Puzzled about current producing devices ??

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Puzzled about current producing devices ??

Post by jalbers » Tue Jan 08, 2002 1:13 pm

I have been studying various op-amp circuits and I have come across the current to voltage convertor circuit. I am slightly puzzled by the term "current producing device" in the circuit. Wouldn't there also be some sort of voltage produced across this device's terminals?

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Chris Smith
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Re: Puzzled about current producing devices ??

Post by Chris Smith » Tue Jan 08, 2002 2:29 pm

You cannot have one with out the other, as far as voltage and current. What your referring to is a "change" in either voltage or current, and the op amp is designed specifically to sense this, but not necessarily the other, [before its too late] and alter the out put accordingly. For example, if you had a long line transmission that needed to keep the voltage above a given volt value, you would use a sense circuit to measure and maintain that exact voltage. On the other hand if you had a sensing circuit that was designed to maintain 80 milli amps to a laser diode at any voltage within the power supply range, you would use a current supply sense op amp. Some things are current sensitive while others are voltage sensitive.

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Re: Puzzled about current producing devices ??

Post by russlk » Tue Jan 08, 2002 2:52 pm

A transistor is a current producing device. In other words, if you change the collector voltage (within it's operating range), the collector current does not change much. A current source is characterized by high output impedance and should produce the same current, regardless of the voltage across it, but real devices have limits. The 1N5305 is a current source diode, which is a FET whose zero gate voltage drain current is controlled and the gate is not accessable.

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