Questions tagged [npn]

A type of bipolar transistor comprised of n-type semiconductor in contact with p-type semiconductor which in turn is in contact with a n-type semiconductor -> thus NPN. The transistor has three terminals, the Emitter, Base and the Collector. Current injected into the base controls the flow of current from Emitter to Collector enabling amplification and switching.

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75 views

Why doesn't the current in a BJT transistor depend on the emitter-collector voltage?

In my understanding, in an NPN junction for example, an increase in the emitter-collector voltage should widen the depletion layer in the base-collector junction therefore increasing the electric ...
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121 views

Why do we need to connect the load before NPN transistor [duplicate]

I am trying to use transistor as switch and getting confused with the workings. For a NPN transistor, Collector needs to be connected to +ve and emmiter to -ve. If we look at the schematics below, in ...
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How to include the Early effect in NPN BJT current equation?

To understand the NPN current equation derivation, I have been studying my lecture handout and the emitter current equation accordingly is found as : \begin{equation} I_E= q n_i^2 A(\frac{D_p}{N_{dE}...
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70 views

How does class AB amplifier biasing work exactly? [duplicate]

Can somebody please explain in both cases the biasing method? I don't understand the following points: It's OK that we need ~0.7V for PNP and NPN base-emitter voltage, that's why a diode is used for ...
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2answers
118 views

A paradox in relay drive circuit

According to the circuit shown below: If we assume a Vbe of 0.7 V for Q1, the base current of Q1 is (5-0.7)/100 = 43 μA, and assuming a β of 100 for Q1, we have a collector current of around 4.3 mA. ...
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Why does this NPN transistor operate differently if it comes before the switched element?

I'm simulating turning an LED on with a transistor switch in LTSpice. To my bewilderment, the current through the LED in Circuit 1 is ~51 mA and the current through the same in Circuit 2 is only ~6 mA....
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2answers
140 views

How can I find operating point in this curcuit?

I made a schematic in LTspice like in the first picture and then solved the question in second picture for Rb=100k. I have found Ib=0.03 mA, Ic=4.5 mA and Vce=-0.836 V but when I checked the graphic ...
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2answers
373 views

Why does this LTspice schematic simulation have five pen plots?

I ran the schematic at the first picture but it gave five graphs like in the second picture. Is there a mistake?
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55 views

Drive NPN from Voltage Regulator?

I am trying to get two disparate camera systems to talk to one another, specifically an Arri camera with a Preston system, and specifically to transmit a tally signal when the camera is recording. ...
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1answer
76 views

'Unknown Voltage Requested' error in LTspice

I made the schematic that is in the first picture and I wanted to plot Vcc vs. Vce but when I tried to plot, it gives an error like in the second picture. How can I fix the error?
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I can't simulate this circuit in LTspice [closed]

I can't simulate the circuit in this picture. When I try to run it gives an error for missing I value. Could you help me? Could you tell me what Vce and Vbe are?
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NPN transistor with no resistance at collector?

I'm trying to work my way through a pretty simple laser-based communication system (full description here) project I found online, just as a learning experience. I'm having issues understanding the ...
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80 views

N-MOSFET and NPN latching switch

I've seen many latching circuits through searching and reading some simple circuits. Most use NPN+PNP combination, some bistable ones use two NPNs, some use DPDT relays. Is the following circuit going ...
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64 views

2N2222 NPN transistor base resistor calculation

I am quite new to electronics. I am trying to control three LEDs with an Arduino using a transistor. I was wondering what the base current limiting resistor value should be. I looked around the ...
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1answer
71 views

How to receive input signal on Raspberry pi GPIO from transistor emitter

I'm trying to build a simple circuit that allows a Raspberry Pi to detect AC voltage in piece of wire inductively. I have the following circuit which basically uses an antenna wrapped around the wire ...
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3answers
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NPN transistor resistor from base to GND. Is this useful?

I have a circuit where many NPN BJTs are interfacing 3.3 V GPIOs with higher voltage circuitry. Each application has a similar setup to the image below (resistor R422 is what's in question). I did ...
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1answer
56 views

Why is the depletion layer near the emitter region narrower than the collector region?

