III. Gm-C Filtering - Epublications - Université de Limoges

Figure 159. Final operational amplifier schematic
Figure 160 illustrates the gain and the phase of the OA without Cstab2 and then with
Cstab2=5pF. This capacitor introduces a zero in the transfer function of the OA. This allows
increasing the phase when OA gain is close to 0dB, thus increasing the phase margin to -180°
[IV.7]. Cstab2 value is tuned so as to obtain sufficient gain and phase margins.
Table 27 has been built using schematic simulations and describes the enhancement of
the stability margins, especially for PVT worst cases (-20°C and 3V supply as it may be seen
later on), reaches +18° of phase margin and +6dB of gain margin. This is very important to
ensure the reliability of the circuit in an industrial environment.
GM and PM are 3dB and 8° above the specification to ensure the robustness
specifications when taking into account process and mismatch dispersion as well as the
parasitics.
However, the major drawback of this solution is the degradation of the common-mode
rejection. Indeed, at high frequency of operation, the circuit progressively becomes pseudodifferential.
Measures of the common-mode rejection have been carried out. The filter rejects
the common-mode by 13dB at 68MHz and by 11dB at 180MHz. These values correlate with
simulations.
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