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Radio Frequency Integrated Circuit Design - Webs

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Voltage-Controlled Oscillators<br />

Figure 8.15 Feedback analysis of a Colpitts common-base oscillator.<br />

vc is the output of the oscillator, we can define forward gain as vc /ve and<br />

feedback gain as ve ′ /v c .<br />

Forward Gain<br />

The forward gain is<br />

H1(s) = vc<br />

v e<br />

= g m Z tank<br />

259<br />

(8.27)<br />

where Z tank is defined in Figure 8.16 and has the following transfer function:<br />

Z tank =<br />

Z tank = Z L ||Rp || Z FB =<br />

1<br />

j�L<br />

1<br />

1<br />

+<br />

R p + j�C 1(1 + j�r e C 2)<br />

1 + j�re (C 1 + C 2)<br />

j�LR p [1 + j�re (C 1 + C 2)]<br />

(8.28)<br />

(R p + j�L)[1 + j�re (C 1 + C 2)] + j�LR p � j�C1(1 + j�re C 2)<br />

(8.29)<br />

Figure 8.16 Definition of Z tank and Z FB in oscillator small-signal model.

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