Analogue design finds analogies between the physical behaviour of groups of electronic components and the operations needed to construct the desired system. To allow this exploitation of the natural behaviours of groups of components, internal signals are mostly of continuous-value, in contrast to digital design. Although not unprincipled, analogue design is often thought of as more of an `art' compared to digital design. Even many predominantly digital systems require some analogue circuitry at least to deal with the interface to the world.
Analogue design can extract more functionality from the components than can digital, because more of the components' behaviour is put to use. Especially potent is the use of real time, rather than representing time as a computational variable like any other, as in digital systems [24,25]. For some tasks (where `task' includes nonbehavioural requirements) analogue design is clearly superior. In other cases, the choice of analogue or digital design is also strongly influenced by the ease of the design process itself, which often favours digital.
The disadvantages of analogue design are the obverse of the advantages of digital design identified above. Although system-level design can be at an abstract level, analogue circuit design must necessarily consider properties of the physical components, and their interactions, in greater depth than for digital design. The second disadvantage is that stability and large noise margins are more difficult to guarantee.
Sarpeshkar [26] shows that as the complexity of an analogue system increases, it becomes essential for signal values to be restored to attractor states occasionally, otherwise all of the signal would eventually be swamped by unavoidable noise. This signal restoration does not happen at every circuit element, as it does in digital systems, and the representation of the analogue signal may be distributed across more than one physical voltage, current, or charge. There is flexibility in the choice of restoration schemes, not limited to two attractor states, and not necessarily uniform throughout the system. Sarpeshkar provides a costs/benefits analysis for some restoration schemes under various resource constraints and operational envelopes, with reference to neural systems; it appears that there is great untapped potential in analogue (or `hybrid') electronics, if new design styles could be developed.
In practice, analogue design is currently based around the use of `building block' subcircuits. These have been carefully designed and analysed in the past to perform generally useful functions, and form a `cookbook' with which a larger design can be constructed. A designer takes building blocks from textbooks, and from looking at other circuits designed for similar problems. Examples of building blocks are amplifiers, filters, oscillators, voltage or current sources, and many others. The original design of a building block can be very challenging, often beyond the capabilities of designers who later use them. Without building blocks, the design of complex analogue systems would be too difficult.
The use of building blocks is not only to avoid `re-inventing the wheel'
for each design. By compartmentalising the circuit into functional
packages with well-defined interfaces, it becomes easier to make the whole
circuit robust across an operational envelope. Taking thermal stability as
an example, Fig. 3 shows a hypothetical system composed of
building blocks. The enlarged subcircuit is a standard current-sink
building block [27]. Transistor Q2 serves solely to compensate for
the thermal variation of the base-emitter voltage of Q1. Under
certain well-analysed conditions, this building block can be used to serve
the function of a current-sink over a range of temperatures, irrespective
of what the rest of the system is doing. If all of the other building blocks
are similarly endowed with thermal stability of function, and are composed
so as not to violate their constraints of operation, then the whole circuit
can be given thermal stability. Although there are still pitfalls, the design
of complex robust analogue circuits would be practically infeasible without
this approach.
