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What is exactly meant by output compliance of a current source? I've read that it's the range of voltage over which the current source behaves well. What exactly does behaving well mean?

I've got a little problem based on this concept, but Im not sure how to go about it.

You have +5 and +15 volt regulated supplies available in a circuit. Design a 5mA npn circuit source(sink) using the +5 volts on the base. What is the output compliance?

How would you do this?

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A well behaved (ideal) current source keeps the current constant no matter what its voltage is.

There are of course lots of different topologies for making current sources, just like there are lots of ways of making voltage sources. However, what this question is referring to is a special property of BJTs that can be exploited to make simple current sources.

Consider these characteristic curves of a common NPN transistor (swiped from here):

Each curve is the collector current as a function of the collector to emitter voltage for a particular base current. Note how the curves are pretty flat after 1/2 a volt or so accross C-E. That means that the current changes little despite large changes in voltage. Sound familiar? Now see if you can exploit this in a circuit to make a well behaved current sink.

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  • \$\begingroup\$ Thankyou for the answer. I've done some research on this property on transistors but I'm having slight doubts.How I can determine the base current for which the corresponding collector current will flatten out at 5mA? \$\endgroup\$
    – Ghost
    Commented May 14, 2013 at 17:27
  • \$\begingroup\$ @Ghost: The trick is to design the circuit so the right base current, or close enough, happens automatically due to feedback. Consider a NPN with a resistor in series with its emitter, then a fixed voltage applied from base to the other end of the emitter resistor. \$\endgroup\$
    – Olin Lathrop
    Commented May 14, 2013 at 21:10
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An ideal current source has infinite output resistance. A reasonable one has very very high output resistance. What this menas is that it will drive, the designed for current, into the load regardless of what that load is. In a simple model if you have a 1A source and you drive a 1 Ohm load the current source has to be able to maintain that current at 1 volt. IF the load changes to 1000 Ohms the current source has to have the compliance to be able to drive 1 A into 1000 ohms at 1000 Volts. If it drops off of the mark, like in only having a 15V rail, then it is not compliant enough.

There is enough hints in your question, a BJT has certain voltage limits that must be met to ensure that it operates correctly. These will limit you compliance especially if the base is at 5V.

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