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CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

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2.4 <strong>Circuit</strong> <strong>Analysis</strong> Techniques 28<br />

Direct nodal analysis is mathematically exact and straightforward to solve with<br />

the aid of calculators or computers capable of matrix operations. However, notice<br />

that the solution required the substitution of numbers into the parameters; had we<br />

kept the parameters and performed a symbolic analysis of the circuit, the resulting<br />

expressions would have been very complex and far too cumbersome to provide<br />

much insight into the circuit’s operation.<br />

Topology-Based Feedback <strong>Analysis</strong><br />

v s ′<br />

R out<br />

Greater insight into the effect of feedback on this amplifier can be obtained<br />

using topology-based analysis. Although other interpretations are possible, the feed-<br />

back amplifier in Figure 2.13 can be seen as series-shunt configuration in which the<br />

output voltage is sampled and then mixed back to the input as a voltage signal. Fol-<br />

lowing the process outlined in [Sedra,1998], we can partition the circuit into a for-<br />

ward amplifier and feedback network as shown in Figure 2.14.<br />

a)<br />

R s<br />

v be1<br />

+<br />

-<br />

g m v be1<br />

r e<br />

R ||<br />

E R f<br />

v c1<br />

R 1<br />

r π<br />

v o<br />

= -------- = 9.43Ω<br />

i test<br />

g m v c1<br />

v o ′<br />

R || 2 ( R f + RE) Figure 2.14 Topology-based feedback analysis: small-signal circuits for<br />

a) forward amplifier and b) feedback network.<br />

b)<br />

v f ′<br />

R E<br />

R f<br />

v o ′

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