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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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Transistors why do they amplify current?

Ok guys I can't understand a thing of the Transistor here is the scheme: My teacher said that when I put 0,7V on the base the PN junction between Base and Collector is direct polarized so the whole ...
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154 views

Is the circuit in this answer wrong?

In the Q/A link below, the circuit in the answer seems to be wrong to me. Answer to "How to gradually light up an LED?" Circuit: simulate this circuit – Schematic created using CircuitLab In ...
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72 views

Dual power supply with BJT. How to do tracking without opamp?

I'm making a 0-±12V 20mA supply for breadboard. Circuit simulates fine with ltspice. How to implement tracking (or tracking ratio) WITHOUT opamp? I found a schematic here https://groenholdt.net/...
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146 views

BJT AC Analysis using r_e model

This is a picture of a Common Emitter Voltage Divider bias and it is been modelled using a CCCS and a diode resistance using the r_e model. Now my doubt is that I get problem in finding the input ...
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1answer
31 views

NTE 4050 buffer and current sourcing capability, datasheet reading woes

I am trying to construct an output that is switched on by the inputs to a buffer. The buffer input, in the final design, will be sourced from an Arduino or Raspberry PI. I needed the buffer because ...
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41 views

how C2 leads to negative feedback and C1 leads to positive feedback

C1 and C2 represent stray capacitances. I was told that C2 leads to negative feedback and C1 leads to positive feedback (pg 371 of Learning the Art of Electronics) and therefore C2 is good while C1 ...
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1answer
51 views

BJT (Common-Collector Configuration)

As we know that for the transistor to operate in the active mode, it should have emitter-base junction as forward bias and collector base junction as reverse bias. So the common-collector ...
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1answer
73 views

Reasoning behind values of CE amplifier

I have the following circuit: I am asked to choose values for the different components such that at the end, the amplifier meets certain requirements. In the solutions, I am told that to minimise ...
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1answer
52 views

Very slow turn-on time BJT (BUV27)

I have been asked to select a replacement BJT for a resonant circuit which uses the now obsolete BUV27. I have selected some based on V(CE) and hFE but all selected types perform a lot worse (low ...
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1answer
68 views

Input resistance confusion

Here's a circuit of an amplifier in the CB configuration: The small-signal equivalent is (using hybrid-pi model) I want to calculate the input resistance \$R_{in}\$. If I'm not mistaken, the blue ...
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1answer
61 views

TTL gate characteristics

In case of BJT for example, the input characteristics can be described with \$I_B=f(V_{BE})\$ and output characteristics with \$I_C=f(V_{CE})\$. How can the input and output characteristics be ...
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1answer
109 views

PNP BJT small signal model

I have been solving BJT circuits on my textbook and stumbled upon a PNP BJT small signal model question. It asks to build the small signal model of a PNP transistor circuit. When I checked the ...
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1answer
93 views

How can I build BJT amplifier with Vin and Vout requirements (peak to peak values)

I have been working on a project but I'm not sure if I'm complying with the requirements that I was given. Design a BJT amplifier such that VIN=0.1 V and VOUT=5 V (peak to peak values). ...
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1answer
120 views

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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1answer
106 views

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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1answer
88 views

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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3answers
57 views

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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136 views

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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85 views

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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2answers
57 views

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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1answer
38 views

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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36 views

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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2answers
205 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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1answer
132 views

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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86 views

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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45 views

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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1answer
67 views

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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1answer
65 views

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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60 views

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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1answer
52 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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42 views

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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1answer
55 views

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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1answer
39 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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1answer
51 views

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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3answers
52 views

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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1answer
57 views

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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37 views

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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1answer
44 views

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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1answer
59 views

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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145 views

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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2answers
77 views

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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61 views

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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1answer
64 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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1answer
57 views

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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1answer
43 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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1answer
62 views

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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1answer
66 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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1answer
112 views

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 ...