Modern RF (radio frequency) communication units, such as cellular telephones and conventional and trunked RF transceivers, generally employ frequency synthesizers rather than depending upon well-known crystal control technology. With a frequency synthesizer, reconfiguring a communication unit for operation on a new frequency is a simple matter of reprogramming, whereas, with crystal control, a new pair of crystals is generally needed.
In cellular telephone communication systems, it would simply be impracticable to provide a communication unit with the enormous set of crystals required to cover a cellular system's 866 possible operating frequencies. Even trunked communication systems often employ a large number of possible operating frequencies. Furthermore, users of conventional RF transceivers have come to view synthesized communication units as desirable, since the ability to reprogram operating frequencies makes it easy to move communication units from system to system.
As is well known, a frequency synthesizer system is only as good as its reference oscillator. A synthesizer's accuracy and stability is directly traceable to the accuracy and stability of the reference. Much work has been done to improve reference oscillator design, to the point where excellent accuracy and stability over temperature have been achieved.
Even after a reference oscillator design has been perfected, however, the reference output must be buffered and distributed to various subsystems within a communication unit. For small, battery powered communication units, such as portable cellular phones, at least two important design considerations immediately surface. First, the reference oscillator and associated buffering and signal distribution circuitry must use minimal power in order to conserve battery life. Second, the distributed reference oscillator signal must have excellent spectral purity to minimize spurious signals and attendant denigration of system operation.
Reference signal buffer amplifiers are generally biased for class AB operation in order to limit output stage power consumption. Furthermore, since space is often at a premium in hand-held communication units, most subassemblies, including reference oscillators and associated output buffers, are implemented using integrated circuit (IC) or hybrid designs. This need to conserve space dictates that transistor geometries, particularly in low current applications such as bias networks, be kept as small as possible.
Unfortunately, the small geometry and low operating current of transistors used in bias networks yield poor noise characteristics that degrade overall amplifier noise figure. Accordingly, a need arises for a buffer stage that will exhibit good noise performance despite size and power constraints.
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