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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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Determine the voltage transfer characteristics of the bjt

How do I start this question, I've just been trying to plot vin vs vout and using EBJ<0.5 and EBJ>0.7 and trying to get a range for vin and plotting? Is this the correct way to go about this?
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Why the input resistance of a common emitter amplifier is like this?

I am trying to find the input resistance of this BJ common-emitter amplifier: I replace the transistor with the hybrid-pi model It appears clear to me that the input impedance will be \$ R_{in} =...
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Base current and Vce relation in NPN transistor output characteristics

Regarding the active region of an NPN transistor and output curves, for a fixed Ib when the Vce is increased the Ic increases; namely Early effect. The reason if Im not mistaken is that the increase ...
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cannot find high cut-off frequency from multisim

for the above circuit i cannot find the high cut off frequency by doing an AC sweep first picture is the frequency response. There is no dip seen for the fH.
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Current spikes on rising and falling edge of NPN BJT switch

I am simulating a simple NPN BJT switch for PWM in LTspice, and I observe some current transients on the rising and falling edges that I do not understand. The circuit is here: Picture of the falling ...
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Polysilicon Emitter equivalent for PNP BJT

I recently read (in the book semiconductor device fundamentals by Pierre about polysilicon emitter technique used in a n-p-n Bipolar Junction Transistor (BJT) to enchance the current gain of BJT. I ...
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Stability of BJT circuit using Re resistor

This is a post I found on Quora. I don't understand why decreasing Vbe decreases the current. Vbe=Vb-Ve To decrease Vbe you could increase Ve, Vb doesn't necessarily have to change, so neither Ib....
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Difference between current gain and overall current gain of BJT common emitter

What is the difference between current gain and overall current gain? Im guessing there is BJT current gain given by Ic/Ib Circuit current gain given by Io/Ii What is overall current gain and why is ...
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Current gain of common emitter bjt

On the right side hes used the current divider rule to calculate the output current Io through the Rl resistor. Looking at how the currents are defined on the diagram, shouldnt there be a negative ...
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Measuring the resistance across Vce BJT

From my current understanding, BJT can be modelled as 2 back to back diodes. From a common emitter characteristics graph, Vce/Ic gives the resistance across the 2 diodes. It is high due the the ...
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What is the importance of the input and output impedance of a BJT

What is the significance of the input and output impedance in a BJT circuit? High output impedance means little to no current will flow at the output, why is this important?
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Output resistance of BJT common emitter circuit

I have found this circuit on a video, he talks about output resistance of the BJT of common emitter amplifier circuit not being infinite but very large so that it must be included in the re model. In ...
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BJT analysis resistance at the base using voltage divider

im currently confused as to why the resistance at the base is R1 || R2, ive done some research and looked at a couple of responses from people. But i still dont quite understand, could someone help me ...
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Switch between charging source for LiPo battery using PMOS?

I have a 3.7V LiPo battery, a usb charger this one and a solar MTTP LiPo charger (this one). The goal is to use the solar charger as default and the usb as a back up. Especially, when charging with ...
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Calculating the Thevenin resistance

In the image below, i have drawn an equivalent circuit. Ive used the voltage divider equation to find the voltage dropped across my 2 points of interest. Have i go the correct calculation for the ...
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How does Dc biasing affect the Q point BJT

In order to amplify an AC signal, its required to provide a DC offset in order for no negative values of current to go through the input. Input signals and output signals oscillate centred on the Q ...
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Choosing a suitable FET for a design project

I'm currently working on a project to produce a high speed and high voltage pulse. I'm trying to find a suitable FET. I intended to supply a voltage of about 50v to circuit. And using the FET to go ...
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58 views

Common emitter amplifier gain with load resistor?

I have a problem I am working on where I have to find the voltage gain of the following circuit with a dc collector current of 0.3mA. My attempt at a solution involved using the formula A=-(RcIc)/Vt ...
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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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What does the DC load in BJT mean?

I see a dc load line, what is the purpose of this line? There are 2 points i see, Ic(sat) and Vcc Does it have something to do with the maximum current through the device and the maximum voltage ...
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Calculating grounded emitter amplifer distortion (Art of Electronics 3rd Ed.)

