on-based (specification) models link to (possible) circuit implementations through a number of well-controlled approximations. A like- wise approach towards constructing good models is pursued in chapters 3 and 4. 2.1.3 The importance of good models in top-down design circuit transfer function small-signal equivalent Figure 2.2: Transfer functions derive their status as a powerful concept to model and specify circuit behavior from the fact that they adequately describe the behavior of a circuit's small-signal approximation. As a final note, it should be stressed that all good models should be as compact as possible. Overly detailed descriptions bear tedious computations and lengthy simula- tions with them. They do not contribute much to design efficiency. Irrelevant model behavior, for instance, involves non-dominant nonidealities or high-frequency tran- sients. Removing irrelevant details from a model is very important, e.g. for efficient optimization-based design. 2.1.3 The importance of good models in top-down design As discussed in chapter 1, minimizing the time necessary to design analog frontends requires the introduction of hierarchy in the design process². As illustrated in Fig. 2.3, a hierarchical, top-down method initially tackles frontend design in terms of low-noise amplifiers, filters, mixers, oscillators, A/D converters, etc. In turn, these blocks are implemented using integrators, operational amplifiers, etc. In a next step, circuit- and transistor-level details are filled out. Beyond the circuit level, there is layout and man- ufacturing. For CAD support of such a hierarchical flow, good models are of utmost importance. They are among the cornerstones in realizing each of the steps in the design flow hi- erarchy. The models are used as interfaces to communicate design decisions between the different levels in the design tree. Many actions in a top-down design flow result in specification and/or implementation models that are passed down/up the design hierar- chy. As illustrated in Fig. 2.1, system