New Port Modeling and Local Biasing of Analog Components

A new approach for biasing analog circuits is introduced in this chapter. This approach is an attempt to address some of the biasing complexities that exist today in biasing large analog circuits. There are three steps involved in this methodology. First, in circuit analysis, the methodology separates nonlinear components (transistors), particularly drivers, from the rest of the circuit. Second, it uses local biasing introduced in the previous chapter to bias the transistors individually and to the specs provided for the design. Finally, the method presents a new way to change the local biasing into normal (global) circuit biasing with choices of DC supplies at right locations in the circuit. It is the last step that will be our main topic of discussion in this chapter. Here we see how we can remove all sources related to the local biasing and replace them with normal circuit supplies without altering the design specifications. These circuit supplies can be voltage sources, current sources or mirrors. In case the supplies are already specified and in place, this method can still maintain the design specs by re-evaluating some of the power-conducting components in the circuit. Power-conducting components are those circuit components, such as resistors, that conduct DC power (current) from the power supplies to the circuit drivers (transistors), for biasing purposes. Limitations in local Biasing We fully discussed local biasing, its properties and applications in the previous chapter. Despite all the advantages that local biasing offers one problem still remains unresolved and that is: how to deal with so many DC sources generated due to local biasing, known as distributed supplies? To see the problem, just take a single bipolar transistor: it normally needs four (voltage and current) sources to get locally biased; however, with coupling capacitors used this number reduces to two current sources and two capacitors (taking care of the voltage drops). Similarly, we may need to use four sources to locally bias a MOS transistor. Again, with coupling capacitors this number can get as low as one source-drain current source. The problem, however, is that for the gate, and possibly the substrate, the coupling capacitors need to have charging paths (a resistive path to a DC supply). One way to handle the case and bring the number of DC supplies down to a minimum of one or two is to use source transformation and replacement techniques, such as voltage dividers, Δ-Y transformation, and current sources/mirrors. Nevertheless, the sheer number of such sources in a fairly complex circuit can get so high that unless we find a shortcut to the final solution the validity of local biasing as an effective methodology is undermined.