  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. The presented approach develops a hierarchical
composition graph based on functional blocks 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. 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. An outlook how
this method can be extended to multi-stage op-amps is given.
