Design considerations for BJT active mixers

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Abstract
This chapter has discussed the design of various types of receiver mixers, starting with basic discrete forms, then moving on to classical topologies. The benefits of emitter degeneration to improve linearity were quantified, after which we showed the advantages afforded by the bipolar 'multi-tanh' concept. This circuit concept has been extensively used by the author, and is now proving useful in extending the dynamic range in many contemporary communications ICs. Multi-tanh gm stages have the further advantage of allowing accurate variable gain through the control of the bias current, not possible with many mixer designs. Finally, the topic of noise in the switching core was briefly discussed. Other novel mixer topologies are now being used to extend the performance of active mixers even further. In the future, it is expected that quite different approaches to mixer design, based on sub-micron CMOS processes, will become commonplace, though it is difficult to forecast whether (and if so, how soon) such techniques will eclipse bipolar technologies in RF applications. With increasing interest in short-range transceivers operating at frequencies of between 2 and 30GHz, and the benefits of heterojunction bipolar transistors in this domain, it will be interesting to see how this age-old rivalry will develop.

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