A 60MHz OTA-based IF filter for wireless wake-up receivers with 115 ns settling time

This paper presents the design and characterization of a fully-differential intermediate frequency (IF) filter based on an operational transconductance amplifier (OTA) with 6th order Butterworth bandpass characteristic. The designed 2-stage BiCMOS OTA can be treated as an operational amplifier (OPAmp) together with its output resistance. The combination exhibits an open-loop voltage gain of 75 dB and high gain-bandwidth product (GBP) of 5.8 GHz, while stability is achieved with a worst case phase margin (PM) of 54° from a low bias-current of only 520 μA. The multiple-feedback filter (MFB) elements for the target Butterworth frequency response are determined with circuit analysis based on an ideal OTA transfer function. The filter center frequency is set to 60 MHz, whereby a measured -3 dB-bandwidth of 15.5MHz is achieved. To enable the operation within an on-time of 200 ns the filter is able to settle at ±1 dB of its steady state amplitude in less than 115 ns. The filter was fabricated in the IHP 130nm BiCMOS technology and requires a current of only 1.6mA from a 2.5V-supply including all bias circuits.