Chapter 4 continues with the implementation of the filter. In chapter 1 it is concluded that one of the basic building blocks is the integrator. In this chapter the design of basic integrator implementations is treated. For this the small-signal models of the transistors (amplifying stages), that are to be used to design the active part (opamp) of the integrator, are treated. These small-signal models are required to analyze the performance of a designed active part.