Electronic – Problem on output compliance


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?

Best Answer

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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