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37 votes
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What is this unusual NPN-PNP transistor pair?

These transistors are devices which are highly optimized for use in ultra-low-cost and high-volume consumer products- off-line fluorescent lamp ballasts. The anti-saturation network (for fast ...
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35 votes

My teacher said a BJT phototransistor doesn't have a base

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 ...
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33 votes

'Leftover' voltage when using constant current

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 ...
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31 votes

When is a MOSFET more appropriate as a switch than a BJT?

When is a MOSFET more appropriate as a switch than a BJT? Answer: 1) a MOSFET is better than a BJT when: When you need really low power. MOSFETs are voltage-controlled. So, you can just charge ...
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31 votes
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Why do CMOS NOT gate designs differ from BJT NOT gate designs?

However, I've always been perplexed as to why people do not use the former method for chip manufacturing. In chip design, resistors take up more area than transistors, and area is one of the main ...
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28 votes
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What's this driving circuit of LED?

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 ...
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27 votes
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Why have both: BJT and FET transistors on IC output?

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 ...
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26 votes
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Why is Vbc absent from bjt equations?

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, \$...
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26 votes
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Effect of bootstrapping in amplifier circuit

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 ...
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  • 69.1k
24 votes

Basic transistor questions

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 ...
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23 votes

Logic behind this fuse status indicator

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 ...
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23 votes
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My BJT audio amplifier circuit isn't working as expected

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 ...
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23 votes

Why do CMOS NOT gate designs differ from BJT NOT gate designs?

It's possible to build this circuit with 2 BJT's or one MOSFET You're reading too much into it. The difference isn't the technology of the transistor, it's method of circuit operation In the first ...
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22 votes
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Help understanding this LED lighting transistor circuit

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 (...
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21 votes

Why would I use 2N3904 instead of 2N4401?

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 ...
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20 votes
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What is the objective of a base-emitter resistor?

R2 is used to prevent a floating base. It gives it a defined state, in case the node labeled 2.8V isn't connected. It's a weak pull-down resistor. A floating pin, ...
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  • 70.8k
20 votes

Why do CMOS NOT gate designs differ from BJT NOT gate designs?

Your circuit with the bjt is RTL (resistor transistor logic). There’s DTL, TTL, ECL, PMOS, NMOS and CMOS. There’s others as well, but you can research these if you’re interested. Basically there’s a ...
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19 votes
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Why would the transistor not switch?

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 ...
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  • 14.4k
19 votes

A question about Vce of an NPN BJT in saturation region

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 ...
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19 votes
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Biasing a second stage amplifier

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 ...
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18 votes
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What do multiple emitters/collectors imply in a BJT schematics symbol?

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 ...
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18 votes
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Why does the current still flow through the collector eventhough the transistor circuit has no Vcc

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 ...
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18 votes
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What is the purpose of this circuit?

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 ...
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  • 21.1k
17 votes

How to determine whether a given transistor is NPN or PNP?

If you know the component is a BJT transistor, you can use a diode tester and test the pins pairwise. If you find two diodes with a common anode, and no conduction between the two cathodes, then you ...
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16 votes
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How does this constant current sink actually work?

The circuit employs negative feedback and utilizes the very high gain of the op amp. The op amp will try to keep its non-inverting and inverting inputs at the same voltage \$V_{\text{set}}\$ due to ...
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16 votes
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How does this BJT transistor circuit works?

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 ...
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16 votes
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Why so many variations of BJT transistors

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 ...
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16 votes

Understanding class A amplifier waveforms

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 ...
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16 votes
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A question about relaxation oscillator

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 ...
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  • 164k
16 votes

Understanding a tachometer signal conditioning circuit I inherited

Q4 provides current gain when the LT1017 output is high. When the LT1017 output is low D11 conducts, bypassing Q4. This arrangement is needed because the LT1017's current source capability (high ...
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