While studying Grob's Basic Electronics chapter 28 I came across this statement, Notice in Fig. 28–2 that the EB depletion layer is narrower than the CB depletion layer. The reason for the different ...
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A question related to a NPN transistor having a DC bias

β = current gain I made the circuit of the above picture with LTspice. (A graph of Vce) (A graph of Vbe) And I was able to get 4.2966V for the value of Vce and 0.6543V for the value of Vbe. However, ...
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2answers
51 views

Input selectivity for a RF receiver pre-amp

In his answer to my question about common emitter amplifiers, @glen_geek commented that the RF receiver pre-amp I used as an example (shown in Figure 1) was "too simple" and that "any ...
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57 views

NPN Transistor Switching - Negative Gain Calculated and Not Switching off Fully = Blown Transistor?

I have the attached NPN transistor being switched by an MCU. I've made some measurements and done my calculations as a result of observing that the NPN is not fully switching off (the relay stays on). ...
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0answers
47 views

BJT common emitter output conductance

I constructed in multisim a common emitter circuit from the following diagram where the lower Keithley is used to supply VBE and the upper Keithley is used to supply VCE. I obtained the IC- VCE ...
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3answers
70 views

Sticky/Intermittent Relay Driven by MCU and NPN Transistor

I have an MCU (1.8 V logic levels) driving an NPN transistor to turn an relay on/off. The relay has a 12 V DC coil with resistance of 1028R per datasheet. This would require a current of 12/1028 = 11....
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Transistor resistor calculation help (design revision 1)

I posted my original design yesterday and got many comments about my errors. This is revision 1 of my design. I am now pretty sure I have the load resistors calculated correctly, but still unsure ...
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1answer
97 views

Transistor Resistor Calculation

I am new to electronics so I'm not too sure about how I should be calculating the base resistor to use in this circuit. I think the load resistor is calculated correctly (data sheet at bottom for ...
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2answers
195 views

When switching an LED with a transistor via a microcontroller, do I have to worry if the load supply voltage is higher than my microcontroller logic?

I want to turn a white LED on and off via my microcontroller GPIO. The ESP-01S board that I have uses an ESP8266 chip, which has a limit of 12 mA GPIO output current. However, I want to drive this LED ...
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1answer
41 views

NPN BJT Operation - What does it mean for the Collector - Base junction to be reversed biased?

According to this screen grab the Collector Base junction must be "reversed biased". I seem to be confused about the meaning of that. I thought that the Collector needed to be at a higher ...
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3answers
3k views

Why NPN and PNP in parallel in this distortion pedal circuit?

I've come across a guitar pedal circuit called a "Tube Sound Overdrive". The schematic is shown below. It looks like a typical common emitter transistor circuit with an adjustable bias (RV1),...
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1answer
61 views

Op-amp LED project

I'm trying to understand this circuit, but I'm having trouble understanding what the second op-amp block does. I understand tha the first one is an oscillator controlled by a trimpot (RL,) but the ...
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4answers
916 views

Why isn't the NPN diode model two diodes connected in series?

I don't understand how we can model an NPN transistor with two diodes with their anodes connected. Shouldn't they be in series, with the base being in-between the diodes? An NPN transistor is in ...
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1answer
2k views

Understanding a tachometer signal conditioning circuit I inherited

I have inherited a tachometer signal conditioning circuit. It has an NPN transistor (2N3904) with a Schottky diode (BAT54) between the emitter and the base. It is preceded in the circuit by an LT1017 ...
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2answers
138 views

Is this a correct way to use an NPN transistor to disable a push button?

I'm using the Raspberry Pi's GPIO to display some information in a LCD (using I2C) and to enable the usage of a few buttons to navigate through the LCD menus. There's another button, the one I'm ...
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1answer
125 views

How does this transistor behave as the base voltage is increased?

Consider this transistor. How do the voltages Vb, Vc and Ve vary as we increase ...
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181 views

Why would an emitter follower be DC-coupled to its input?

