Sub-circuit analysis for power supply rejection ratio in regulated cascode operational transconductance amplifiers and filters

Operational Transconductance Amplifiers (OTA) form an integral part of OTA-C filters. The authors have experimented with OTA-C filters for frequency shaping purposes in micropower all-CMOS hearing aids as against conventionally used switched capacitor filters, as switched capacitor filters present problem due to switching noise and require a double polysilicon technology in a low voltage process. The power supply rejection ratio (PSRR) of such filters becomes important in hearing aids, as the negative supply voltage for the OTA is derived from the low voltage battery by the use of a dc to dc converter through capacitor switching where a ripple voltage on supply rail is inherent due to charging and discharging mechanism in the converter circuit. A regulated cascode (RGC) differential stage is used for OTA to increase the linearity of the differential input stage and ensure an acceptable level of CMRR. An important finding that has emerged in the present investigation from the viewpoint of application of OTA-C filters in hearing aids is that PSRR of high pass filter is much higher compared to that in low pass configuration. The results have been validated both by SPICE simulation and measurement of OTA-C filter chips fabricated for hearing aid application.