Micropower Rail-to-Rail Adjustable Voltage Source
Category: Basic Circuits
Designer: RANA MUHAMMAD ADNAN KHALIL
Notes
This circuit is a variation to the basic connection of a unity gain buffer amplifier. The unity gain amp is used primarily to isolate the i/p from the o/p. In this circuit, the o/p is connected back to the -ve i/p terminal of the amp. Therefore any voltage at the o/p would be forced upon the -ve i/p as well. As the op-amp has very high open loop gain, the o/p would tend to be forced to a value very close to Vin voltage. Any difference in voltage b/w the +ve i/p and -ve i/p terminals would result in an equivalent i/p applied across the inputs of the op-amp, which is then amplified by the open loop gain of the op-amp. By using rail-to-rail op-amp, the i/p voltage Vin can be at any voltage b/w V+ and V- supply rails. The o/p of the rail-to-rail op-amp generally has a very low o/p resistance, which, when connected to a load resistor, would result in an o/p voltage that can reach from within a few millivolts to a few 10th of millivolts from the supply rails. The actual o/p voltage achievable depends on the o/p impedance of the op-amp selected and the current it has to supply to the load resistance. I/p to the +ve i/p terminal is set by the potentiometer. The potentiometer can be of almost any available value, due to high i/p impedance of the op-amp (Rin >~ 1000Gohm). Generally a potentiometer 1Mohm or higher is used to limit the power consumption across the potentiometer. Recommended IC's are ALD1706, ALD1721, ALD1726, ALD2711, ALD2706.