Many electronic systems require a constant voltage reference to operate reliably, hence the need for power supplies and voltage regulation circuits. The figure below is an example of using a voltage regulator to maintain a stable output voltage with a changing input voltage. This module focuses on using op-amps with feedback mechanism as well as switching regulators. We first learn voltage reference specifications including load and line regulation, then we take a look at some basic op-amp voltage reference design principles.
With op-amp circuit characteristics, and various feedback configurations, op-amps become a good choice in designing voltage regulation systems. The figure below demonstrates the concept of low dropout regulator (LDO) to maintain a stable output voltage at the load. A low drop-out regulator is one type of linear regulator that operates by a feedback network with sensing circuits. As the load changes, so will the sense voltage (Vsense). The feedback circuit captures this "error" and corrects it in a closed-loop circuit.
The figure below shows the detailed operation of a LDO regulator using an op-amp, sense resistor network, and transistors. For a given input voltage (Vin), the output (Vout) connects to a resistive load (RLoad). When the load changes, in this particular case, due to system variations, Vout falls. This in turn causes Vsense between R1 and R2 to decrease. The op-amp (error-amplifier) is used to capture the error voltage, which controls transistor Q1 and Q2. Because Vsense is reduced, and it compares to an internal reference voltage, the op-amp ouput goes up further enhancing Q1 and Q2. As a result, the load current flowing to the load increases. The final result is that Vout increases to the desired value. This feedback mechanism occurs throughout the whole time as long as line and load regulation are within the specifications.