Figure 12.26 shows CG Colpitts oscillators. In ICs, the transistor is usually biased by a current mirror connected in series with either the source or the drain. The current source in Fig. 12.26 represents the current-mirror current sink connected in series with the source of the oscillator transistor. The gate-to-source capacitance is absorbed into capacitance and the drain-to-source capacitance is absorbed into capacitance . The gate-to-drain capacitance is not included in the basic topology of the Colpitts oscillator, and for simplicity it is neglected in the subsequent analysis.
Figure 12.26 Colpitts oscillators with a transistor operating in the common-gate (CG) configuration and biased by a current source suitable for ICs. (a) Colpitts oscillator ...
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in Fig. 12.26 represents the current-mirror current sink connected in series with the source of the oscillator transistor. The gate-to-source capacitance 
is absorbed into capacitance 
and the drain-to-source capacitance 
is absorbed into capacitance 
. The gate-to-drain capacitance 
is not included in the basic topology of the Colpitts oscillator, and for simplicity it is neglected in the subsequent analysis.