Physical significance of current source

Hi,

Throughout the elementry electronics text we come across the term current source. I have never seen a current source in real life....so can anybody elaborate what actually a current source is and what sources in real life can be regarded as current source...Whats the physical significance of current source and how it is different from voltage source...

thanks

Hi!
I've seen lots of current sources.
Current sources are very useful in analogue electronics, for so many different reasons that it seems deceptive to start with any particular one at the head of a list!
.
Current sources can be made using op-amps and transistors, and in lots of other ways. I'll stick with op-amps and transistors and make up a circuit for you.



You'll see I've drawn a negative feedback circuit in which a resistor is supplied a current determined by the setting on the potentiometer. Of course a load of some kind is connected to the square pads in the collector circuit of the transistor.
The current in the load is determined solely by the pot, within a range of values set by the voltage of the supply.

Here's another constant current source: Think of a 100V supply with a 10 Meg resistor in series with the load. For a large range of loads, the current doesn't change much.

What was the text you used, please?
[/img]

Current sources are quite common in IC design, not so much in normal design. There are specific uses in normal circuit design, but in IC design they are a common building block. That is partly because different design principles need to be used in IC design. Absolute accuracies are poor but matching can be good. A current source as a load looks like a high impedance so can be useful for getting significant gain out of an amplifier stage. A current can be replicated easily due to transistor matching. A current source load will usually be smaller than an equivalent sized resistor.

Keith

From your explanation i have got the idea that for current sources, the ouput current is not dependent on the load impedances. It always remains constant..but there has to be some limits..obviously u cannot put an infinite impedance at the load and think that the current will still be the same..Is this the case...also if the current remains constant ..can the voltage of the current source change?...

What about the voltage source...how are they different from current sources...

I am not really able to comment on what may happen in the case of an infinite load, but I think you mean a load that tends to infinity and I think you are on the right track. A current source is a quantified and limited thing.

When you think of a voltage source you should remember that voltage and current are reciprocal quantities.

A zener diode is a good voltage source when fed with a suitable current. We could use a voltage follower to get quite large currents, and whatever resistance we put in the load would receive the same voltage, subject to the limits of the design.

Practical current sources have a finite resistance instead of infinite. A good current source could be several meg ohms. As the load connected to the current source varies the current source must adjust its voltage to maintain the current. A similar thing is true of voltage sources (e.g. A battery). When the load resistance varies the current changes to keep the voltage constant.

If you think of a single transistor amplifier, the gain usually depends on load resistance. So if that is a current source then the gain can be quite high.

A zener diode isn't really a voltage source - it is a voltage shunt. While its effect may appear similar (relatively stable voltage) it achieves it in a different way to a series voltage source.

Keith

thanks for nice expalnations...i got the idea now..

I forgot to mention - with a current source, it eventually cannot manage to maintain the current because it doesn't have enough supply voltage. A similar thing is true of a voltage source - it eventually cannot supply enough current to maintain the voltage.

Keith.