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- From: carmijo@hpnmdla.sr.hp.com (Chris Armijo)
- Newsgroups: sci.electronics
- Subject: Re: Help: What is an Independent Current Source!?
- Message-ID: <7380093@hpnmdla.sr.hp.com>
- Date: 9 Nov 92 17:30:01 GMT
- References: <Nov.9.00.11.39.1992.9368@clam.rutgers.edu>
- Organization: Hewlett-Packard, Santa Rosa, CA
- Lines: 35
-
- In sci.electronics, steuer@clam.rutgers.edu (robert Steuer) writes:
-
- I understand an Independent Voltage source is like a battery and I
- understand that an Independent Current source can supply a current,
- but what is a REAL-LIFE example of an Independent Current Source?
- I have asked fellow students, my teachers, even a teacher with a
- Doctorate Degree and still I get hmmm, thats a good question!
-
- I can think of two examples of current sources:
- 1) A bench power supply which has a constant-current output. It's
- true that the vast majority of power supplies are set for constant
- voltage (like a battery), but you *can* purchase supplies which
- will output a constant current (over some given voltage range).
- You may have seen adds for the new HP-E3610 supplies. These
- have constant voltage or constant current outputs.
-
- 2) The most common example of a circuit-level current source is
- the collector output of a bipolar transistor. Bipolar transistors
- typically have output resistances on the order of 10-20k ohm.
- While this doesn't seem like a lot, it's enough for them to
- be considered "current sources" to a first approximation.
- If you look at a schematic for an op-amp, you will see many examples
- of this -- where the collector of a transistor is feeding another
- circuit with a relatively constant current.
-
- The second example brings up a good point. In reality, there isn't any
- such thing as an ideal current source, just like there are no ideal
- voltage sources. All sources will have some finite impedance. Whether
- you call a source a "voltage source" or a "current source" really
- depends on how you are using it. In general, if the source impedance
- is much lower than the circuit you are driving, then you call it a
- voltage source. If the source impedance is much higher (like the
- second example above), then you call it a current source.
-
- Chris
-