CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

4.5 Analyzing Transistor Circuits 102
recognizing the feedback can be traced to the unintended use of the Source Absorp-
tion Theorem. Using this theorem has the unfortunate consequence of hiding the
feedback because the transconductance element which feeds back a current in
response to a voltage is represented only as a resistance. DPI/SFG analysis allows
us to make a distinction, in essence bringing out lost details in the circuit’s opera-
tion.
EXAMPLE 4.5 A CASCODE CURRENT MIRROR
This example illustrates the type of insight that DPI/SFG analysis provides in
contrast to traditional nodal analysis. We wish to determine the output impedance of
the MOSFET cascode current mirror shown in Figure 4.27. We begin by setting the
dependent current source of transistor M 2 to zero, and obtain the small-signal dia-
gram shown in Figure 4.28.
Solution using Nodal Analysis
tions:
I ref
V bias
V out
Using Kirchhoff’s Current Law at nodes out and x, gives us the following equa-
M 1
V x
M 3 M2
I out
Figure 4.27 MOS transistor cascode current mirror.
iout + gm1v x + ( vx – vout) ⁄ rds1 = 0
– gm1v x + ( vout – vx) ⁄ rds1 =
0
(4.17)
(4.18)