Rail-to-Rail Voltage Follower (Buffer)
Category: Basic Circuits
Designer: RANA MUHAMMAD ADNAN KHALIL
Notes
The unity gain amp is used primarily to isolate the input from the output. The key characteristics of this circuit are that the i/p impedance is required to be very high at the i/p and the o/p impedance at the o/p is very low. In this configuration, the o/p is connected back to the -ve input terminal of the amp. Thereforre any voltage at the output would be forced upon the -ve i/p as well. As the op-amp has very high open loop gain, the o/p voltage would tend to be forced to become very close to Vin voltage. Any difference in voltage b/w the +ve i/p and the -ve i/p terminals would result in an equivalent i/p voltage applied across the inputs of the op-amp, and which then is 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 the V+ and V- supply rails. The o/p of the rail-to-rail op-amp generally has a very low o/p resistance when, when connected to a load resistor, would result in an o/p voltage that can reach from within a few mV to a few 10th of mV 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. Recommended IC's ALD1701, ALD1702, ALD1704, ALD1706.