# 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.

1,345 questions
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### SPICE BJT simulation results

I'm trying to simulate simple BJT circuits to verify my hand calculations. Following is my circuit, I'm just trying to get the calculated DC collector current with the shown voltages and parameters. ...
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### BJT: why does Ic increase when temperature increases? [closed]

When the temperature of an BJT increases, the thermal voltage $$V_T = \frac{k_BT}{q}$$ where $k_B$ is the Boltzmann constant, $T$ is the absolute temperature of the junction, $q$ is the ...
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### Convention regarding BJT collector current calculations

Using the equation Ic=Is*e^(Vbe/Vt) yields impossible values for me. I am using Vbe = .7v & Vt = .025 A. Is in circuit is 3.4mA. I am trying to determine Ic as an initial value for analyzing ...
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### A confusion about the formula for transistor Vbe Ic characterisitcs

Below is NPN BJT transistor Vbe Ic characteristics and the formula: Many texts approximate this equation as: Ic = Is*e^(Vbe/Ut) and following this they say when Vbe=0, Ic becomes equal to Is. ...
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### Please explain what is the right answer for this question [on hold]

How on increasing emitter's doping Icbo decreases...
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### Can a BJT base-collector junction be safely used as a diode?

In a (hobby) project I'm doing, I noticed that the diodes I needed were more expensive than the NPN transistors, which left me wondering if I could substitute the transistors for the diodes. I'm ...
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### To add a reasonable parasitic in a BJT circuit, how much resistance should be added in series with an inductor?

I am working on a tunable image reject notch filter using Agilent_HBT SiGe BJT model in ADS software of Keysight. To consider parasitics in the circuit performance I need to put a value of series ...
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### 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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### Drive Nixie tube with BJT

I'm trying to replicate the Nixie clock designed by Kevin Lee (https://0x7d.com/2017/nixie-tube-clock/). It uses a NXP PCA9685 PWM controller and a MSD42WT1G BJT to drive each tube cathode separately. ...
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### BJT Clapp Oscilator for 3-6MHz

I designed a Clapp VFO to aid in my Ham Radio projects (Non-superhet SSB and CW recievers for 80M and hopefuly 40M). I chose the clapp topology for it's frequency stability. I want it to tune at least ...
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### BJT differential amplifier: What causes common mode voltage gain?

In any normal op amp, if it was ideal it would have a gain such that the output voltage would be However in reality this is not the case as there is a common mode voltage gain and hence in reality ...
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### Emitter follower and input impedance

I'm designing an active bandpass filter (using only BJTs, voltage sources, resistors and capacitors) with the following properties: Corner frequencies $8 ~ \text{kHz}$ and $800 ~ \text{kHz}$ (...
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### High Input BJT Differential Amplifier

This is a basic differential amplifier. I want to modify it in order to have high input impedance (a few $M\Omega$ from DC to at least $200~kHz$) I thought about replacing the CE pair input with ...
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### Where to find details of dimensions and doping of a typical bjt

I have been searching for dimensions and doping levels of base, emitter and collector for a typical bipolar junction transistor presently in use. I have tried searching on internet and seen datasheets ...
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### The BJT characteristic curve

Lets say you want to operate at 20uA base current. To move along that 20uA graph, you need to vary Vce. But to vary Vce, you either vary Vb (voltage at base) or change the values of the resistors at ...
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### How can I have an ECL logic input for a CMOS logic gate

I'm working on a triggering system that uses a comparator to check if the signal is below a threshold value. I found a great comparator for the job, except its output is ECL logic. I wanted to use the ...
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### Common Emitter Stage BJT Amplifier with Early Effect Inclusion

How can I find Rin, Rout and Gm parameters of a BJT amplifier in common emitter stage without neglecting Early Effect? We have such homework and I couldn't find the exact same example in Behzad ...
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### How to choose resistors for a multiple resistor voltage divider circuit?

I am trying to create a voltage divider circuit to off offsset two BJT input voltage is for a push pull amplifier configuration, and I am having trouble choosing the proper resistor values for my ...
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### Bipolar Junction Transistor problem. Where did I go wrong?

$\beta = 100$, Transistor Q1 is PNP and Q2 is NPN. Trying to find $V_1, V_2, V_3, V_4$, and $V_5$. I assumed active mode for both. $\beta = 100$ $V_{BE} = 0.7 \mathrm{V}$ Then  \alpha = \...
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### Changes when $V_{be}$ is increased in npn transistor

For the given circuit, a statement has been given in my book, saying It is obvious that if Vbe is increased by a small amount, both hole current from emitter region and the electron current from ...
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### Can I apply Kirchoff's law in the open contour in the picture

So is it right if I apply Kirchoff's law : Vcc=I1R1+I2R2+I3Re if I1 is the current that flows in R1 resistor, I2 in R2 and I3 in Re
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### BJT amplifier design with input resistance, output resistance, and open-loop gain requirements

so currently i'm designing a BJT Amplifier with these requirements: Open-loop gain = 35 dB Input resistance = 600 Ohm Output resistance = 200 Ohm Another thing is the only BJTs I can use have hFE of ...
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### Why doesn't the emitter resistance of a BJT depend on the AC current?

When designing BJT transistor amplifiers, one calculates the emitter resistance of BJT: $r_e=\frac{26mV}{I_c}$ where the voltage is the thermal voltage at room temperature and $I_c$ is the bias ...