The Basic Linear Shunt Regulator is similar to the basic linear regulator with a few differences. The main difference is that the Linear regulator uses a series pass transistor for a control component, while the Basic Linear Shunt Regulator uses a transistor in a parallel with the load for its control component. There are 4 main parts to this type of regulator which are; a reference voltage, error detection, a control element (the shunt), and a sample circuit.
R1 is placed in series with the load, and works like a series regulator but instead regulates by regulating the current passing through the control element (Q1). When the value of the output goes to decrement due to the change in input voltages or load current made from the variation in the load resistance it can be observed from resistors R3 and R4. The resultant voltage decreases the op-amp output to decrease its collector current value while the collector voltage rises. This allows for the variation in voltage to be overcome and keep the value of the output steady and constant.
H. (2021, January 30). Introduction to Linear Shunt Regulators – The Engineering Knowledge. The Engineering Knowledge. https://www.theengineeringknowledge.com/introduction-to-linear-shunt-regulators/