# Tag Info

72

Many people think that the answer to this question is related to the width of Base region in BJT transistors - it is incorrect. The answer got quite long. You can read starting from "Tricky Question" section if you want the bottom line. I believe that you were led to ask this question due to something like this picture: This is a standard practice of ...

35

I think you and your teacher are both right - you are just not using the same definitions and thus your wording disagrees. From your description: Shouldn't be transistor without base be just a semiconductor (PP,NN)? You define the "base" to be the semiconductor in the middle of a normal BJT - the P-doped region in a NPN or the N-doped region of an PNP....

31

This circuit is particularly useful for a variety of reasons. So I wanted to do it a little more justice than just two sentences' worth. OVERVIEW In the following circuit, I've intentionally boxed up the circuit so that it appears to have three pins. There is a good reason for this choice, as will become clearer later on (if not already.) simulate this ...

29

Power isn't "across" something. Power is the voltage across something times the current going through it. Since the small amount of current going into the base is irrelevant in power dissipation, calculate the C-E voltage and the collector current. The power dissipated by the transistor will be the product of those two. Let's take a quick stab at this ...

28

As Ignacio Vazquez-Abrams mentions it's a constant current driver though the designer put the switch in the wrong spot. Theory of operation with these drivers is the LED current path is through the right transitor and the right current sense resistor, and in this case through the right switch. The current through the LED rises to the point that the voltage ...

27

Paragraph 2 of the description says: FAN3100 drivers incorporate MillerDriveTM architecture for the final output stage. This bipolar-MOSFET combination provides high peak current during the Miller plateau stage of the MOSFET turn-on / turn-off process to minimize switching loss, while providing rail-to-rail voltage swing and reverse current capability. ...

24

Your question seems to be about beta or hFE. Yes, this can vary significantly between parts, even from the same production batch. It also varies somewhat with collector current and collector voltage (using emitter as the 0 V reference). However, for any one transistor, its gain actually varies rather little as a function of collector current across a ...

24

You are incorrect in your titled assertion. But I can guess where it comes from. Most people use the simplest concepts they need to get the job done. They are concerned about the forward voltage, $V_{BE}$, which is somewhat impacted by the collector current and very much impacted by temperature... so that's important... and $V_{CE}$ is immediately ...

24

You've framed some good questions and I've upped you for that. To address (1) and (2), let me avoid the small-signal linearization model and just have you look squarely at the circuit itself, as it stands. I've redrawn the schematic a little. Not so much because I think it will make things clearer than your own schematic. But because perhaps drawing it ...

24

Q1 and Q2 form a astable multivibrator which will output a square wave (12V, 0 V, 12 V, 0 V, ...) voltage via D2. If D1 wasn't there then that square wave would turn on/off Q3 and with that also the LED. So the LED would blink. However D1 is there and via R5 it keeps Q3 on even if the square wave voltage is 0 Volt. So the LED will not blink, it will be on ...

23

Partial answer - could get very long - may add more later: Choices in this context are usually bipolar or MOSFET. Once you get to JFET you may also wish to think about SCR/TRIAC, IGBT, ... . You may wish to throw bipolar-Darlington into the mix. Short: Goes something like - Small bipolars up to say 500 mA and 30 Volt load voltage are low cost, can be ...

23

Intro This post started out as a direct answer to the OP's question. But I want to expand on the original answer. My apologies it has grown so long. (Keep in mind that I'm just a hobbyist who enjoys learning.) There are a variety of different types of audio amplifiers. Most of them today will be based upon ICs, as they are quite common, cheap, and perform ...

22

This circuit is designed to provide a constant current to the LED independent of the supply voltage. The MOSFET is turned on by the voltage at the collector of Q1. As soon as the current through R1 (which is the same as through the LED) results in a drop of about 0.6V, Q1 will start to turn on and divert current through R2. This will then reduce the ...

21

No. Two back to back diodes is NOT a transistor. The special property that makes a PNP or NPN sandwich a transistor rather than just two diodes is that the base layer is very thin. In semiconductor physics terms, there aren't two separate depletions regions in the base. The depletion regions from the two junctions overlap in the base, which is necessary ...

21

When electrons flow through a forward-biased diode junction, such as the base-emitter junction of a transistor, it actually takes a non-zero amount of time for them to recombine with holes on the P side and be neutralized. In an NPN transistor, the P-type base region is constructed so as to be so narrow that most of the electrons actually pass all of the ...

