# 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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### How to measure collector current in BJT?

In the following circuit: I know the expression for the collector current is $$I_C =\frac{V_{CC}-V_C}{R_L}$$ Say I have a potentiometer for $R_L$ and I decide to set it to 0 Ω. What would $I_C$ ...
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### Do Germanium BJTs have lower impedance than Silicon BJTs?

What brings this up is the fact that Germanium has a higher electron mobility than Silicon(as well as lower barrier potential) and does that translate to greater admittance(including susceptance)?
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### Designing a constant current pulse circuit

I'm looking for help designing a constant current pulse through a variable load impedance of 10-100k (R4). The ideal response is shown below. This waveform was measured from an off the shelf ...
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### Can I use an NPN BJT transisor as a standard switch?

Is is possible to use a NPN transistor as a normal switch? If I put 5 V on the transistor base the switch is closed and if there is 0 V on the base the switch is open. Does the transistor need to ...
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### Double BJT circuit with one BJT off and one in saturation

In the circuit below, due to the base voltage, the circuit becomes a common-emitter setup. However, assuming a reasonable value for the base current the BJT should be in saturation. However the value ...
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### Discriminate MOS from BJT easily

I have a transistor for which no datasheet is available the transistor is supposed to be NPN (which implies BJT). (BD247C-Transistor-NPN, 100V 25A, 120W in TO220) how can I find out easily if it's ...
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### NPN BJT Transistor Circuit Analysis Problem: Is This Question Solvable?

I've been doing some circuit analysis questions as practice but came across one that seemed very odd: As far as I can tell, this question is missing information and therefore unsolvable? I was able ...
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### Transistor Reverse Biasing - AC

I am working on a project which require to activate a 24V Rated relay using a 3.3V rated IO. To provide adequate isolation I have added a Opto-isolater stage. In order to increase the current output ...
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### How to find the output impedance of a two BJT small signal circuit?

Like for instance, the picture above. How to find the output impedance? My thought was like draw the small signal circuit of them, then do the equivalent. But I am not sure if I need to short the ...
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### Using a zener diode and a bjt to regulate voltage received by a controller

Okay, here goes… I’m trying to create a circuit that will prevent low conductance (from rain water exposure) which occasionally shorts a particular switch, from triggering a controller input that the ...
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### Is it the quantum tunneling that causes the charges to go from emitter to collector rather than to base in BJT?

I was trying to figure out the working of a transistor and I find few things in the transistor action analogous to quantum tunneling like the width of base (barrier in case of tunneling) need to be ...
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### Simulation result of bjt cascode amplifier

I use square wave as the input source to my BJT cascode amplifier. However, I got my output voltage as I used transient analysis in PSPICE to obtain the simulation result. Can someone tell me whether ...
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### Transistors - What is re model?

I would like to know an in depth definition of the re transistor model in BJTs. I know most diagrams very well, but the definition does not. Also, can you guys help me differentiate the small scale ...
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### Does the reverse saturation current aka “Is” in the Ebers-Moll equation have any link to Iceo or Icbo

I can understand what are the leakage currents Icbo and Iceo but I don’t know and cannot find whether one of them is the same thing with the reverse saturation current in Ebers Moll equation namely Is....
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### How does this circuit output 25kHz?

This circuit doesn't use a gate drive, so I am trying to understand how it can output 25kHz. A friend of mine told me that it does so with the use of LC resonance; the capacitor and inductor next to ...
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### Is the resistance between collector and emitter is lower that of the path between collector and base? [closed]

If no, pls tell me why. And if yes, my question is how can the path C-B-E be have lower resistance than that of B-E? Shouldn't the resistances of BE and CB add up cause they're in series?
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### Why can't a combination of two diodes connected back to back act like transistor? [duplicate]

I saw an explanation of the transistor working here by Bill Beaty. I absolutely loved it. But I'm not entirely sure why the action cannot be achieved by two diodes connected back to back by a metal ...
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### Why is the transconductance of a BJT greater than that of a MOSFET?

During my classroom lectures, I came across this doubt of why the transconductance ($g_m$) of a BJT is greater than that of a MOSFET, i.e.: $$g_{m(BJT)} > g_{m(MOSFET)}$$ It would be very ...
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### what is bootstrap capacitor in this circuit how does it improve linearity and how does divide the collector resisitance?

what is bootstrap capacitor in this circuit how does it improve linearity and how does divide the collector resisitance?
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### How to calculate the input and output capacitors of a common emitter amplifier using BJT?

Sorry if the question is stupid, but my head is still hard as a coconut and I am learning... I am dealing with this circuit that I have calculated from scratch. This circuit was designed to have ...
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### Biasing high frequency BJT cascode amplifier

So far, no one in the course has gotten both primary spec: 14.125 (with 1 V/V swing either way) nor 3dB-bandwidth of 500kHz to 20MHz using the 2N4427 (datasheet). Vcc = 9V I_CQ = 10mA-15mA I have ...
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### Midband output voltage of cascode BJT amplifier in PSpice not right

I made a cascode BJT amplifier. $V_{AC}$ = 10e-3 V (ACMAG) From an AC sweep at the output, $R_L$, I need to get a voltage between 200 mV and 250 mV. However, I got 0 V. Why is this so? Do I ...
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### High-side switch with BJT for 24V controlled by logic level

Earlier I have asked another question and I got very good answers but I couldn't get a solid success as per my size considerations and an unexpected solenoid type change. And I am now giving another ...
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### Base resistor configurations, are there any differences?

