# What is this electronic symbol labelled "Current Source"?

What is this electronic symbol? How do I physically implement this?

simulate this circuit – Schematic created using CircuitLab

• A quick Google search revealed this. Feb 9, 2014 at 19:40
• A floating controlled current source, even a non-ideal one, is pretty bothersome to implement. Restricting it by grounding one side of it or the load usually makes things a bit easier. Feb 9, 2014 at 20:23
• This is a dependant current source. The value of the current is a function of something in your circuit usually. You will see this symbol alot in models of BJTs and FETs Feb 9, 2014 at 22:29

When dealing with circuit schematics, an arrow inside a source means that it's a current source, as opposed to a voltage source. The diamond shape means that this is a controlled source. Since the gain makes no mention of amps/volt, this is a current-controlled current source. The gain is one, so this is actually a unity-gain current-controlled current source. An example of a device like this would be a current mirror, where current running through one leg of a device causes an equal current to run through another leg of the device. An advantage of this device is that it acts as a buffer, preventing the output circuit from loading the input.

From wikipedia (http://en.wikipedia.org/wiki/Current_mirror)

simulate this circuit – Schematic created using CircuitLab

Basically, the current mirror shown here acts as a current to voltage, then voltage to current convertor. Current flowing into Q1 is converted to a voltage at the base terminal. This voltage then determines the current flowing through the base of Q2, which sets the current flowing through the collector of Q2. It's a fairly common circuit, and you should be able to find it in any undergraduate level electronics textbook, as well as any decent resource online. Other implementations exist, and a search online will probably turn up many circuits you can use.

Edit: Resistor values, transistor part numbers, and voltages are just the defaults in the schematic capture program StackExchange uses. You'll need to do a little math to get a circuit that will work right for you. Note that the above circuit is a very basic circuit, like you would find in a textbook. More practical circuits definitely exist.

• My first resource is always Wikipedia, followed by Google. For a general introduction to current sources: en.wikipedia.org/wiki/Current_source There's mention in there of some improved current mirror circuits, such as the Widlar and Wilson current sources. Depending on your needs, these circuits may be right for you, or overkill. Feb 9, 2014 at 20:08
• Circuit Lab schematic editor has a part with an arrow in a circle. It falls under the ideal source. The diamond with an arrow falls under controlled source. Feb 9, 2014 at 22:26
• @The Photon That's interesting. I'm from Europe and if I saw an arrow in a circle, I would have thought it to be an ideal current source. I've seen a symbol for ideal voltage source showing a circle with a line connecting the terminals, which is supposed to show that it's different from non-ideal source, because it acts as a short-circuit. It's combined with +,- and ~ for polarity and phase. Feb 9, 2014 at 22:52
• @AndrejaKo, That's probably the symbol I'm thinking of. Feb 9, 2014 at 23:16
• @AndrejaKo, No I'm wrong twice. This question shows an example of an arrow inside a circle to represent a voltage source. Since the caption is in Cyrillic, I'd guess it's from an eastern European source. Feb 9, 2014 at 23:55

This symbol stands for a dependent current source. It's also called controlled current source. The dependent current source is still ideal, even though it's not constant.

This symbol is often used for illustrating how active components (transistor, OpAmp) function. A schematic with this symbol should explain what the current depends on and what the dependency equation is. Here's an example from a textbook.

Schwarz & Oldham Electrical Engineering: An Introduction ISBN 0-19-510585-0

It's a constant current generator and the Howland current pump is usually reckoned to be the best because it can both source and sink current: -

Set Vin+ and Vin- and I out (for this particular set of resistor values) is $\dfrac{V_{IN+}-V_{IN-}}{1000} amps$.

Another one I like is this: -

It only supplies current from the top rail but it can supply several amps if the resistor values are chosen correctly and a heatsink is applied to the final transistor output stage.

Here's a random collection of other current sources that can be found: -

Number 3 is quite useful because it uses hardly any compoennts at all and the LM317 is easy to get.

• What does it mean when the Holland Current Source circuit "sinks" current? Is that what is expected of an ideal current source, e.g. it should also "sink" current for cases where there are reactive loads connected to it when the current load can actually go against the current source's indicated direction? Feb 9, 2014 at 21:38
• The symbol OP asked about is used more often for a controlled current source than for a constant current source. The notation "Gain=1" in OP's drawing also gives a (strong) clue that a controlled source is intended. Feb 9, 2014 at 21:39
• @Hobbyte it means it can be a negative current generator - it works equally well driving current out of the op-amp as taking current back into the op-amp output. A current generator when setup to produce current in a particular direction will always do that irresective of load, even an open circuit - it will produce a terminal voltage of infinity to satisfy its definition to feed current. Obviously op-amps and transistors can only produce output voltages limited to their supply rails so that is the realistic weakness implementing one! Feb 9, 2014 at 21:42
• @ThePhoton very true - which is why I focussed initially on the H-current pump - it is a voltage controlled current pump. If we had the full picture, I suspect that what the OP showed was a current dependant current generator (hence unitless gain) BUT I took the decision (correctly or not) to ignore this. Feb 9, 2014 at 21:46

It's a current source.

An ideal current source will supply a specified current to any load and change the output voltage to make sure the current is delivered. In reality there are limits.

In practice the current source can deliver to a load with:

i) one connection at ground.

ii) one connection at Vdd (power).

Here's a good description on Instructables.

It's ground referenced.

For a source with a Vdd referenced load, swap the power and ground (but obviously not of the op-amp power) and change the transistor to NPN.

• As a new contributor to this site, can I get some guidance on why this answer was down voted? It is accurate, answers the question and provides a link to a circuit that can be built and simulated. Is it that answers without a schematic tend to get down voted? Feb 11, 2014 at 10:05
• Your answer's a bit lacking. For example, it's not just a current source, but a controlled current source. Also, the answer itself is at a pretty elementary level of detail. Also, it's usually held to be bad form here to link to a website that answers the question. Sites move and links die, but your answer here is eternal. Better to summarize here. Thanks for asking how to improve, though. Feb 16, 2014 at 5:34