A linear regulator is a type of integrated voltage regulator that uses a transistor or FET operating in the linear region to reduce excess voltage from the input, providing a stable and regulated output voltage. Its primary function is to maintain a consistent DC output voltage despite variations in input voltage or load conditions, ensuring safe and reliable long-term operation of the circuit.
One common type of linear regulator is known as a standard linear regulator or NPN linear regulator. These devices typically consist of key components such as a reference voltage source, a sampling circuit, an error amplifier, and an adjustment transistor. The basic working principle involves feedback control to regulate the output voltage effectively.
The standard linear regulator operates based on a feedback loop. As shown in Figure 1, it includes a series regulator made up of NPN transistors VT2 and VT3, while VT1 serves as a drive transistor using a PNP configuration. The input voltage is U1, and the output voltage is U0. Resistors R1 and R2 form a voltage divider that samples the output voltage, generating a sample voltage UQ, which is fed into the non-inverting input of the error amplifier. A reference voltage UREF is applied to the inverting input. The difference between these two voltages is amplified by the error amplifier, producing an error voltage Ur, which controls the voltage drop across the series regulator to stabilize the output.
If the output voltage U0 drops, both UQ and Ur decrease, leading to an increase in the drive current and a reduction in the voltage drop across the adjustment transistor, thus increasing U0. Conversely, if U0 rises, the drive current decreases, causing the voltage drop of the adjustment transistor to increase, which brings U0 back down. This continuous feedback ensures that the output voltage remains stable, with U0 essentially equaling UREF.
Several important points need clarification:
First, the feedback circuit requires compensation to ensure stability. Some regulators include internal compensation, eliminating the need for external components, while others require an external network for proper operation.
Second, the feedback loop monitors the output voltage via a resistor divider, sending the sampled voltage to the non-inverting input of the error amplifier, while the reference voltage is connected to the inverting input. The error amplifier adjusts its output to balance the two inputs, ensuring the output voltage is proportional to the reference voltage.
Third, the current through R1 and R2 is typically much smaller than the load current, making their impact negligible.
Fourth, the use of a PNP transistor for VT1 is essential because its base-emitter voltage (UBE) is negative, allowing the base voltage to be lower than the emitter, which is necessary for proper operation.
Finally, the circuit shown in Figure 1.2.2 is a simplified version. In real applications, additional circuits such as a start-up circuit, overcurrent protection, and thermal shutdown are required to enhance reliability and safety.
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