IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS-II: ANALOG AND ... Research Council of Canada (NSERC) and the Canadian Network of Centres.
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IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS-II:
ANALOG AND DIGITAL SIGNAL PROCESSING, VOL. 43, NO. 1, JANUARY 1996
A Novel Continuous-Time Current-Mode Differentiator and Its Applications Ezz I. El-Masry and John W. Gates A b s ~ a t t - A novel continuous-time current-mode differentiator with a frequency range extending from dc to 100 h4Hi is presented. This circuit is constructed using a capacitively coupled current mirror. General first- and second-order circuits, using the differentiator, have also been introduced. To demonstrate how the proposed differentiator can be used to construct higher order filters a sixth-order band-pass filter with a center frequency at 2.5 MHz and a 1 dB pass-band ripple has been realized and simulated. All simulations were performed using HSPICE for a 1.2 pm CMOS process. The total power dissipation for the sixth-order filter is 23 mW from a single 3 V supply.
Im
I
]
integrator
Fig. 1. Single transistor current integrator.
I. INTRODUCTION Continuous-time filters are well suited for high-speed and lowpower applications as compared to switched-capacitor or digital filters. In the last decade there was a great effort to integrate these filters using digital CMOS technology [ l ] and [2]. In one approach operational transconductance amplifiers and capacitors (OTA-C) are used to realize continuous-time filters [3]-[5]. Another approach uses a grounded capacitor at the gate of a transistor to approximate an integrator [2], [6], and [7]. The OTA-C approach to current-mode filtering is well established but results in circuits that are usually larger and operate at lower frequencies than the second approach. The drawback of the second approach is that large capacitors are required to maintain the integrator approximation at lower frequencies. In this paper we propose a continuous-time current-mode differentiator circuit that operates for frequencies between dc and 100 MHz and utilizes a single 3 V power supply. The circuit is simple, smdl in size and consumes very little power. The basic current integrator is shown in Fig. 1 [ l ] and its small signal model is shown in Fig. 2. The transfer function for this circuit is obtained as tout -gout ' gm 7 ( s )= (1) tin (gds $. gout) . (gin S '
+ c).
This can be approximated as 5when s >, 9 and gds > (gds, +gds,) the transfer function of this circuit can be approximated as (see (4) at the bottom of the page). when s > gds, +gds, and gln
