1. Three-terminal voltage regulator:

The output transistors of a typical three-terminal regulator (also known as a standard regulator) use NPN transistors or N-channel MOSFETs.

This type of regulator requires a difference in input-output voltage, called dropout voltage (VDO), to operate, as shown in the diagram below. For NPN voltage regulator, the minimum value of VDO must satisfy VIN-VOUT>RIN×IIN+2×VBE.

For regulators using MOSFETs, the minimum value of VDO must satisfy VIN-VOUT> RIN×IIN+VGS. Suppose RIN = 1kΩ, IIN = 1mA, VBE = 0.7V, VGS = 1V. Then, the calculated minimum input voltage required to generate a 5V output voltage is 7.4V for an NPN regulator and 7V for a MOSFET-based regulator.

2. LDO regulator:

LDO regulators mainly use PNP transistors or P-channel MOSFETs as output transistors. The minimum dropout voltage of this LDO regulator is determined by the collector-emitter voltage (VCE(sat)) and the drain-source voltage (VDS=RDS(ON)×ID). Therefore, LDO regulators operate with less dropout voltage than three-terminal regulators.

There is also a newer LDO regulator with even lower dropout voltage. It uses an N-channel MOSFET with an on-resistance smaller than that of a P-channel MOSFET as an output transistor, and is equipped with a power supply pin dedicated to a control circuit (including a MOSFET drive circuit).

3. Construct an oscillator:

LMS1587CT-3.3 is originally a series voltage regulator integrated chip, which maintains a stable 5V voltage between its output and GND. Actually the output forms a follower with GND. Using this one, LC can be added on the basis of its output to form a sinusoidal oscillator circuit. The following experiment will verify this idea.

schematic diagram

Experimental results

LMS1587CT-3.3 is originally a series regulated power supply, but it can actually be regarded as an emitter follower with a large current gain between its output and input. Therefore, taking advantage of this feature, an LC circuit can be used to form a Colpits oscillator based on it.