  This paper presents a method to automatically synthesize the structure of an
operational amplifier. It is positioned between approaches with fixed design
plans and a small search space of structures and approaches with generic
structural production rules and a large search space with technically
impractical structures. Based on [1], the presented approach develops a
hierarchical functional composition graph that spans a search space of
thousands of technically meaningful structure variants for single-output,
fully-differential and complementary operational amplifiers. The search
algorithm is a combined heuristic and enumerative process. The evaluation is
based on circuit sizing with a library of behavioral equations of functional
blocks [2]. Formalizing the knowledge of functional blocks in op-amps for
structural synthesis and sizing inherently reduces the search space and lessens
the number of created topologies not fulfilling the specifications.
Experimental results for the three op-amp classes are presented.
