A 1 Volt CMOS Pseudo Differential Amplifier Apirak Suadet and Varakorn Kasemsuwan Department of Electronics, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Ladkrabang Dist., Bangkok, 10520, Thailand Phone: 66-2-326-4222 Ext. 102, Fax: 66-2-739-2398, e-mail: kkvarako@kmitl.ac.th Abstract-This paper presents a 1 V CMOS pseudo differential amplifier using simple rail-to-rail CMFB circuit. The proposed circuit employs the complementary common mode feedback (CMFB) consisting of common mode detector, transimpedance and transconductance amplifiers. The simulation results using HSPICE under a 0.18 gm CMOS technology shows that the rail to rail output swing is achieved with low common mode gain (-36 dB). The differential output swing of the circuit is ± 0.7 V. The power dissipation of the circuit is 0.23 mW. I. INTRODUCTION At present, a high performance analog circuit using low voltage is strongly demanded. This is mostly due to the advance of the large scale integration with complicated circuit systems and the increase of the demand for battery- operated portable equipments. However, supply voltage reduction in analog circuit causes several performance degradations and, therefore, new approaches in the design are needed to obtain analog circuits with enough bandwidth, gain and good linearity. Operational transconductance amplifier (OTA) is one of the most frequently used basic cells as OTA finds many applications in many analog circuits such as operational amplifier, voltage comparators, A-D and D-A converters and high frequency filters. Several approaches have been proposed to design low voltage OTA [1-10] using both fully differential (FD) and pseudo-differential (PD) configurations. FD is typically based on a differential pair with a tail current source while PD is based on two independent inverters without tail current source. It is known that avoiding the voltage drop across the tail current source, in a PD structure, allows wider input and output ranges and makes the architecture attractive for low power- supply applications. However, PD structure requires an extra common-mode feedback (CMFB) circuit which serves two purposes: 1) to fix the common-mode voltage at high impedance nodes and 2) to suppress the common-mode signal components. Several approaches have been proposed to achieve CMFB [1-10]. [1] used switched-capacitor (SC) circuits result in lower power consumption, but the SC- CMFB circuits introduces clock-feed through error making circuits not plausible for continuous time applications. [2] used simple resistive divider to sense the voltage of two differential nodes. However, resistors have to be large resulting in a large chip area. To solve the problem, methods of employing MOS transistor as CMFB circuit have been proposed [3-5]. The CMFB consists of CM detector and one stage amplifier. As a result, the common mode gains are quite high and, in addition, the output swings are limited. [6-8] employs transistors with two stage common mode amplifiers. The resulting common mode gain is low. The problem with this structure is that the circuit has limited output swing and potential oscillation problem. In addition, bipolar transistor technology in [7] is not compatible with the well known CMOS technology. [9- 10] propose the complementary CMFB which can achieve both low common mode gains with good output swings. However, both NMOS and PMOS in the circuit are required to have the same threshold voltage and transconductance which may not be true in general. [11] proposed positive feedback technique to increase the differential gain. However, the common mode gain is low (Acm 1/2). In this paper, a 1 V pseudo-differential amplifier (PDA) using a new common mode feedback (CMFB) is proposed. The CMFB consists of a common mode detector, transimpedance and transconductance amplifiers. The common mode gain is low (-36 dB). The positive feedback is also employed to increase the differential gain. The differential output swing of the circuit is ± 0.7 V. II. CIRCUIT DESCRIPTION The proposed PD is based on the configuration shown in Fig. 1. As seen, PDA consists of the input transconductor GM(IN) and common mode feedback network (CMFB). When the outputs from GM(IN) are differential signals, the currents through resistors R are of the same value but opposite phase. These currents will flow to each resistor and be mirrored to the OutlA and OutIB. Because these currents are of the same amplitude but opposite in phase, there will be no input current to the transimpedance amplifier and no voltage variation at node C. The current through resistor R are also mirrored and positively feedback to the output of the input transconductor GM(IN). As a result, the output impedance of PD at node Vo1 and Vo2 are given by Zout When the outputs from PD are common mode signals, the common mode current will flow through nodes A and B with the same amplitude and phase. As a result, the summation of these two currents are added and passed to the common mode amplifier (A) which consists of transimpedance amplifier and output transconductor GMO. The output current of GMO is fed back to the output node of input transconductor GM(IN) to eliminate common mode signal. From Fig. 1, it can be easily shown that the common mode output impedance at the output nodes (Vo1 and Vo2) are 1-4244-0549-1/06/$20.00 (2006 IEEE.