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CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

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4.4 Determining Port Impedances 93<br />

sented by the various transmittance branches. For instance, transmittance rep-<br />

resents the overall transfer function from the controlled source w to the short-circuit<br />

current node of the port. In practice, these transmittance branches are determined<br />

through collapsing appropriate portions of the signal-flow graphs of actual circuits.<br />

i p<br />

i SCp<br />

Y pp<br />

Z DPp<br />

Figure 4.15 General SFG of feedback amplifier in Figure 4.14.<br />

To relate the SFG in Figure 4.15 to Blackman’s Impedance Formula, we need to<br />

re-represent the graph in terms of the open-loop impedance, Z° . When the feedback<br />

is disabled, we obtain the graph shown in Figure 4.16a. The graph can be simplified<br />

down to Figure 4.16c where Z° is given by<br />

i p<br />

i SCp<br />

a)<br />

Y pp<br />

Z DPp<br />

vp Txp T px<br />

Z o<br />

i p<br />

v p<br />

T xp<br />

T wp<br />

T px<br />

The feedback amplifier can now be represented by the alternative SFG shown in<br />

Figure 4.17. <strong>Using</strong> Mason’s Direct Rule to find the port impedance, v p ⁄<br />

i p,<br />

we see<br />

x<br />

Z DPp<br />

T wx<br />

k<br />

w<br />

= --------------------------------------------------------------- .<br />

1 – Z DPp( Y pp + T pxT xp)<br />

x<br />

i SCp<br />

Z o<br />

c)<br />

i SCp<br />

Z DPp<br />

T wp<br />

Figure 4.16 Collapsing the SFG of the feedback amplifier with feedback<br />

disabled.<br />

v p<br />

i p<br />

T px x<br />

Y pp +T px T xp<br />

b)<br />

v p<br />

T px<br />

x

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