The Laws of Circuit - you can learn and practice by just reading

Copyright. Charles Kim 2006

 

 

Dependent Source?

 

Consider this situation. You put $1000 into your bank account. After books and burgers and Starbucks and Multiplex with your debit card, you check your balance, and lo and behold, you still have $1000! Unbelieving but equally with greed, you try the same spending pattern again. And again, $1000 is still there. No letter from the bank. No phone calls. Your bank account keeps the same purchasing power of $1000, all the time. No matter what. You can say your account is "independant" of whatever is happening. Or a magic world. The bank account I have is in the real world. My balance is "dependant" on my spending and my deposit. Your real-world account is not unlike mine. The balance may be dependent on your dad's (hopefully timely) deposit. Or it may be dependent on your cell-phone bill. Or whatever. In the circuit world, there are 2 types of sourses (which have supplying power): independant and dependant (voltage and current) sources. An independant source is either voltage source or current source which keeps its supplying power at a value, like 10[V] or 2[A], and the value does not change no matter what. When connected to a simple load (which is anything that consumes power) or a heavy load (which consumes lot more power), or disconnected from it, the value does not change. Now you're asking if there is such a thing. Not in the real world. But it does exist in the paper. In the classroom and in the book. Then, is it OK to use in the class such non-existing source? Yes. Why? It's because of the beautiful and magical hand called "approximation" or "modeling." Think about your cell-phone battery. Probably you charge more often than I do. Not because my battery was made in the Neverland but because simply I talk on my cell-phone much less. Anyway, for the first day or two, your phone is working perfect. Even under a harsh condition of hours of non-stop talking. Seemingly not unlike to the definition of the independant source. Quickly ignoring what will happen tomorrow morning, we move ("model") the cell-phone battery onto the paper as an independent voltage source. A power supply which is connected to an electric outlet can be easily modeled as an independent voltage source without worrying about tomorrow. Another question? Yes.

Where and what are these dependant sources? When transistor is moved (again "model") onto the paper, it contains a dependent source. When operational amplifier is modeled, it contains a dependent source. The magnitude of a dependent source is dependent on (or "controlled by") either voltage across of an element in the circuit or current flowing through an element. This means a circuit component determines the magnitude of a dependent source.  Remember independent sources which are cruely independent.  By the way, in modeling, an independent source is indicated by a circle, while a dependent source by a diamond.

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