Analysis, Design and Optimization of Active Devices

The current-feedback operational amplifier (CFA) is an active device which uses the supply-current sensing technique in order to achieve high performance features for analog signal processing applications. However, under certain load conditions, the CFA exhibits an unstable behavior. This work presents a new frequency model which rakes into account the non-unilateral characteristics of the CFA buffer stages and includes the load-dependent instabilities. The model can be easily incorporated into a standard macromodel for simulation purposes or used analytically for accurate circuit design based on CFAs. As a consequence of this approach, some improvements in the CFA structure could be suggested.

The use of pathological elements (i.e., nullators, norators, and the voltage mirror-current mirror pair) as universal active elements opens up the possibility to model the behavior of active devices with either differential features or input-output multiples, in order to be used in analysis tasks of linear(ized) analog circuits. This brief tries on the modeling this class of active devices, which is carried out by considering the impedance characteristics along with the behavior equation of each active device, the kind of signal to be processed and by applying the pathological element properties. In order to obtain a more realistic model, not only parasitic elements associated to the input-output terminals of each active device are considered, but the tracking errors of voltage and current differential followers modeled with grounded pathological elements and admittances are also included. Due to that the gain and tracking errors are finites and of limited bandwidth, a two-pole model is used for each of them in order to include their contributions on the symbolic expressions computed. Because the behavior of fully-differential amplifiers is modeled with grounded pathological elements, a standard nodal analysis can be performed. This imply that not only the size of the admittance matrix is smaller than those generated by applying modified nodal analysis (MNA) method, for instance, but also the number of nonzero elements and the generation of cancellation-terms are both reduced. As a result, the computational complexity during the solution of the system of equations is reduced when recursive determinant-expansion techniques are applied. Examples are described and compared with the MNA method, in order to show the usefulness of the proposed models to compute fully-symbolic small-signal characteristics of analog circuits containing fully-differential active devices and/or with input-output multiples.

In recent years, many articles came out where one could find the analysis of oscillatory systems of electric sinusoid signals generators with amplifiers called CFOA—current feedback operational amplifiers. As a rule, the analysis of such systems is made by applying mathematical modeling methods on the basis of the amplifier linear model, which does not allow estimating advantages and disadvantages of the systems realized with those amplifiers in comparison with classical systems. A nonlinear model of a current feedback operational amplifier (CFOA) is introduced in the paper; nonlinearity of “current mirror” is reflected in the form of current double limiting. The analysis of two known oscillatory systems has been carried out with the use of this non-linear model. Dependence between current limiting level, output voltage amplitude, and maximum oscillation frequency has been obtained. The paper shows that output current limiting under current output connection of capacitive load reduces frequency range and output voltage amplitude considerably and increases harmonic distortions in comparison with classical oscillatory systems. The research done has found that the application of new amplifiers does not give considerable advantages to the oscillatory systems with CFOA.

The realization of two integrator loop oscillators using Operational Amplifiers (Op Amps) is reviewed and a new additional circuit to provide independent control on the oscillation condition is proposed. Four new grounded capacitor oscillator circuits using unity gain cells and having independent control on the condition of oscillation and on the frequency of oscillation are introduced. The link between the Op Amp based two integrator loop oscillators and three Current Feedback Operational Amplifier (CFOA) based oscillators is detected and clearly explained. This paper serves also as a tutorial paper in introducing the subject of second order two integrator loop oscillators using state variable matrix equation and Nodal Admittance Matrix (NAM) equation.

In this paper, a circuit technique to extend the measuring range of a linear variable differential transformer (LVDT) is proposed. The transfer characteristic of the LVDT contains the odd function form of the cubic polynomial. Therefore, the measuring range of a commercial LVDT is linear in a narrow range compared to its physical dimensions. The wide measuring range of the LVDT requires a large structure of the LVDT, which increases the scale and the cost of the measurement system. The measuring range of the LVDT can be linearly extended to the maximum of the stroke range using the proposed technique. The realization of the proposed technique is based on the use of the hyperbolic sine (sinh) function of the electronic circuit building block, named the class AB bipolar amplifier. The class AB bipolar amplifier can be obtained by the current feedback operational amplifier (CFOA). The circuit of the proposed technique requires two CFOAs and an operational transconductance amplifier (OTA) as the active devices and all devices used in the proposed technique to synthesize the sinh function are commercially available. The proposed technique exhibits an ability to compensate for the nonlinear characteristic of the LVDT without digital components. The proposed technique is attractive in terms of its simple circuit configuration, small size, and low cost. The linear range extension of the LVDT used in this paper is significantly increased with a maximum error of about 18.3 μm of 6.2 mm at the full stroke range or the full-scale percentage error of about 0.295%. The results indicate that the proposed technique provides excellent performance to extend the measuring range of the LVDT without modifying the LVDT structure.