21

There may be good reasons for choosing the 2N3904 when working at lower current. The 2N4401's higher current capability comes at a cost - higher capacitance and much longer turn off time at low current. Emitter-Base Capacitance:- 2N3904 = 8pF 2N4401 = 30pF Storage time:- 2N3904 = 200ns @ 10mA 2N4401 = 225ns @ 150mA

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Amplifier System Specifications The first step is to organize the specifications you provided in an earlier version of your question. end-to-end system specifications Two BJT voltage-gain stages with some independent specifications (see later.) Bandwidth: $20\:\text{Hz} - 20\:\text{kHz}$. Input impedance: $R_{_\text{IN}}\ge 5\:\text{k}\Omega$. Output ...

18

It's to do with the structure of a BJT transistor. Lets look at an NPN: Image Source You have a collector region made of N-type semiconductor, a base of P-type, and an emitter of N-type. I'm not going to go into detail as it is beyond the scope of the question, but lets suffice it with a question - don't the collector and emitter look similar? What you ...

18

The opamp in the circuit is actually a voltage comparator, it's designed to output only 2 voltage levels - high and low. The positive feedback makes it Schmitt trigger -circuit (=a comparator with hysteresis) to make the comparator a little faster and to increase noise immunity. The transistor is pulse amplifier. The capacitor makes it have low pass filter ...

17

You could successfully build a audio amp from many different types of BJTs. It will be the circuit, not the transistor, that makes the amp work well. I'd pick jellybean parts like the 2N4401 (NPN) and 2N4403 (PNP) and stick with them for everything except for the final power output transistors. Lots of parts could fill that role. If you have your own ...

17

These are multi collector (or multi emitter) bjts. Their operation is quite similar to the normal ones. Multiple emitter bjt's do not conduct only if all the $V_{BE}$ are below $V_\gamma$, it is a sort of wired AND. Multiple collector bjt's collectors total current is set as usual by $I_B$, and if all the collectors are the same size (area at silicon ...

17

For Q3 to switch on, the voltage drop between its base and emitter must be about 0.6 V, which means that the same voltage must be dropped over R3, which means that the current flowing through R3 must be at least I3 = 0.6V / R3. When there is less current flowing through R3, the voltage drop over R3 is smaller than Q3's minimal voltage drop, and Q3 will stay ...

17

The transistor going into saturation isn't a property of the transistor itself, but instead a property of the circuit surrounding the transistor and the transistor, as part of it. The simplest case to imagine is an NPN switch. I'll present two different such switch circuits to make the above point concretely clear: simulate this circuit – Schematic ...

16

Low low low cost is the aim. A small volume maker with no expertise in anything other than manual soldering (if that) can make these. They can be even made at home by workers if desired. Single side phenolic board. Cheap. Not only manual assembly but component size and hole spacings not well matched and leads are hand bent and nobody cares. A design ...

16

The zener with series resistor R3 has about 10V on the anode wrt ground. It is seeing 50mA so the actual voltage will be a bit higher than the 10V nominal, maybe one percent on average. That voltage is buffered with Q7 and used to create a ~17mA current source for the current mirror composed of Q6 and Q5, which feeds the differential amplifier composed of ...

16

In the early days of BJTs, every year the processes would improve and there would be new applications for these parts with new demands. As a result, semi-conductor companies were frequently coming out with new parts. Some were just better versions of older parts, and some were to get into a new niche that didn't have a BJT for it before. Now consider that ...

16

In active mode operation, the BJT collector will be 180 degrees out of phase with the base. But when the BJT is saturated, its collector will tend to follow the base when the emitter resistor is present. So the interpretation here is that for part of the cycle your BJT is saturated and for the rest of it you have way too much gain for the signal, winding up ...

16

Collector-emitter breakdown voltage isn't the relevant parameter in this circuit. Note that the B-E junction is reverse-biased, and the B-C junction is forward biased, so the relevant parameter is the B-E breakdown voltage, which is just 6.0 V. When the B-E junction breaks down in reverse bias, charge carriers are injected into the base region, which ...

15

Well, you're not really minimizing power dissipation here with the pre-regulator. Your pre-regulator is a very simple linear regulator that is designed to provide a lower input voltage to your LM317 regulator. The Zener diode and R3 will provide a relatively stable 14 V. Transistor Q1 is in an emitter-follower configuration, so you get a diode drop down ...

15

You need to understand three things in order to understand a Schottky transistor. In an ordinary BJT, the saturation state collector-emitter voltage falls below or equal to 0.2 V. In an ordinary BJT, the base-emitter voltage must be equal to or above 0.8 V. The saturation state voltage drop of a Schottky diode is around 0.3 V. Now, if we ...

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