Internal and External R-C Networks

Any external and internal R-C networks cause phase shifts which affect the gain and phase margins. For each R-C roll-off, the gain reduces by 20 dB/decade. For example, take an op-amp connected as an inverting amplifier. There are capacitors in the op-amp circuits externally and internally. The internal capacitors are mainly caused by parasitic or as a compensation network. The external capacitor is usually the loading of the op-amp as a capacitive load. Each capacitor will present a pole to the circuit causing additional phase shift and gain roll-off, hence affecting the circuit stability criteria. The second effect from the pole is signal attenuation (low-pass filter), where the signal is reduced by –20 dB per decade. Each time a signal passes through a pole (capacitor), it attenuates by yet another –20 dB. The larger the capacitor, the more attenuation is asserted. Both phase shift and signal reduction as a result of the R-C networks are explained by the following bode plot. The very first gain roll-off is caused by the dominant pole as shown in the accompanying figure.