In this chapter, Active filters using only OTAs and capacitors are described. Cascade design of OTA-C filters needs high input impedance for each block, as well as first-order and second-order blocks. Various structures that can be used to implement these will be discussed in detail. The subject of ladder filter simulation using component simulation as well as operational simulation also will be discussed in detail. Finally, the multiple loop feedback technique for attaining high-order filters will be considered.

Operational Transconductance Amplifier-C Filters

In the previous chapters, the design of current-mode filters using transconductances, CCs, SI memory cells, and log-domain cells were studied in detail. While SI and log-domain designs yielded transistor-level implementations, the OTA and CC (which were treated as block boxes) need to be implemented using bipolar or MOS or BiCMOS technologies. Extensive work has been done on the design of transconductance elements in all these technologies, which will be covered in the first part of this chapter. The second part deals with the design of CCs in bipolar and MOS form. Some of the emerging areas of applications of current-mode filters and voltage-mode filters using OTAs are PLL loop filters, video filters, magnetic disk drive read channel ICs, DVD read channel ICs, ADSL analog front ends and RF circuits, e.g., for paging receivers. Anti-aliasing filters for digital audio are good candidates now for mixed-signal design. These will be considered in detail in this chapter.

Practical Design and Applications of Current-mode Systems

The design of a tunable high-frequency fully differential bipolar operational transconductance amplifier (OTA) is presented. Techniques resulting in tunability and broadbanding are discussed, as are unavoidable tradeoffs resulting from the lack of a vertical pnp device. Using an 8-GHz bipolar transistor array process, the simulated -3-dB frequency of the OTA is over 2.7 GHz, the maximum linear input range is +or-2.5 V, and the power dissipation is 28 mW for a power supply of +or-5 V. The OTA can also operate at a +or-2.5 V. The design of a third-order elliptic OTA-C filter with cutoff frequency of 250 MHz and tuning range from 200 to 290 MHz is presented. Filter nonidealities are analyzed, and predistortion and compensation techniques, are discussed. Temperature variations from -30 to +100 degrees C and supply variations from +or-4.5 to +or-7.5 V change the cutoff frequency of the filter by less than 10%. The Q-factor can be electronically adjusted for all frequencies in the tuning range. >

Paul van Halen

Papers

Design of a 2.7-GHz linear OTA and a 250-MHz elliptic filter in bipolar transistor-array technology