Operational amplifier (op amp) current source works on a few main principles.
- Load is connected between the output and inverting input (-).
- The output voltage will keep rising until it hits the voltage drop/division of the load plus the voltage at the non inverting input (+).
- Remember, no current goes in or out of the (-) input, other then a very small amount of leakage. It just looks at the voltage, and the output voltage stops rising when (+) and (-) inputs are at the same voltage.
- Inverting input also has a resistor headed to ground, which is what sets the current. Thar resistor is in series with the load.
- Although the (-) is connected between load and current setting resistor, it does not take any noticeable amount of current (just a small amount of leakage), and therefore does not affect them electrically other than to set the output’s voltage across them.
- Since the (-) input has the same voltage across it as the (+) input, thanks to negative feedback, the current setting resistor headed to 0V ground also has the same voltage across it.
- The current through the current setting resistor and the load will be the voltage at (-) divided by the current setting resistor’s resistance.
Video and other important points to be aware of:
- Remember that a single supply op amp maximum output voltage usually falls short of the positive supply voltage by a volt or 2.
- Therefore, the supply voltage should be at least the voltage that will be applied to the current setting resistor, plus the voltage dropped/divided by the load, plus 2 volts due to output loss.
- Negative feedback is when the voltage at the (-) input is affected by the output voltage. A higher output voltage raises the voltage at the (-) input, while a lower output voltage lowers the (-) input.
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