What is this electronic symbol labelled "Current Source"?

Question

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

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

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.

Answer 2

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

Answer 3

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.

Answer 4

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.