45 votes
Accepted

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 ...
  • 166k
23 votes
Accepted

What's the point of a current source?

The current in any 2 terminal device is always the same, there's no way for a 2 terminal device to provide "extra" current. You can't have 0A in and 4A out. What it does do is force the current to ...
  • 21.1k
20 votes
Accepted

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/...
  • 12.8k
19 votes
Accepted

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 ...
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 ...
15 votes

What's the point of a current source?

A current source is a voltage source which - in ideal circumstances - creates as much voltage as needed for the specified current to flow. Think of it as a power supply with an adjustable voltage and ...
  • 317
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 ...
  • 166k
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 ...
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 ...
13 votes
Accepted

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 ...
  • 51.8k
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 ...
  • 64.8k
11 votes

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 ...
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 ...
  • 412k
10 votes
Accepted

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 ...
10 votes
Accepted

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 ...
  • 8,216
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 ...
  • 76.3k
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, ...
  • 1,738
9 votes

How does Stitching capacitor provide shortest return current path between plane splits, as we know that it blocks DC current?

We're only talking about high frequencies here. If there is a DC connection, then there would be no need to split a plane. If you split the plane there is a loop created that has some area. That means ...
9 votes
Accepted

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 ...
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9 votes
Accepted

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} ...
  • 124k
9 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 ...
9 votes
Accepted

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 ...
9 votes
Accepted

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 &...
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9 votes
Accepted

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 ...
  • 412k
8 votes
Accepted

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 ...
  • 3,149
8 votes
Accepted

PSU Current on negative line

It would probably make more sense if the label were adjusted sligthly: Those -5V and -12V outputs really are negative voltages (not the ground returns for +5V and +12V) and they do sink current.
  • 14.1k
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 ...
  • 166k
8 votes
Accepted

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 ...
7 votes

what is this current source

When we say an ideal current source has infinite resistance, we are not talking about series resistance, we are talking about shunting resistance. The model of a non-ideal current source looks like ...
  • 124k
7 votes
Accepted

Power ATMEL ATMEGA328P directly from USB

TLDR; Yes, but it is always a good idea to put a smoothing capacitor across the Vcc/Ground pins of the IC, and try to put that capacitor as close to the pins as possible. Longer answer: The USB ...
  • 9,668

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