# 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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### Question about drawing the small-signal equivalent circuit of an amplifier exercise

From Razavi's Fundamentals of Microelectronics book: I don't understand where did this ground come from? The resistor r_pi2 is connected between the emitter and base and then the base is grounded (...
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### Simple BJT circuit problem

This problem is from Transistor circuits and applications by Cowles: Attempt at solution The circuit diagram seems incomplete to me, because they assert a +12 V potential at the emitter but the ...
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### Chip Identification - "+A0V" & "LBSS"

Would anyone know what the +A0V chip that looks like a BJT is. Measuring with a ruler the dimensions are approx. 1.5mm by 1mm. It is connected to this chip that I also don't know what it is, with ...
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1 vote
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### Determination of early voltage from the Ebers-Moll equation

I know that it can also be estimated by extrapolation, but I would like to determine the Early voltage from the Ebers-Moll equation. The equation I want to use is: Where Is = saturation current. Ut = ...
• 297
67 views

### Where should the base resistor be placed in a PNP switch?

I would like to understand the difference between placing a base resistor at R2 versus R4 when implementing a PNP switch. It seems there is no difference whether I place a 4.7K resistor at R2 and ...
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### Are the steps in my solution for this BJT circuit correct?

I am faced with this question: Calculate and determine the value of the resistor RE that will ensure that VC=6 V. Redraw the transistor model of the equivalent circuit for small amplitude AC signals. ...
60 views

### Why input signal of negative feedback amplifier is Vs - Vf?

The block diagram below is from my textbook. I don't understand why the input voltage Vi = Vs - Vf. Take an example as the image with numbers on it, I know its not correct in reality but just assume ...
41 views

### Inclusion of R_E in AC and DC analysis of BJT

we agree that the value of Re exists when analyzing the DC analysis of this circuit, right? But when we do the AC analysis of this circuit, Re resistance does not exist, because there is a parallelism ...
82 views

### Level shift a digital 1.8V signal to split supply rail, using BJT

I have designed a DC/DC converter (based on Zeta technology) that will generate a split supply with a single buck controller and coupled inductor. The controller's GND pin is the negative output. ...
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2k views

### What is the purpose of the BJT in this circuit? [closed]

What is the purpose of Q1? Is it just to amplify the current? What region will Q1 be operating in and how do I determine this?
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### How to Properly Select Resistors and Capacitors for a Voltage Reference Circuit with BJTs

I am working on a reference voltage circuit and I am having trouble when selecting the appropriate resistor and capacitor values. The circuit involves OP07 operational amplifiers and transistors (...
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### How much current will a BJT transistor draw through its base?

If a BJT transistor has a voltage Vb at its base, its nominal emitter voltage Ve will be Ve = Vb - 0.7. If we know Re, we know the current through the emitter Ie = (Vb - 0.7)/Re. If beta is high, ...
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### Why should the Vce be half the value of the supply source?

For a BJT amplifier biased to operate in the active region; I think that the statement "the voltage Vce is half the value of the supply source" is incorrect. Because I know that such a thing ...
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### Arduino can not drive a BJT connected to a relay

I'm trying to make a simple 24V relay driven by an Arduino Leonardo as per the following picture: The Arduino code simply toggles the output every 2 seconds. When I measured the pin voltage alone, ...
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51 views

### Electric field in power transistors

I had a question about power transistors, why are small fields used in power transistors? I think this question could be because the voltage is high, but mostly in terms of the transistor structure I ...
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### Voltage and current gain in two stage BJT amplifier

In the two stage amplifier bellow, by doing the DC analysis you will find the bias currents $I_{C,T1}=3.71mA$ and $I_{C,T2}=5.48mA$, if I am correct. In order to find the voltage gain from Base ...
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### The LED that is series with relay is destroyed after a couple of activations

I've tried to make a bidirectional relay circuit with a pair of relays that work with 24 volts. I tested the circuit and it works fine and both LEDs turn on when the relay is activated. After a ...
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130 views

