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CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

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4.2 <strong>Circuit</strong> <strong>Analysis</strong> <strong>Using</strong> Driving-Point Impedances 79<br />

The representation of a node voltage as a Z DP × I SC product can be used to<br />

analyze entire circuits. As a first attempt, the procedure can be expressed in the fol-<br />

lowing steps:<br />

1. Attach auxiliary voltage sources, vn , to each node of the linear circuit network.<br />

2. Determine the driving-point impedance and short-circuit current of each<br />

node.<br />

3. Finally, the circuit equations are represented by the DPI relations for each<br />

node,<br />

vn = Z DPn × I SCn<br />

(4.3)<br />

and by the relations that dictate the values of all sources, both dependent and<br />

independent.<br />

Ultimately, the concept of placing auxiliary voltage sources is not essential. The<br />

idea is simply a convenient paradigm for guiding our analysis and allowing us to<br />

apply our traditional understanding of superposition.<br />

EXAMPLE 4.1 A SIMPLE LINEAR CIRCUIT<br />

We can demonstrate DPI analysis with a simple example. Consider the circuit in<br />

Figure 4.3, and determine the node voltages given that = 1Ω , Rb = 5Ω , and<br />

I dc<br />

= 1A .<br />

I x =<br />

2V b<br />

V 1<br />

Begin by placing auxiliary voltage sources at nodes 1 and 2. When current source<br />

I dc is zeroed, nodes 1 and 2 are isolated. As such, the driving-point impedances are<br />

R a<br />

R a<br />

I<br />

1 dc<br />

2<br />

Figure 4.3 Simple circuit example to demonstrate DPI analysis.<br />

V b<br />

+<br />

-<br />

R b<br />

V 2

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