# 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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65 views

### Which PNP is broken?

I have a circuit with 2 PNP transistors, probably one of these is broken. One of these tested gives me a diode between Collector(+) and Base(-) while open circuit if I change the polarity of the test. ...
121 views

### NPN Transistors Not Working When Emitter Is Grounded

I'm trying out transistor logic for the first time using PN2222 transistors that came in a kit that I got from Amazon. I've been running into some problems with making a basic NOT Gate (as shown in ...
645 views

### Differential and Multistage Amplifiers(BJT)

I want to determine the currents in each of the branches. I could analyse the current mirror portion and the differential amplifier portion, but facing problem thereafter. I could find the ...
43 views

### Relationship between base resistor and collector current in this unbalanced differential amplifier

For this BJT circuit, I was wondering if there was a solution for how the collector currents change with respect to the value of the single base resistor. This arrangement popped up in a circuit I ...
794 views

### Oscillation won't pass voltage buffer

This is the full schematic of the circuit theremin http://img443.imageshack.us/img443/9292/fulltheremin.png Full size: http://img443.imageshack.us/img443/9292/fulltheremin.png It didn't work on the ...
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### How do I calculate the currents and voltages in a circuit comprising of a transistor and resistors using a model like the Ebers-Moll model? [on hold]

I wish to know how to calculate the voltages and the currents at each of the pins in a BJT using either Ebers-Moll or Gummel-Poon model For reference I have managed to obtain some important values ...
39 views

### Why does the common collector specifically have a high input impedance?

When looking at a chart comparing different BJT amplifiers, the common collector/emitter/base, it is usually said that the common collector has a high input impedance, therefore it is suitable for use ...
30 views

### BJT output characteristics in negative Vce region?

How do the BJT output characteristics in negative Vce region look like? Is it zero current or a reflection of the usual characteriscs in positive Vce region about origin?
96 views

### High Input BJT Differential Amplifier

This is a basic differential amplifier. I want to modify it in order to have high input impedance (a few $M\Omega$ from DC to at least $200~kHz$) I thought about replacing the CE pair input with ...
79 views

### Why does this amplifier not work past a given Vcc?

This circuit works well enough in simulation, I've built it and I can't get the final stage to put more than around 16V across the load. The last stage itself is performing like a voltage follower in ...
36 views

### Electronic Circuit with Opamp and Transistor to pull down a reference voltage via a diode. (Voltage Limiting)

I have a circuit which I have designed to pull down a voltage via sinking current into the opamp output. When the source signal is of higher output impedance this works fine, but when the source ...
97 views

### Why is this current source with both sides grounded still supplying current

Now if we write a node equation at $C$ we get $I_C = g_m − sC_μ V_π$ But my question is, is it correct, how do we define the current through the branch with $g_mV_π$ since both of its ...
100 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 ...
36 views

### Thermal voltage of a BJT vs. diode voltage drop

I'm learning about the hydrid-pi model of the BJT transistor: In this model, the base and the emitter have a resistance $r_{\pi}$ in between. The voltage between the base and the emitter is ...
85 views

### Common Emitter Stage BJT Amplifier with Early Effect Inclusion

How can I find Rin, Rout and Gm parameters of a BJT amplifier in common emitter stage without neglecting Early Effect? We have such homework and I couldn't find the exact same example in Behzad ...
35 views

### Examples of Common Emitter Circuits [duplicate]

I want to practice more bipolar transistor, more specific Common Emitters. Can someone give me any source where there are exercises with solutions of Common Emitter Circuits . i just want to go ...
63 views

### Switching speed of MOSFET and BJT

I've read on the internet that MOSFET is faster switching device than bipolar transistor, however, i don't understand why. Considering the fact that transconductance of bjt is grater than ...
146 views

### Common collector - bias resistor?

[Original question: CCD output buffer; seemed best to make the transistor part a separate post] Taken from Wikipedia, the circuit in Fig. 1 acts as a common collector (high input impedance - low ...
797 views

### How to add a multi-emitter BJT to LTSpice?

I want to simulate the standard TTL NAND gate which contains a multi-emitter BJT transistor in LTSpice. I've searched the components available in LTSpice but didn't found this component. How can I add ...
193 views

### What are these transistors doing and how are they working in this VU Meter?

I was thinking of making KITT's voice box from Knight Rider, which is essentially a VU meter. I did some researching and found an appropriate IC, LM3916, http://www.mouser.com/ds/2/405/lm3915-443929....
611 views

### PNP / NPN Reverse current and voltage (Vce/Vec and Ice/Iec)

I discovered that these two circuits behave in an identical way from a loose perspective (only the following schematics tested): simulate this circuit – Schematic created using CircuitLab R1 ...
80 views

### what is the difference between these two configurations?

I can understand this circuit configuration: But i don't understand when signal is placed in Qp's base. In this configuration we don't have VBB/2 in the two base... Sorry about my english and thanks ...
44 views

### How can U_CE be smaller than U_BE in this circuit?

Recently I was wondering why UCE can be smaller than UBE. I've drawn a small diagram with the typical values of the BC547 NPN transistor. This amazes me all the more as UBE shares its PN-regions with ...
200 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 ...
127 views

### Changes when $V_{be}$ is increased in npn transistor

For the given circuit, a statement has been given in my book, saying It is obvious that if Vbe is increased by a small amount, both hole current from emitter region and the electron current from ...
200 views

