As mentioned earlier, a linear regulator is a type of integrated voltage regulator that uses a transistor or FET that operates in a linear region to subtract excess voltage from the applied input voltage to produce a regulated output voltage. The function of the linear regulator is to provide a stable DC output voltage when the input voltage or load changes within the specified operating range, and to ensure that the regulator circuit can work safely and reliably for a long time.
A common integrated linear regulator is also called a standard linear regulator or an NPN linear regulator. The standard linear regulator is mainly composed of a basic part such as a reference voltage source, a sampling circuit, an error amplifier, and an adjustment tube.
The basic working principle of standard linear regulator
The basic principle of the standard regulator is shown in Figure 1. The series regulator is a Darlington tube composed of NPN transistors VT2 and VT3. VT1 is a drive tube that uses a PNP transistor. U1 is the input voltage and U0 is the output voltage. R1 and R2 are the sample-receiving resistors, and the sampling voltage UQ is applied to the non-inverting input terminal of the error amplifier. Compared with the reference voltage UURE applied to the inverting input terminal, the difference between the two is amplified by the error amplifier to generate an error voltage Ur. Adjust the voltage drop of the series regulator to stabilize the output voltage. For example, when the output voltage U0 decreases, both UQ and UR decrease, and as the drive current increases, the voltage drop of the adjustment tube decreases, causing the output voltage to rise.
Conversely, if the output voltage U0 rises, the drive current output from the error amplifier decreases, and the voltage drop of the adjustment tube increases, causing U0 to drop, and finally U0 remains stable. Since the feedback loop always tries to equalize the potentials at the two inputs of the error amplifier, ie U0=U REF , according to the above figure, we can get:
Need to explain a few points:
First, the output voltage is controlled by a feedback circuit that requires compensation to ensure loop stability. Some linear regulators have a built-in compensation circuit that eliminates the need for external compensation components to achieve stable operation of the linear regulator. Some linear regulators require an external compensation network.
Second, the feedback loop for controlling the output voltage "judges" the output voltage through the sampling resistor, and sends the sampled voltage to the non-inverting input of the error amplifier, and the reference current is connected to the inverting input. This means that the error amplifier will equalize the sample voltage to the reference voltage by constantly adjusting its output voltage and adjusting the tube current. The output voltage of a linear regulator is typically several times the reference voltage.
Third, the current through the resistor dividers R1 and R2 is negligible compared to the load current.
Fourth, to squib the tube VT1, a PNP transistor must be used. This is because the emitter junction voltage UBE of the NPN transistor is a positive voltage, and U B >U E , that is, U B >U 1 is obviously unreasonable; The U BE of the PNP tube is a negative voltage and can satisfy the requirement of U B <U E , that is, U B <U 1 .
Fifth, Figure 1.2.2 is only a simplified circuit. In the actual circuit, a starting circuit, an overcurrent protection circuit, and an overheat protection circuit are also required.
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