IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 35, NO. 12, DECEMBER 2000 1853 30–100-MHz NPN-Only Variable-Gain Class-AB Instantaneous Companding Filters for 1.2-V Applications Mourad N. El-Gamal, Member, IEEE, Rola A. Baki, Student Member, IEEE, and Amir Bar-Dor Abstract—Two variations of a continuous-time instantaneous companding filter were integrated in a 25-GHz bipolar process. Their 3-dB frequencies are tunable in the ranges of 1–30 and 30–100 MHz. The dc gains are controllable up to 10 dB. The mea- sured dynamic ranges for a 1% total harmonic distortion are 62.5 and 50 dB, for the 30- and 100-MHz filters, respectively. At max- imum cutoff frequencies, the filters dissipate 6.5 mW from a 1.2-V supply. Index Terms—Analog integrated circuits, companding, contin- uous-time filters, current-mode circuits, log-domain, nonlinear cir- cuits, tunable filters, low power, low voltage, VHF filters. I. INTRODUCTION C OMPANDING can be used to maintain reasonable dynamic range (DR) in integrated analog signal pro- cessors where the allowable voltage swings are limited by the low-voltage supply requirements of modern low-power applications. As shown in Fig. 1(a) for the case of syllabic companding [1], the input signal is compressed before being processed, which ensures signal integrity over a large range of input levels. At the output, the signal is expanded to restore its DR. This can result in a higher DR compared to conventional analog signal processors [2]. Unlike the latter, this does not come at the expense of increased power dissipation or chip area for a given bandwidth [3]. The DR of low-voltage com- panding analog circuits can further be extended using class-AB current-mode based circuit topologies [3]. The objective of the work presented in this paper was to design very high-frequency (VHF) and low-voltage contin- uous-time filters with cutoff frequencies in the 30–100-MHz range, suitable for low-power applications with moderate linearity and signal-to-noise ratio (SNR) specifications (e.g., high-frequency digital communications), and requiring a wide frequency tuning range [4], [5]. Log-domain filters, which constitute a special class of instantaneous companding signal processors [1], were considered. They have already been used to realize programmable integrated filters reaching cutoff frequencies up to 220–435 MHz [6], [7]. The power consumption of those filters were relatively high due to their 2.7–5-V power supplies, and the DRs were limited by class-A Manuscript received April 9, 2000; revised July 11, 2000. M. N. El-Gamal and R. A. Baki are with the Department of Electrical and Computer Engineering, McGill University, Montreal, QC H3A 2A7, Canada. A. Bar-Dor is with PMC-Sierra, Inc., Montreal, QC H3R 3L5, Canada. Publisher Item Identifier S 0018-9200(00)10059-9. operation. Two low-voltage class-AB implementations have already been proposed: The first one is based on the bipolar integrator introduced by Seevinck [8], and the second one is the BiCMOS realization proposed by Punzenberger and Enz [9]. Seevinck’s circuit is a good candidate for high-frequency applications, since it employs n-type devices only in the signal path. However, it needs a minimum supply voltage of 1.7 V, compared to the 1.2-V requirement of the BiCMOS circuit of Punzenberger and Enz. In this paper, we propose a 1.2-V bipolar realization which does not employ pMOS or PNP devices in the signal path, making it suitable for VHF appli- cations. It is based on a new differential class-AB integrator [10], which is an extended version of the single-ended class-A prototype reported in [11]. The bipolar filter presented in this paper compares to the BiCMOS realization in [9] as follows: 1) The maximum 3-dB cutoff frequency is extended from about 1 MHz, for a 1.2-V supply, to 30–100 MHz. 2) The input preconditioning circuitry, which is crucial for the success of a fully integrated filter and for testing VHF circuits, is incorporated on-chip, unlike the off-chip dis- crete-components preconditioner used in [9], [12]. 3) Each individual integrator is differential, in the sense that it would not respond to a common-mode signal applied to its positive and negative ports simultaneously (refer to (8) and (9)). That was not the case for the integrator in [9]. 4) The integrator, and all the input and output interface circuitry, do not employ p-type devices in the signal path. This extends their applicability to the many bipolar processes which feature low-quality low-bandwidth PNP transistors, and does not restrict them to BiCMOS processes or to the special bipolar processes featuring high-quality PNP transistors. 5) Finally, unlike in [9], the filter’s gain in our implementa- tion is programmable. This, in addition to the wide fre- quency tuning range, can be a highly desirable feature in many applications (e.g., [13]). This paper is organized as follows. Section II summarizes the concept of log-domain filtering. Sections III and IV describe in detail the operation of the proposed class-AB integrator and of the supporting input and output interface circuitry. Sections V and VI present the design and measurement results for two in- tegrated filter prototypes. Section VII concludes with a discus- sion and performance comparison with other filters reported in the literature [9], [11]. 0018–9200/00$10.00 © 2000 IEEE