In my experience, an emitter-follower amplifier is usually AC-coupled to both its input and output: However, I recently came across such an amplifier which is DC-coupled - the capacitors C1 and C2 ...
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1answer
87 views

NPN transistor replacement verification

I am trying to replace several transistors on my PCBs due to chip shortages. One such unit is a particular NPN transistor datasheet I cannot get my hands on these little guys anymore. But I figured ...
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0answers
85 views

PNP- NPN light flasher

I understand that R1 and R2 make a voltage divider. initially Q1 is not conductive, so is Q2? initially C1 acts as short circuit, so flashes the light L1 for a moment? Please correct me if wrong. ...
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1answer
79 views

P Channel MOSFET gate controlled by N Channel MOSFET (rather than NPN). Crazy?

I needed a generalized circuit that would switch on/off any device on the Vcc side with varying voltage (3.3 to 24 V) and current (0 to ~10 A). A P Channel MOSFET is an obvious choice, but I didn't ...
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1answer
77 views

Relay does not switch off

I have what I thought was a simple relay circuit but the relay is always on. This is the part of the project that is not working Q5 never switches off. To simplify it further I removed the relay and ...
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1answer
78 views

identify transistor

Does anyone know of a website or online tool to help identify a transistor-like component by its markings? For instance, one cracked and from the pieces, I can't be sure what it used to read (partial ...
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1answer
55 views

Can I use a BC338 in this application?

I want to build this FM transmitter that runs on 1.5-3V. I've got 2 questions so far: Can I use a 2N2222A NPN in this application? (I think I can but want to double check.) How can I calculate (or ...
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2answers
117 views

How to calculate base voltage and current for NPN transistor required to drive a P-channel MOSFET gate

I'd like to use a P-channel MOSFET as a on/off switch. I can't pull the gate down directly from my controller because the voltage of the load is 12v, thus I have to use another transistor, as far as I ...
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5answers
432 views

Transistor collector emitter saturation voltage

I am trying to choose a transistor to use in a current mirror to prevent thermal runaway of parallel LEDs (yes I have to connect them in parallel. I know connecting in series or using separate ...
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4answers
732 views

How can the transistor be in the cut-off region?

Imagine we have an NPN transistor and the base voltage Vb = 0V. How can the transistor be in the cut-off region? I've started by assuming it to be in the active region and then calculated the emitter ...
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1answer
57 views

Tachometer frequency problem

I have an NPN tachometer sensor. I want to measure the rotation speed of a three phase motor, but the output frequency of the tachometer changes. For example, if the actual value of the motor ...
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1answer
66 views

Help Needed with LTSpice Multistage Cascode Simulation

I'm using this online calculator to get -50 gain for a cascode amplifier: https://www.daycounter.com/Calculators/Cascode/BJT-Cascode-Calculator.phtml However, I failed to duplicate the value in ...
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1answer
99 views

How to modify this circuit so that the second LED turns off completely?

Circuit simulation is here There are two parts to this circuit. The first part is a switch that is either floating or grounded, controlling a large number of relays via pnp transistors. (Many relays ...
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3answers
771 views

How can I solve this simple BJT circuit?

Looking at this circuit (first circuit): Choose a value for V2 between 0V and 10V. Then how could we know the IB, IC, IE, VB, VC, VE? Can we solve it by hand (by hand means the value IB, IC, IE, VB, ...
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4answers
1k views

Basic electronics: Transistor switch

I am new to electronics. I am reading this book called Practical Electronics for Inventors. This is a portion about bipolar transistors from the book: There are two things I don't get from this: ...
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1answer
60 views

Totem pole driver topology (inverting amp)

Hello guys this question is regarding a gap in my knowledge regarding transistors specifically the driver of the totem pole configuration. Based on the following figure And these results here: When ...
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2answers
42 views

How to choose a transistor for PWRKEY on SIM808 design?

I am using a 3.3V digital output signal from a microcontroller. The design documentation suggests using this circuit. How do I determine the transistor that I need?
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1answer
192 views

What is the purpose of the D4 diode between base and emitter of PNP transistor?

I am having problem with understanding the purpose of the Q7 transistor. . The anode of the D4 diode is connected to the base and cathode - to the emitter which makes Veb = -0.7V. This will always ...

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