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48 votes
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What's the intuition for reading transistor/resistor circuits?

What's a step by step way of thinking about the flow of electrons in such a circuit? How can we tell it would provide constant current for instance? The first step is to forget about flow of ...
Transistor's user avatar
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21 votes
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op amp+mosfet = current source.Why do we need a feedback resistor?

R2 (10k R4 in my diagram) is there to form together with C1 (1nF capacitor) a Miller Integrator to prevent unwanted oscillation. And yes, this circuit will sometimes oscillate, mainly due to poor PCB/...
G36's user avatar
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20 votes
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What are some practical examples of a constant current source?

Photovoltaic cells and betavoltaic cells are examples of devices that produce more-or-less constant current up to some maximum open-circuit voltage, just as a battery produces more-or-less constant ...
Spehro Pefhany's user avatar
17 votes

Why are current sources not drawn explicitly in the datasheets' schematic diagrams?

Most likely just to simplify the schematic. They would likely be implemented with various scaled and interconnected current mirrors as well as some sort of reference current generation circuit. One ...
alex.forencich's user avatar
14 votes

How do we create current sources?

Constant current sources ... there are no such devices in nature; almost all of sources are (constant) voltage sources. So we have to make them. I think you are correct. Due to physics chemical ...
Transistor's user avatar
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14 votes

What's the intuition for reading transistor/resistor circuits?

What's the intuition for reading transistor/resistor circuits? More precisely speaking, "reading" means "understanding" here. So, the question is, "How do we intuitively ...
Circuit fantasist's user avatar
13 votes

Reversed DIP Switch

This is what your circuit looks like: simulate this circuit – Schematic created using CircuitLab When you close the DIPSW the LED will be shorted thus the current will flow through the DIPSW ...
Rohat Kılıç's user avatar
13 votes
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I don't understand current source, voltage source, infinite resistance, zero resistance etc

If something has infinite resistance wouldn't nothing happen because everything is blocked? I also don't understand what an "open circuit" really means (to my eye it just looks like a ...
DKNguyen's user avatar
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12 votes

Why do PNP transistors source current and NPN transistors sink it?

Have you ever heard of an emitter follower? An NPN transistor connected in the emitter follower configuration sources current: This figure is from the Wikipedia page on the common collector (emitter ...
JRE's user avatar
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11 votes
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Why do I get two different answers for the current through the 47 kΩ resistor when I do a source transformation?

If you transform a source and a resistor then the only equivalence is the V/I characteristics at the two terminals of the transformed circuit, not for the elements inside the transformed circuit. If ...
Elliot Alderson's user avatar
10 votes

op amp+mosfet = current source.Why do we need a feedback resistor?

You don't need a feedback resistor and neither do you need C1. I guess the "designer" has some strange perception that the circuit will oscillate without them but it won't. Oscillation will occur if ...
Andy aka's user avatar
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10 votes
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Tube cathode biasing with CCS

The 12AX7 double triode is connected as a differential amplifier (long-tailed pair). In the Wikipedia article you see a similar circuit implemented with BJTs, but the principle is the same: consider ...
LorenzoDonati4Ukraine-OnStrike's user avatar
10 votes
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Why is my transistor beta lower than expected?

The beta value you are getting is rather typical for a transistor in reverse active mode. That is, using the collector as an emitter. Assuming every thing else is ok (vendor, supplier, and ...
Edgar Brown's user avatar
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10 votes

How can I make a current source that can supply 1uA?

D2 generates a stable voltage, while the forward voltage of Q1 varies with temperature (by about -2 mV/deg. C). You can compensate for this by putting a standard diode in series with D2 -- e.g. a ...
Spehro Pefhany's user avatar
10 votes

Why do I get two different answers for the current through the 47 kΩ resistor when I do a source transformation?

I gather you find the correct answer from this transformation: simulate this circuit – Schematic created using CircuitLab One way to see why is that the transformation on the left side doesn't ...
jonk's user avatar
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10 votes

Sensor datasheet shows op-amp circuit with capacitor to ground from inverting input. How does this work?