### What does this value mean in a BJT datasheet?

In the switching characteristics section of a BJT datasheet I often see test conditions like "Ib1=Ib2=1mA". I understand this is base current but what exactly do they mean by Ib1 and Ib2? Example: ...
155 views

### Designing a Class A Audio Amplifier

I am currently working on a project where I am required to design a Class A Audio Amplifier using BJTs. The requirement of the amplifier is as follows: - Acquire a variable gain of at least 10 (Av >=...
153 views

### BJT-Finding Base Current

I have attempted the following question on this circuit with a bjt. As I have learned in my electrical engineering class, I can replace the transistor with the following circuit and assume it is in ...
44 views

### How to calculate small signal parameter for a BJT when early voltage is taken into consideration?

I have proved one formula of output impedance ro = (Va+Vce)/Ic = Va/Ic' where Ic is collector current and Ic' is collector current neglecting early effect. But reading from standard texts other ...
456 views

### Square wave to sawtooth wave using two BJT

I have two circuits that are supposed to be square wave to sawtooth wave converters: Both circuits receive a positive square wave as their input and generate a positive sawtooth as their output. Do ...
133 views

### How can I have an ECL logic input for a CMOS logic gate

I'm working on a triggering system that uses a comparator to check if the signal is below a threshold value. I found a great comparator for the job, except its output is ECL logic. I wanted to use the ...
49 views

### Need help with 2 Transistor Current Limiter Circuit in LTSpice

I am new to LTSpice. I have attached a picture of my circuit below: The goal of the circuit is to provide current to the R1 until It is too high. Then the BJT will activate, shutting the FET off. ...
45 views

### Stable current source with discrete components

The purpose of this is to compare and learn different circuit topologies, not use of the shelf solutions which obviously provide cheaper, simpler and better results. Considering this requirement (no ...
61 views

### In this BJT circuit why can I apply a voltage divider in the resistors?

I presume that I can remove the capacitors since the circuit uses DC, but then I don't understand why I can apply a voltage divider in the 30K and 20K resistors since the base is connected between ...
40 views

### 2VCC in a BJT C.E. amplifier

It is known that it is possible to get 2Vcc output voltage in a BJT Common Emitter amplifier by using an RF Choke (instead of a simple resistor) connected to the collector. Precisely, we will get at ...
18k views

### BJT in Reverse Active Mode of Operation

What will happen if for a BJT transistor, it's emitter terminal is treated as collector and collector as emitter in a common emitter amplifier circuit?
68 views

### What's the voltage transfer characteristic of this power driver circuit?

[Exam problem for an instrumentation course] Can anyone help me understand how this circuit works? I'm not too familiar with power circuits... This somewhat reminds of a BJT class B amplifier (but ...
77 views

### How to proove if any circuit have negative feedback?

Queries regarding this circuit I am able to say that this circuit has negative feedback but my procedure is disorganized I want a step wise procedure to identify the polarity of feedback In this ...
51 views

### What is topology of feedback for circuit?

I am confused between Voltage Shunt and Current Shunt Also mention why R2 shouldn't be considered as feedback as it is emitter degeneration resistance Consider the above circuit why can't I consider ...
43 views

### Effect of base doping on early voltage,base transport factor and emitter injection efficiency?

According to me as base doping increases recombination will be less at base region so base transport factor and emitter injection efficiency will increase and as effective base width will decrease due ...
2k views

### Why are MOSFETs better than BJTs in application of Voltage Controlled Resistor?

Why are MOSFETs better than BJTs in application of Voltage Controlled Resistor ? Thanks for your responses.
41 views

### What will be vo/i for given circuit?

Solve using both approaches first using small signal analysis and then using feedback analysis.
92 views

### What will be small signal trans resistance gain of shown circuit?

Take beta = 99. I analysed this circuit using first considering this circuit as feedback circuit with shunt-shunt type of feedback and then did the same analysis considering that the circuit is a ...
63 views

### How to find voltage gain of below analog circuit without using Nodal analysis?

This is a theoretical circuit just made to see that if I can find voltage gain of circuit and it took me much calculations then anticipated to get voltage gain.I just want to find if there is better ...
114 views

### Can I, Bi-Directional Level Shifter as Motor(3V) controller with Arduino Lilypad (3.3V)?

Basically this could be easily done by using a BJT NPN, but since I want to control two motors(3 V, 45-60mA) and 5 LEDs was thinking of using a Bi directional level shifter instead of using ...
109 views

### Why it is said that second transistor cannot enter saturation mode in Darlington pair?

When I made equations it came out that for both of transistors to be in saturation Vc-Ve2<.2V Vc-(Vb2-.7V)<.2V Vc-Vb2<-.5V ............{eqn a} As for first transistor to be in ...
127 views

### Collector currents of different BJT Models and Ebers-Moll

Per Horowitz&Hill book, the Ebers-Moll equation for the collector current in a BJT is: Ic = Is (e ^ (Vbe/Vt) - 1) where Ic is the collector current and Is is the saturation current. Now, I am ...
56 views

### What are limitations of Millers theorem for amplifier?

I mean to say that are the two circuits one original and other its miller/ drawn exactly equal in all respects or their are some difference in some respect if so please mention. I am not able to ...