CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

esistances of the feedback amplifier are
2.4 Circuit Analysis Techniques 30
and we arrive at the same results obtained using nodal analysis. In addition, how-
ever, we also gain some degree of insight into the role of feedback in this circuit,
and have determined the loop gain to be a measure of the amount of feedback used
in the circuit.
Rin = ( 1 + Aβ)RinA
– Rs = 36.62 × 4678Ω – 1kΩ = 170kΩ
R out
R outA
345.5
= ----------------- = ------------Ω =
9.43Ω
1 + Aβ 36.6
Unfortunately, there are numerous limitations with this technique. Firstly, we
face the challenge of trying to make all circuits conform to the classical feedback
structure. As such, this technique is limited to circuits with a single or dominant
feedback loop. In addition, we must identify the topology that best reflects the feed-
back sampling and mixing mechanisms of the circuit. Often this is not apparent. For
circuits that do not approximate the ideal feedback structure, all nonidealities such
as loading effects must be accounted for by adding port parameters to the principle
blocks. The values of these parameters are obtained through a process of modifying
the original circuit and deriving various characteristics under short-circuit and open-
circuit conditions. This process is involved and prone to mistakes. Lastly, this analy-
sis implicitly assumes that both the forward amplifier and feedback network are uni-
lateral. In other words, we assume that the signal traverses forward only through the
amplifier and the output is fed back only through the feedback network. With many
practical circuits, this is an assumption whose validity is difficult to ascertain, and
should the underlying assumptions prove to be inaccurate, there is essentially no
recourse. With this method, the accuracy of every analysis must be verified using
exact analysis or computer simulation. As such, this method is not exact and is best
thought of as an intuitive aid to understanding feedback circuits.