IEEE JOURNAL OF SOLID-STATE CIRCUITS> VOL. 26, NO. 7, JULY 1991 Realization of a 1-V Active Filter Using a Linearization Technique Employing Plurality of Emitter-Coupled Pairs Hiroshi Tanimoto, Mikio Koyama, and Yoshihiro Yoshida 937 Abstract —A low-voltage (1 V), low-power (100 pW), and low- frequency (9 kHz) fifth-order fully integrated active low-pass filter (LPF) using a bipolar technology is described. Novel highly linear transconductors consisting of N emitter-coupled pairs were designed for low-voltage operation. The linear input range is expanded to about 100 mVP.P at 1% error with N = 4, which is about twice that of the conventional linearization technique. The filter is basically a gyrator-capacitor type, in which gyrators are implemented by using the linearized transconduc- tors. Large time constants were realized with very low current (540 nA/transconductor) owing to the high transconductance- to-operating-current ratio of the linearized transconductors. Measured results show a passband ripple of 1.5 dB, a minimum stopband rejection of 70 dB, and a dynamic range of 56 dB, despite a very high nominal impedance (400 kfl). Practical limitations of this approach are also discussed, such as the sensitivity of the linearized transconductors against process variations, noise, and frequency limitations. I. INTRODUCTION R ECENT advances in hand-carried electronic equip- ment demand smaller and smaller components. Typ- ical examples are hand-carried radiotelephones and radio pagers. Filters are very important components in these telecommunication equipments. Reducing the equipment size and power dissipation are very important in realizing such equipment, which is small and easy to maintain. As the size of batteries is now becoming the limiting factor for such equipment, it is not sufficient to reduce the size of bulky components such as filters by integrating them; the reduction of their power dissipation is also very important. A key point is to develop, simultaneously, both low-power and low-voltage operating LSI’S in order to reduce the battery size. Here, low voltage means a single battery operation, i.e., about 1 V. This paper describes a low-voltage (1 V) and low-power (100 wW) fifth-order fully integrated active low-pass filter using a bipolar technology. Manuscript received November 30, 1990; revised March 14, 1991. H. Tanimoto and M. Koyama are with Toshiba Research and Devel- opment Center, 1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan. Y. Yoshida is with the Semiconductor Division, Toshiba Corporation, 1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan. IEEE Log Number 9100257. There are several types of active filters that can be integrated; however, the transconductor–capacitor type of filter is one of the few choices considering a 1-V opera- tion [11. The well-known gain-cell type variable transcon- ductors are not applicable because they require an extra V~E for logarithmic compression, and thus need at least about 2V to operate. Another possibility is to utilize a class-AB variable transconductor [4]; however, this transconductor requires buffers at the input or output in order to absorb its relatively low input resistance. This will lead to a larger power consumption or will require a similar power supply voltage to the gain-cell type transconductor. In contrast, the ordinary bipolar emitter-coupled pair with current source loads can operate from a 1-V power supply as a variable transconductor. Also, this circuit has a high transconductance-to-quiescent-current ratio, which is essential for a low-power operation. Examining the above emitter-coupled pair, the output voltage can swing only about 0,3 V peak to peak in this situation. Unfortunately, a conventional emitter-coupled pair has only a 10-mV linear input range and cannot fully exploit the limited dynamic range, Although a conven- tional linearized differential pair using two emitter-cou- pled pairs with 1:4 ratioed emitter sizes [2] has about a 40-mV linear input range, it is not sufficient when used in practical circuits, which often produce tens of millivolt offsets. Thus, further linearization of the differential pair as a transconductor is of foremost concern. This paper introduces highly linear transconductors consisting of IV bipolar transistor differential pairs and gives the conditions to implement such transconductors in Section II. For the IV= 4 case, a linear input range of 100 mVP.P at 1% error can be realized theoretically. Sensitiv- ity analysis results on the proposed circuit structure indi- cated that the circuit is very robust against changes in processing parameters (Section III). Section IV describes the practical implementation of a 1-V filter IC and re- lated design considerations such as low-voltage operation, noise, and frequency limitations. The measured results of the experimental active-low-pass filter IC using the lin- earized transconductors are described in Section V. The test chips have demonstrated the usefulness of the ap- proach for low-voltage and low-power realization of filter IC’S. 0018-9200 /91/0700-0937$01 .00 01991 IEEE