Research Of Current Differencing Transconductance Amplifier And Its Application On High-order Filter Design

Recent decades, current-mode circuit techniques have become an importantresearch direction in the field of integrated circuits due to its excellent performance ofhigh slew rate, wide bandwidth, low voltage and low power consumption etc.Current-mode continuous-time filter is a front subject in microelectronics, integratedcircuits and system. It has good wide prospects and development in instruments,electronic measurement, automatic control, communication, etc. As the latestdevelopment in current mode device, current differencing transconductance amplifier(CDTA) is the current mode circuit in strict sense. It inherits various advantages ofthe current-mode circuit. Domestic and foreign researchers have great interests inCDTA and its application circuits. It showed a greater advantage in analog signalprocessing, especially in the design of continuous-time current-mode filter. Thispaper studies CDTA’s circuit implementation and its current mode high-order filter’sdesign theory, design method and implementation. Innovative work and mainachievement include the following aspects:(1) A novel low-voltage high-linearity wideband CDTA is presented. The circuitis composed of a NMOS current differencing circuit and a NMOS cross-couplingtransconductance circuit. The proposed CDTA has all NMOS structure in AC path.The PSpice simulations of the proposed circuit show a good performance:-3dBfrequency band-with is about900MHz, low power consumption is2.48mW, inputcurrent linear range is±100μA and low current input resistance is less than20, highcurrent output resistance is more than3M. So the design of IC circuit is in goodagreement with the port property of CDTA.(2) Three kinds of current-mode high-order filter based CDTA have beenproposed. According to the actual situation that researches on CDTA high-order filterare not adequate apparently, based on the researches of polycyclic feedback filter’ssystem design method, this paper proposes three kinds of current-mode high-orderfilter based CDTA: current-mode nth-order universal filter based on CDTA;current-mode nth-order leapfrog ladder filter based on CDTA; current-mode fullbalanced nth-order leapfrog filter. Among, the current-mode nth-order universal filterhas current-mode multi-inputs single-output structure. Through a rational changing of the amount and mode of joining external current signals, circuit can realizes low-pass(LP),high-pass (HP),band-pass (BP), band-stop(BS) and all-pass (AP) filter functionswithout changing the topology. On the basis of analysis of passive ladder filter’sstructure and signal flow diagram, a current-mode nth-order leapfrog ladder filter hasbeen presented. The proposed circuit, which shares simple configuration and containsminimum active component, passive component, can realize nth-order filter’sfunction. It adopts only n active components and n capacitors. So it enjoys thefeatures of low power consumption and suitable for integrated circuit (IC) fabrication.In order to meet the design requirements of current signal’s double-ended input anddouble-ended output, a current-mode full balanced nth-order leapfrog filter has alsobeen proposed. The circuit has full balanced structure and can effectively inhibit evenharmonics and common-mode interference signals.(3) A novel current-mode active device, current differencing cascadedtransconductance amplifier (CDCTA), has been presented. Meanwhile, a single inputmultiple output current-mode nth-order filter based on the device is proposed. Inorder to improve the flexibility of current-mode circuit design, CDCTA has beenproposed and it simplifies the design of the high-order filter circuit considerably. Theproposed nth-order filter has current-mode single input multiple output structure. Itcan realize multiple filter functions without changing the topology, such as LP, BP,HP. The proposed circuit of simple structure contains a single CDCTA, n groundedcapacitors, so it is convenient for IC fabrication.