CMOS Logic and Current Mode Logic

This chapter describes basic logic circuits. This can be seen as largely background material for applications in later chapters in the design of dividers and phase detectors in frequency synthesizers. The types of logic discussed will be CMOS rail-to-rail logic, CMOS current mode logic (CML), bipolar CML, and bipolar emitter coupled logic (ECL). Note that CML is a general term and applies to both bipolar and CMOS; however, with metal oxide semiconductor (MOS) transistors, it is often called MOS current mode logic (MCML). ECL is the name often given to bipolar CML that has emitter followers as the output stage transistors. At low frequencies, CMOS rail-to-rail is preferred for its simplicity and low static power dissipation, while, at higher frequencies, CML or ECL is used, as they can operate faster with lower power because of the reduced output swing. As shown in Figure 5.1, when it is not switching, CMOS rail-to-rail logic does not consume any current, while CML does. CMOS rail-to-rail logic consumes current only during transitions, and its power consumption is proportional to the operation frequency. CML bias current must rise as the speed of switching increases, just as CMOS rail-to-rail logic does, but it does so at a slower rate. Thus, above some frequency, CML becomes a lower-power solution. CMOS rail-to-rail logic is differential and, therefore, has good power-supply rejection, which is preferred in many synthesizer applications. Various types of logic and some important design trade-offs are summarized in Table 5.1.