Referring to section 2.3.4 A1 (pages 94-95) of the Art of Electronics 3rd Ed., I'm having difficulty understanding how to calculate distortion of a grounded emitter amplifier. The book says: ...
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How do the areas of the emitter, base and collector affect their respective currents?

I want to know what will be the effect on current if we change(increase or decrease) the area of the emitter, base, and collector. For example, if we double the emitter area what will be the effect on ...
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In practical BJTs, why can't Vce become 0 when the transistor is saturated?

Practically it is about 0.2 V when saturation starts. Why can't it go to 0?
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Bipolar transistor as a switch. Cut off mode?

I have a question regarding bipolar transistor and it's operation as a switch. I know that, in order to make transistor work as a switch, we need to avoid active region, meaning, we need to make ...
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BJT circuit current analysis

Why is the current into the base Ib=Vbb-Vbe/10k? And not just Ib = Vbb/10k?
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Confirm re model of transformer coupled BJT amplifier

Could someone confirm my conversion of this transformer coupled BJT amplifier into a \$r_e\$ model? simulate this circuit – Schematic created using CircuitLab I get \$I_E\$ = 3.11mA and \$r_e\$...
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BJT: Why do we need Rc and Re?

The below graph shows a graph for a BJT with Ic against Vce. Depending on Vce, there is saturation region, active region and breakdown region. These are resposible for different operations. In the ...
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48 views

Analysis of BJT transistor circuit

in the image, 2 loops are formed, is it possible to form a loop for the last one? If so, what equation would arise?
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What happens to the low cut off frequency when replaced with a larger bypass capacitor?

When the emitter capacitance is increased to 100 microfarads. What would happen to the low frequency cut off of this amplifier?
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Trouble analyzing BJT circuits when we change β from very high to 100

For example we have this circuit: Originally we assumed β to be very high and we assume VBE = 0.7. So for this, we have IB = 0 and therefore V1 = -0.7V and V2 = 3.4V. Now that we have β at 100, IV1 ...
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33 views

BJTs to drive N-Fet

I stopped using BJTs for a while and now I cant figure out why this configuration does not give me a 12V PWM at the gate of the mosfet? I am only getting slightly more than my PWM voltage about 4.2V ...
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Logic behind this fuse status indicator

I was doing this circuit which I found on the Internet. My question is what is the logic behind the functioning of this circuit? Thanks!
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Confusion with a simple two stage amplifier design example

I have been following a lecture on net about a two stage amplifier example. It is basically a BJT amplifier with a PNP emitter follower as shown below. I named some of the nodes for clarity: The ...
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For MMBT2222; How to get hfe value to calcluate minimum base current to drive transistor in Saturation?

If my collector current VCC = 3.3V & Ic = 31.82uA then how to calculate minimum base current is required put transistor in saturation mode? Depending on it I will calculate base series resistor ...
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Transistor as switch (in Saturation mode) - Why transistor saturation appears early with increase in collector resistor

This image is a simulation of a 2N2222 transistor as a switch to level shift 12V I/O signal to MCU compatible 3.3V I/O input. Simulation is done on LTSPICE XVII with DC sweep analysis of V2 (12V) (...
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MMBT2222 Datasheet doesn't mention maximum base current. How to calculate it?

The MMBT2222 Datasheet doesn't mention the maximum base current. How to calculate it? So that design should not exceed it. I want to use it for switching in common emitter configuration; where the ...
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Bjt for trigger MOSFET

I my project MOSFET need 13 v 65% PWM to trigger but controller output is 5 volt then how can I amplify this in voltage
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Analysis of voltage regulator circuit

I have found out the output voltage to be 9V. To find power, we need \$I_CV_{CE}\$ We can find \$I_E\$ from the output circuit. \$ I_E=\frac{6}{24k}+\frac{9}{10} \$ But can we find \$I_C\$? Or do ...
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How to determine operating mode of BJT transistor?

In the lab, I worked with this circuit: I varied RL over a range of values and measured VC and VE. Then I got the IC values and plotted the results down below: I am very confused as to how to know ...
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Virtual Short Concept in an Op-Amp

By virtue of the open-loop gain being infinity for an ideal op-amp, the two input terminals are said to be virtually shorted. Yet, the virtual short concept is only applied to op-amps in negative ...
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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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61 views

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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40 views

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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54 views

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 ...