Current source programs an output current as defined by the user, regardless of the load resistance. We know that when load resistance is larger, the voltage across it also increases. An ideal source would maintain a constant amount of current even if the load resistance changes, or in other words even if the output voltage changes. Such behavior describes a very good current source that has high output impedance. Remember, the output voltage changes, but very little or no change in output current. That means high output resistance or impedance. We use the term impedance when dealing with changes. Howland circuit in the figure is a classic current source circuit. When R 1 is made equal to R 2 , R F equal to R 3, the output current is given by V IN /R 1 where V IN is V 1 minus V 2 . A simple implementation is grounding V 2 and taking V 1 as the V IN . Figure 1 Basic Howland Current Source This circuit is such a clever manipulation of the op a...
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