### Collector and Emitter of BJT are always shorted [closed]

I am testing a circuit and i have an npn BJT in that. I am supplying a 5V pulsed source to the base of the BJT at node C. And a constant -5V input at node A. Note the frequency must be 10KHz with 10% ...
106 views

### BJT circuit questions

I have landscape lights. The load is 3 VDC 100 mA for each filament. I have two of them so the load is 200 mA at 3 VDC. I have solar panel which varies voltage from 0 V to 1.5 VDC. I want the ...
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### BJT not switching properly

Image-1 Image-2 Image-3 Note:- The query is regarding the signals SHUTDOWN, TRIP_TEMP. Ignore the signals GATE_EN, MON_TEMP, ALARM_TEMP Image-1, Image-2 and Image-3 are part of a single schematic ...
2k views

### What causes signal distortion in the BJT circuit I designed?

I set up the circuit as above and ran a simulation. Then, I noticed that the negative part of the signal was flattened, almost like a rectifier. However, when I reduced the capacitance of the ...
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### Is LTspice AC analysis trustworthy at RF when using low impedance sources?

I'm an amateur at circuit design but I've been reading a lot and trying my best. I've edited the question to try to make the main problem more coherent and add a simple schematic and plots. My setup ...
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### Practical Implementation of A Two Quadrant Transconductance Analogue Multiplier [closed]

Currently cannot simulate a functional Two Quadrant Transconductance Analogue Multiplier. Understood constraints: Vx is small (Vx << Vt ~ 25mV) Vy is greater than VBE(on)(0.6V) (a DC offset ...
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### Voltage Controlled Amplifier using Oscillator

I have found an example of a voltage Controlled amplifier using a square wave control on this page I'm trying to modify it using my own control signal, a triangle wave from Low frequency oscillator. ...
161 views

### Design of a BJT Translinear-Based Square-Rooting Circuit

I'm trying to make a simple sqrt circuit to act on an input voltage and provide an output voltage without using log and antilog amplifiers. I'm trying to base my design of the translinear combination ...
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### Shockley Equation in relation to circuit analysis with PNP and NPN transistors

I have been looking online to create a Sqrt(Vin) circuit using analog electronics. I have stumbled across a statement in youtube videos that from the shockley Eq. Vbe = Vt Ln( Ic/Is ) However from a ...
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### Why don't these resistors affect the gain in voltage divider bias?

Our textbook says that voltage gain is not affected by the R1 and R2 resistors, but I don't understand the reason behind it. Also, don't we get the 1/gm that is in the denominator from r(pi) resistor ...
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### Simulation of h11 parameter bjt

I want to find out value of h11 parameter, and I know that Vcc= 10V and Ib=10uA. I have tried to simulate it, but I guess it is wrong because I should be getting 8k ohm and I am getting 2,7k ohm. Is ...
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### Breaking "rule of thumb" for common emitter?

I'm trying to understand this common emitter amplifier that is part of the of a guitar pedal. The image is take from the ElectroSmash analysis from the Big Muff Pi . The values for R6 and R4 are ...
1k views

### How is a BJT in the active region able to vary collector current without depending on the collector and emitter resistances?

I'm currently struggling with the idea of Ic = β x Ib as applied to BJTs. I am not understanding how a relationship like this is possible without accounting for collector and emitter resistances. At a ...
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### how do I add turn-off to a latching circuit?

so, I made this latch circuit : Falstad Simulator simulate this circuit – Schematic created using CircuitLab It turns on and stays on. How do I add turning off using single momentary switch? I ...
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### Battery Charging Circuit - What is the operational analysis of device Q1?

simulate this circuit – Schematic created using CircuitLab V1 is a 24 volt dc 210 milliamp power adapter. V2 is a 14.4 volt nominal nicad battery in a handheld vacuum. V1 is almost 30 volts open ...
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### Op amp as a comparator output turning partially high when it's supposed to be low

