How does Derrida's discussion of signs (signifier/signified) differ from de Saussure's notion of the sign?

Saussure laid a ground work for the field of semiotics, or the study of signs, in advancing his concept of signifier/signified to create the sign. To Saussure, the signifier is the sensorial form (word, spoken or read, image, physical thing, etc.), whereas the signified is the concept. These two things interrelate to become the sign, which creates meaning for the interpreter of the signs.
Derrida went a little further with his analysis of the concept, presenting several key concepts that are crucial to understanding his theory of Deconstructionism:
Logocentrism refers to a hierarchy of sorts in the interpretations of signs. He argues there is a primacy of speech, based on the immediacy of the transaction between speaker and hearer(s). Writing is a step removed, in that there is a transactional space between a signifier being written down and a reader encountering it to imbue it with meaning. It is not signified until it is read, which delays the transaction of meaning-making.
What complicates this further is Derrida's concept of trace, which is the absence surrounding a sign. In other words, when a signifier is written, the signified carries meaning derived from both the context (other words surrounding it or relative concepts such as tree/bush), as well as all the other meanings that are not present. To Derrida, the sign is perennially elusive, in that meanings are always changing for the person encountering the sign. With regard to writing, this is even more true as the transactional delay between signifier/signified (time of writing/time of being read) allows for more time and meaning-making to occur that can shade the understanding of the sign.
He coined the term différance to highlight these concepts, as it is an intentional misspelling of différence. In French, differer can mean to defer or to differ. Because there is no auditory distinction in words, there is abstraction in the meaning. However, in misspelling the term for his purposes, Derrida calls attention to the deferral of meaning-making in the transactional delay of written language, as well as the ever-changing associations that make up the signified.
In sum, while Saussure's foundation of semiotics creates the framework to understand that a symbol derives meaning from the representation of it (which we may sense through sight, sound, touch, etc. as well as the contextual understandings of that signifier), Derrida creates a framework of micro-analysis which examines the elements of meaning-making so thoroughly that it allows for the concepts influencing the meaning-making to remain in permanent flux and establishes a rationale for signs being interpreted not only on the basis of the elements of the sign that are present, but also as much by which elements are absent.