Sorry for the noob question. From my understanding, R1s acts as pull down resistors for the base and R2 will limit the current flow to the base. I have seen both configurations, but couldn't work out ...
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### Soft latching power switch

I have stumbled upon a soft power switch with low part count on EEVBlog, which I found very interesting for my hobbyist projects: https://www.youtube.com/watch?v=Foc9R0dC2iI Since it uses a PMOS and ...
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### Transistor as variable resistor in LFO

I've constructed a basic dual op-amp LFO as in the below circuit simulate this circuit – Schematic created using CircuitLab The variable resistor R5 is used to alter the rate of the LFO, I'm ...
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### Operation of BJT with low values of Vbe (“cutoff” region)

Many sources claim that when the base-emitter voltage becomes smaller than approximately 0.6V-0.7V, the bipolar junction transistor goes in the cutoff region and there is zero base and collector ...
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### PMOS High Side Switch

I have been working on a project and one of the ideas was to control power supply of LCD 16x2 module and IR sensor via I/O pin of microcontroller, basically to have them on/off when i need them to be. ...
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### How to make BJT/MOSFET work in RF?

I am trying to design an amplifier with a transistor model. I followed some basic tutorials online (http://microwave.eecs.utk.edu/ECE545_files/02_Lab_2.pdf) and designed a transistor model for BJT/...
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### Derive terminal voltages for a BJT

Question: A BJT for which $BV_{CBO}$ is 30 V is connected as shown in the figure below. What voltages would you measure on the collector, base and emitter? simulate this circuit – Schematic ...
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### Input impedance of a one stage voltage amplifier?

This is not a school/university assignment; I'm a physicist trying to learn some electronics. I'd like to calculate the input impedance of the following circuit. simulate this circuit – ...
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### Analysis of BJT transistor - is a linear analysis possible?

Since semiconductors aren't passive components, can I realistically derive analytic solutions for a circuit with BJTs? I know this may seem like a very naive question, but am I on the right track with ...
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### Should analogue multivibrator and comparator pullup circuit have separate supplies?

I have managed to produce a design within the falstad circuit simulator for an analogue "LED chaser" using an analogue multivibrator and 3 comparators inspecting the voltage of one side of the ...
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### How important are BJT characteristic curves when designing amplifiers?

I was going to make a voltage amplifier for weak signals with 2SD786, but I noticed that the manufacturers haven't included the characteristic curves in the datasheets. How can I optimize the circuit ...
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### How to determine the most appropriate DC amplification value for a BJT?

The BJT diagram is shown below: The voltage source at the base side is increased incrementally from 1V to 10V, with the voltage source at the collector side being constant. The Beta values are ...
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### Two-port model of a BJT

In electronic books, the small signal model of a BJT is given as: I've just studied two-port networks theory and, as an application, another book I'm reading shows this picture: 1st question: why do ...
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### Circuit operation and component substitution questions for stroboscopic timing light

I'm trying to make a piece of circuitry, for a tool used to set the ignition timing of my old car. I've found the schematic and description here: http://citroen.tramontana.co.hu/en/ignition/...
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### Self-made Schottky transistor doesn't behave as I'd expect

For a project of mine I need to generate UART-like signals with higher voltage (up to 18V) and a baudrate of 115200 bps and later possibly 250000 bps. I tried a basic BJT level shifter which worked as ...
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### Problem with the fall time of a NPN transistor

I have the below circuit to translate a slow digital MCU on/off (3.3V) signal to a higher voltage signal (range from 12 to 24V). I have a specific attention for the Vin=12Vdc case, the voltage drop ...
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### Why are two maximum power dissipation mentioned for this transistor?

I am wondering why manufacturers mention two different maximum power dissipation values for transistors like 2N1613: Does it have anything to do with whether a heatsink is used or not?
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### Calculation of collector resistance for maximum output swing

simulate this circuit – Schematic created using CircuitLab Question from Sedra and Smith (7th Edition) Question: The given bias arrangement is to be used for a common-base amplifier. Design ...
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### what is the difference between using mosfet and bjt for op amp based current source

I have planned to design a op amp based current source driver for laser,but i came across two design one used mosfet1 at the output of op amp and other used bjt2 what is the difference What is the ...
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### Base resistor for BJT Transistor [duplicate]

I'd like to use PBSS4350SS transistors for driving multiple low power solenoids, but looking at the datasheet I can't figure out what value the base resistor (R1) should be? simulate this circuit &...
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### I wanted to convert 0-1v, 2ns pulse to 0-7v, 2ns pulse

I wanted to generate a 2ns pulse width signal in particular 2ns ON time and 6ns OFF time signal. I can generate a 0-1v signal having 2ns pulse width using FPGA(by any means we cannot have more than 3....
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### Make a one-way switch with a BJT or relay?

I've been trying to figure out how to make a one-way switch with some analogue electronics. i.e.: my first idea was to use a BJT. What I'm trying to find is a design that when the base current is ...
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### LC oscillator emitter current doesnt go to 0(class C)

Im trying to create a 433MHz oscillator with the help of ADS for the simulation. From the lectures i have had on transistor LC oscillators, i know the are forced into class C operation, and that ...
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### BJT: Equation of Vce for Saturation Region

In Sedra and Smith: Microelectronics Circuits (6E), it is mentioned, on page 185: Recalling that the CBJ is much larger than the EBJ, the forward-voltage drop $v_{BC}$ will be smaller than \\$v_{...
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### ESD ratings of diodes and bipolar transistors

While it's easy to find ESD ratings for ICs (bipolar, CMOS, and any mix) and to some extent for MOS transistors (for the inherent reasons), I don't find ESD rating for many common diodes (left apart ...
I found an expression that explains how a circuit with op-amp responses to a distortion: $$y = \frac{1}{\beta}x + \frac{\delta}{A\beta}$$ Now consider the circuit: Here, the green, blue and red ...