If you're referring to the 2.2 μF cap, it was probably placed for the purpose of not amplifying a DC signal (including the op-amp's own offset). Think about it: at DC, that capacitor is an open, ...
Designalog's user avatar
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10 votes
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How do I explain this regulation action in terms of control theory?

There is no closed loop feedback system evident That's because you didn't spot it. It's the base-emitter region of the 2N3904 (Q2) that acts like a comparator at around 0.5 volts to 0.7 volts (...
Andy aka's user avatar
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10 votes
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Practical example of controlled voltage and current sources

Short answer The difference between the ordinary constant sources and dependent sources is that the former have only output (the input is implicit, constant) while the latter have both input and ...
Circuit fantasist's user avatar
9 votes
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PIC Micro - Can I use both sinking and sourcing maximum current ratings if some pins sink while others source?

The maximum ratings are usually about two things, broadly speaking. One is about local dissipation -- the port pins are often arranged near each other on the die and if you dissipate too much in a ...
jonk's user avatar
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9 votes
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LM317 intermittent current source

To drive 50 mA through 330 ohms, you need 16.5 V. For 50 mA through 1330 ohms you need 66.5 V. In the constant current mode, the maximum output voltage of the LM317 circuit is \$V_{in} - 3\ {\rm V} ...
The Photon's user avatar
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9 votes
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How do we create current sources?

I will expose my philosophy in a few consecutive steps each of them illustrated by a colorful picture. The left part a is a conceptual electrical circuit illustrating the basic idea; the right part b ...
Circuit fantasist's user avatar
9 votes
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How to reach ~50nA p-p current noise in ~200mA current source?

The main theme is: eliminate sources of 1/f as far as possible and limit bandwidth as desired. I suggest the following principal circuit (details added further down): Low Noise Resistor The main ...
tobalt's user avatar
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9 votes
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Why do PNP transistors source current and NPN transistors sink it?

This is because it's more typical to connect the load to the collector. You can use the emitter as an output, but that has certain limitations...the voltage gain will always be 1 in an analog emitter ...
Cristobol Polychronopolis's user avatar
9 votes
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Question about electrical power in circuits

By example I can demonstrate how a current source can both absorb and emit energy. Take this simple circuit consisting of a single voltage source (a battery) and a resistor: simulate this circuit &...
Simon Fitch's user avatar
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9 votes
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Current source with MOSFET

In my understanding the 1K along with the 200pF create an integrator topology to provide some stability Correct and, particularly when using a MOSFET in these configurations there can be a ...
Andy aka's user avatar
  • 460k
9 votes

Passing 10 A through a nichrome wire (2.3 Ω) with a 4.2 V Li-ion battery source

There is nothing to figure out, because it is not possible. A 2.3 ohm load on a 4.2 V battery will draw a current of 1.8 A. And the battery is almost never that full to have 4.2 V, typical battery has ...
Justme's user avatar
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9 votes

Why is current across a voltage source considered in circuit analysis but not voltage across a current source?

I'm not sure why you think KCL concerns itself with voltages anywhere, or KVL somehow needs to know anything about current anywhere. When applying KCL, we consider only current. When applying KVL, we ...
Simon Fitch's user avatar
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8 votes
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Source Transformation of dependent sources

Since this is quite within reach I will try to go through the whole story. Let's take a generic network N and spot one generator (either current or voltage), for now we will not consider it as ...
carloc's user avatar
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8 votes
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How does the resistor at the tail act as a current source in a differential pair?

An ideal current source has infinite output impedance (i.e. \$dV/dI = \infty\$). A real current source is equivalent to an ideal current source in parallel with some finite output impedance Ro. A ...
τεκ's user avatar
  • 4,279
8 votes

How to make a constant current sink in the pA-nA range

Linear have a Precision Nanoamp Bidirectional Current Source application note that may be of interest. Figure 1. This circuit sources and sinks just nanoamps of current with precision due to the low ...
Transistor's user avatar
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