to compare and learn different circuit topologies, not use of the shelf solutions which obviously provide cheaper, simpler and better results. Considering this requirement (no ...
1answer
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### In this BJT circuit why can I apply a voltage divider in the resistors?

I presume that I can remove the capacitors since the circuit uses DC, but then I don't understand why I can apply a voltage divider in the 30K and 20K resistors since the base is connected between ...
2answers
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### 2VCC in a BJT C.E. amplifier

It is known that it is possible to get 2Vcc output voltage in a BJT Common Emitter amplifier by using an RF Choke (instead of a simple resistor) connected to the collector. Precisely, we will get at ...
2answers
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### What's the voltage transfer characteristic of this power driver circuit?

[Exam problem for an instrumentation course] Can anyone help me understand how this circuit works? I'm not too familiar with power circuits... This somewhat reminds of a BJT class B amplifier (but ...
1answer
57 views

### What is topology of feedback for circuit?

I am confused between Voltage Shunt and Current Shunt Also mention why R2 shouldn't be considered as feedback as it is emitter degeneration resistance Consider the above circuit why can't I consider ...