SummaryRecently, abstract current mirror and voltage mirror elements have been proposed for behavioral modeling of active analog blocks. Such artificial elements and the traditionally used nullor element together are called pathological elements in the literature. Pathological elements are very useful in modeling and analysis of active network. Hence, researchers have been motivated to study symbolic analysis methods for networks containing pathological elements. However, so far, only nodal admittance matrix analysis has been formulated. In this work, an alternative two‐graph method is formulated, which has the advantage of providing a compact intermediate form for subsequent symbolic term generation. With a compact two‐graph representation, either a matrix method or a tree enumeration method can be employed. For completeness, the classical two‐graph theory has been extended in this paper to encompass all four types of dependent sources and all pathological elements. Illustrative examples are presented to demonstrate the principle of compact symbolic term generation by the presented two‐graph method. Copyright © 2014 John Wiley & Sons, Ltd.

ABSTRACTIn this paper, we present an exact analysis of finite gain inverting voltage amplifiers using current feedback operational amplifiers (CFOA) for use as stand-alone amplifiers or for being used in second-order active resistance capacitance (RC) filters such as Tow–Thomas biquads. The proposed circuits require one or two additional CFOAs/resistors/capacitors. The effect of finite output impedance at z terminal, input resistance at x terminal, and current mirror pole of the CFOA are studied. The circuits considered have been simulated using PSPICE using a behavioural macro-model of the CFOA as well as that of a practical CFOA AD 844.

The current paper presents a novel Schmitt trigger using two second‐generation current conveyors and four resistors and its application as a relaxation oscillator. The performance of the proposed circuit is examined using Cadence and the model parameters of a 0.6µm CMOS process. The obtained results demonstrate excellent agreement with the theoretical values. The measured results based on commercially available current feedback operational amplifiers (AD 844 AN) are included and the non‐idealities are also examined. The topology reports low sensitivities and has features suitable for VLSI implementation. Copyright © 2010 John Wiley & Sons, Ltd.

Synthesis of active inductors using SFG stamps

In this paper, a new grounded inductor simulator employing two current feedback operational amplifiers (CFOAs), three resistors and one grounded capacitor which is suitable for IC fabrication is proposed. Also, a new voltage-mode (VM) universal filter with three input and single output is given as application example of the grounded inductor simulator. The universal filter with low output impedance can be easily cascaded with other VM circuits. Both of the proposed circuits have the feature of improved low frequency performances, and can be easily constructed by two commercially available active devices such as AD844s. However, both inductor simulator and universal filter need a single passive component matching condition. Simulation and experimental test results are given to demonstrate the performance and workability of the proposed grounded inductor simulator and filter structure.

This research article comes with three novel topologies of Voltage Mode (VM) third order Quadrature Sinusoidal Oscillators (QSOs) using Current Differencing Buffered Amplifier (CDBA) as an active device with grounded and virtually grounded passive components. An implementation perspective of the prototype circuits follows a specific class of filter specifically Low pass (LP), High pass (HP) and All pass (AP) including an integrator/ differentiator in a closed loop. To justify the feasibility of the proposed QSO configurations, time response and frequency response outputs are generated through PSPICE simulation using 180 nm CMOS process parameters. In addition, the proposed configurations are experimentally tested using commercially available Current Feedback Operational Amplifier (CFOA) as IC AD844AN for the implementation of CDBA active block that validates the theoretical propositions and computer simulation results. The other performance analysis viz. sensitivity, non-ideality, frequency stability, phase noise analysis and Monte-Carlo analysis are also discussed for the proposed structures. An application workability of the proposed QSOs configuration is used to determine the transmission and reception signal of Quadrature Amplitude Modulation (QAM) and Quadrature Phase Shift Keying (QPSK) techniques.

A five-port general purpose analogue building block, termed as an operational floating current conveyor (OFCC), is described. The OFCC combines the features of a current feedback (CFB) operational amplifier, second generation current conveyor and operational floating conveyor (OFC). A simple implementation scheme of the OFCC is described and its terminal operational characteristics are used to yield a working model. The OFCC is then used as a single block to realize the current conveyors (CCII+ and CCII-). Experimental results are presented and show good behavioral agreement with simulation results.

A new five-port general purpose analogue building block, termed as an operational floating current conveyor (OFCC), is described. The OFCC combines the features of current feedback (CFB) operational amplifier, second generation current conveyor and operational floating conveyor (OFC). A simple implementation scheme of the OFCC is described and its terminal operational characteristics are used to yield a working model. The OFCC is then used as a single block to realize the four main amplifier types and the current conveyors (CCII+ and CCII-). Computer simulation results are presented and discussed. >

MOS-C realizations of the Kerwin–Huelsman–Newcomb (KHN) circuit using the commercially available Voltage Operational Amplifier (VOA) and the Current Feedback Operational Amplifier (CFOA) are reviewed in this paper. Additional MOS-C KHN realizations using the Operational Transresistance Amplifier (OTRA) and the Differential Current Voltage Conveyor (DCVC) are also included. MOS-C realizations of the KHN circuit using CMOS operational amplifier, CMOS current feedback operational amplifier and CMOS operational transresistance amplifier are also given.Spice simulation results using 0.18 CMOS technology model from MOSIS are included together with detailed comparison tables to demonstrate the differences between MOS-C KHN circuits using both of the commercially available active building blocks and CMOS integrated building blocks. A comparison with the Gm-C KHN circuit is also included.

Low-voltage subthreshold CMOS current mode circuits: Design and applications