IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 32, NO. 2, FEBRUARY 1997 245 A 200 MHz Steered Current Operational Amplifier in 1.2- m CMOS Technology Eyad Abou-Allam and Ezz I. El-Masry Abstract— In this paper, we present the design of a CMOS current operational amplifier (COA). A design technique based on current steering is proposed to enhance the frequency capabil- ity of the amplifier and achieve higher bandwidth of operation. A complete COA was designed and fabricated using a 1.2- m CMOS technology, the COA occupies an area of 0.13 mm . Results from HSPICE simulations and measurements indicate an open-loop gain of 70 dB, a gain-bandwidth product exceeding 200 MHz, and a settling time of 5.1 ns. The amplifier operated under 3 V dc voltage supplies, and the power dissipation is approximately 4.5 mW. Index Terms— Amplifier, CMOS, current-mode, current- steering, operational amplifier. I. INTRODUCTION V OLTAGE operational amplifiers (op-amps) are well doc- umented and their basic behavioral model is well stan- dardized. An ideal voltage op-amp is modeled as a three (possibly four) terminal device providing a high gain dif- ferential voltage-controlled voltage source with infinite input resistance and zero output resistance. As may seem simple, a current operational amplifier (COA) is basically a current- controlled current source that exhibits very low (ideally zero) input resistance, very high (ideally infinite) output resistance, and very high (ideally infinite) current gain. Several behavioral models have been proposed for the COA [1]–[5]. Although they all follow the above description, they differ in the way their terminals are arranged. A single-input differential- output COA, shown in Fig. 1, is obtained by applying the adjoint networks theorem to the voltage op-amp. This COA is particularly useful in transforming almost all voltage-mode RC active circuits into their equivalent current-mode circuits [6]. Since the COA is a reciprocal device to the voltage op- amp, all sensitivity analysis of the original RC-active networks remains valid for their current-mode equivalent circuits [6]. Furthermore, using this type of COA eliminates the negative resistance needed when using current conveyors to transform some circuits such as the Deliyannis bandpass filter [6]. Recently, various CMOS implementations of the COA have been reported [4], [5], [7]–[10]. Unfortunately, most of these Manuscript received July 5, 1995; revised August 23, 1996. This work was supported by grants from the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Canadian Network of Centres of Excellence in Microelectronics (Micronet). This work was supported in part by KFUPM E. Abou-Allam is with the Department of Electrical Engineering, Technical University of Nova Scotia, Halifax, NS B3J 2X4, Canada. E. I. El-Masry was with the Department of Electrical Engineering, Technical University of Nova Scotia, Halifax, NS B3J 2X4, Canada. He is now on sabbatical with the Electrical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia. Publisher Item Identifier S 0018-9200(97)01123-2. Fig. 1. Ideal model of the COA. Fig. 2. Block diagram of two-stage COA. Fig. 3. Mirrored current amplifier. COA’s do not offer the frequency advantage expected from a current-mode system. The following section presents general analysis of the frequency limitations of these designs. A design technique that relies on current steering to enhance the frequency capabilities of the COA is proposed in Section III. The proposed technique is then applied to design a very high frequency COA in Section IV. Simulation and measured results of the COA are presented in Section V. II. MIRRORED CURRENT AMPLIFIERS To date, CMOS COA’s are designed using two or more stages. In general, the first stage is a transimpedance stage and the second stage is a transconductance [4]. A compensating capacitor is inserted at the high impedance node between the two stages as shown in Fig. 2. In the transimpedance stage, current mirrors are used to transform the input current to the interstage voltage. A simple circuit that represents COA’s designed using this method is shown in Fig. 3, and will be referred to as the mirrored current amplifier. In this 0018–9200/97$10.00  1997 IEEE