CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

v i
R in
v o
R out
4.4 Determining Port Impedances 89
Figure 4.10b shows the schematic of the unity-gain buffer complete with the op
amp’s internal model, and test current sources i ti and i to used to measure the input
and output resistances respectively. The circuit has three nodes so we use the exist-
ing voltage source v x , and place two auxiliary sources v i and v o at the input and out-
put of the amplifier. In contrast to EXAMPLE 4.2, all voltage sources are referenced
to ground. As such, calculating the short-circuit currents is much easier, and the
intermediate SFG that is shown in Figure 4.11 can be readily determined by inspec-
tion. The final SFG shown in Figure 4.12 is obtained by explicitly representing the
constraint on voltage source v x ,
i ti
-
rid vid
+
v i
vx =
Avvid a) b)
Figure 4.10 a) Unity-gain buffer, b) Schematic showing internal op amp model, and
the test current sources for determining the input and output resistance.
vx = Avvid = Avvi – Avvo 1 ⁄ ro iti isci rid vi 1 ⁄
rid r || id ro isco v x
i to
1 ⁄ rid Figure 4.11 Signal-flow graph for unity-gain circuit prior to adding constraints
on dependent sources.
.
v o
r o
v o
i to