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What actually is a current source?

In circuits we talk about voltage sources and current sources. Now, a voltage source is a battery, I know what a battery is and I can visualise it and understand how it works. But what actually IS a current source? Can somebody name me a physical object that is a current source or give me a picture of one or is it just an abstract idea of current just being 'added' to that point in a circuit by some device? Thanks.

Reply 1

uberteknik

Original post by Aiden223

From the physics point of view, anything that can supply moving charge must by definition, be a source of current. i.e. free electrons or ionic atoms.

From an engineering perspective, the definition of voltage and current sources are indeed rather abstract.

Voltage source: is envisioned as a two terminal black-box that maintains a fixed voltage drop across it's own terminals regardless of the load resistance placed across the terminals. i.e. the voltage source supplies whatever current is necessary to maintain the p.d. across the load.

In practical reality, this can only be achieved for a limited maximum load current. e.g. a battery approximates a voltage source over a limited range of currents. But because it has an internal resistance, as the current demanded by the load increases, voltage is lost and the output voltage across the terminals droops.

In practical circuits, feedback loops around operational amplifiers are used to maintain accurate reference voltage sources.

Current source: is envisioned as a two terminal black-box that maintains a constant current regardless of the load resistance. i.e. the current source develops any necessary p.d. across the load in order to maintain a constant current.

Again, in practice, the range of loads over which constant current can be maintained is limited by the physical implementation of the circuit.

Circuit symbol:

Practical implementation is a semiconductor current mirror:

The BJT devices need to have closely matched characteristics and are therefore selected specifically to comply.

R_biasis fixed and sets up a constant reference current in the LH transistor which is mirrored through the RH load irrespective of the load resistance (within the circuits compliance limits).

Reply 2

Aiden223

Thank you for your detailed response. Could you please give me a name of a device that IS a current source? Like, a battery is a voltage source, what is a current source? I'm curious what it is in a real life situation, so say you were building a circuit and you wanted to add a current source, what physical object would you use for that purpose? Thanks.

Reply 3

uberteknik

Original post by Aiden223

I gave you an example of how they are fabricated using the current mirror above.

Here are others:

Reply 4

Aiden223

Oh ok so you can't get an object which is a current source, but more it's a specific setup of multiple components that supplies extra current?

Reply 5

uberteknik

Original post by Aiden223

Oh ok so you can't get an object which is a current source, but more it's a specific setup of multiple components that supplies extra current?

You got it. :smile:

The tighter and wider the range over which you want to control the current source, then the more complex and difficult to implement the circuit becomes.

Reply 6

Aiden223

Original post by uberteknik

You got it. :smile:

The tighter and wider the range over which you want to control the current source, then the more complex and difficult to implement the circuit becomes.