I'm using an AD823 op amp, as a comparator as part of a feedback loop in a boost converter. Although the boost converter and feedback portion functions as desired, the problem is when I check the 13 V ...
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177 views

### Multi-stage amplifier help

I designed a multi-stage amplifier (I know, so original) for my circuits class. It follows all the parameters that are needed: Vs (sine wave) of 10 mV pk-pk Vdc of 15 V Gain of -250 without load (2.5 ...
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### What is the difference between using op amp and transistor for small signal amp? [closed]

I have a question. As you know, not only the transistor can be used for switching level or amplifying small signal (single or multiple stage design), but also op-amp can be used in same purpose. So ...
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### I'm trying to design a CE BJT amplifier with 260 gain and Bandwidth of 5.5Mhz

I'm struggling to design a common-emitter BJT amplifier for my college project. for this i selected a BC547 transistor and I have done the calculations by Assuming my Vcc as 12V. then I took Vce = 5V ...
390 views

### Why is the frequency and voltage of my Colpitts oscillator lower on breadboard than LTSpice?

My colpitts oscillator I made on LTSpice oscillated at 5 MHz at a peak-to-peak voltage of 260V. When I put all the components down on my breadboard and measured it with a real oscilloscope, the ...
1 vote
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### How to calculate values for current and voltage limiting resistors in variable power supply?

I have an assignment, where I have to calculate the values for a current limiting resistor (R3) and a voltage limiting resistor (R14) for a variable power supply. The input voltage to the circuit is ...
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### I don't understand how transistors are used as a switch

When I see circuits with transistors used as switch I see this circuits.The thing I dont understand is why is this transistor is so important ? Wouldn't we have the same circuit with just a simple &...
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### How to improve gain for CC-CE-CC amplifier

I'm trying to design my first amplifier; my configuration is CC-CE-CC. My Vcc is 10V and Ic for 3 transistors are 0.5mA. When i designed this, i wanted the overall gain to be 50 times, ( and around 90%...
67 views

### NPN transistor as a switch configuration

I have a npn transistor configured as a switch which is normally on and at times I have it switch off and on at 7Hz-8Hz, with R1 5.10k and R2 100k and 0 V to 5 V MCU pin and max $I_C$ current from ...
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### If a BJT can be modelled as a current-controlled current source, is there any limit to the load resistance?

I have read that a BJT can be treated as a current-controlled current source. I am guessing that it can't provide the same current to an indefinitely high resistance. What is the maximum load ...
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### BJT four-resistor bias circuit analysis, parallel resistors

I don't understand why R1 and R2 are in parallel. I need to find the Thévenin resistance of the circuit, and my textbook says that R1 and R2 are in parallel, but I don't understand why. I really ...
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### Class-A power amplifier magnitude and phase

I just did a simulation in Multisim of a class-A power amplifier, but the phase graph of the AC sweep (AC analysis) is not what I expected. With the reference circuit diagram, I simulated the circuit ...
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### Bipolar Junction Transistor operating modes

If I use a BJT transistor as a switch, the transistor operates in the saturation or cut-off region, not in the active region. However, some sources use the formula IC = β·IB. However, this formula is ...
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### Reconciling High Drain Voltage with Typical V_BE Drop in nMOS+PNP IGBT Configuration

I'm examining two design methodologies for IGBTs (Insulated Gate Bipolar Transistors) and have encountered a puzzling issue regarding the drain voltage in the nMOS part of an nMOS+PNP IGBT ...
644 views

### How to prevent this op-amp from railing

I have the following circuit: An input that can go negative (+/-5 V sine wave), and two inverting op-amps. The second op-amp is "current buffered" and is driving the base of a BJT. Here is ...
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### What is the difference between a BJT's DC and AC gain and why are they different [closed]

Could you please explain what is the difference between the DC and AC gain of a BJT and why is the gain different for DC and AC? Where would the different gains be used?