4.4 Class AB Log Domain Filters

Because of their internally nonlinear behaviour, log domain filters can be designed with features that no purely linear filter could ever possess. One such feature is that of class AB operation. Circuit designers familiar with the design of amplifiers, and especially power amplifiers, are used to the idea of so-called 'push-pull' or class AB output stages. These are essentially output stages that have two halves that share the work of driving the load. One half drives the load during positive half cycles while the other half is idle, then the two halves of the circuit switch roles during negative half cycles of the signal. When there is little or no signal present, the halves of the circuit consume very little power. Only when the signal is large does the circuitry work hard to drive the load. This operation results in very power efficient designs. In contrast, class A circuits will have parts that are never idle whether or not the load is being driven. Such circuits are generally quite power inefficient.

Until fairly recently, it was assumed that only 'memoryless' circuits that is, circuits without reactive elements could enjoy the benefits of class AB operation. Seevinck [12] proposed the first class AB dynamical circuit in 1990. At that time he proposed a class AB integrator based on the translinear design principle. Subsequently, this circuit was refined and, ultimately, was shown to be a special case of A log domain filter possessing the...