Design of Z-copy controlled-gain voltage differencing current conveyor based adjustable functional generator
Introduction
Applications of electronically controllable active elements [1] in the field of signal generation allow interesting benefits such as: simple controllability of oscillation/repeating frequency, independent control of oscillation condition, control of output amplitudes (or ratios of amplitudes), duty cycle control, etc. by electronically adjustable parameter that is controlled externally by DC bias current or voltage [1]. We focused our attention on electronically controllable functional (triangular and square wave) generator with differential square wave outputs in this work because square wave signals are important in many analog and digital communication subsystems (clock generators, pulse width modulators, DC–DC converters, TTL and CMOS logic, etc.). Differential outputs are beneficial mainly in low-voltage technologies (output level rapidly decreases with decreasing power supply voltage – even tens of mV in some cases) due to availability of two-times higher output level and lower additional common-mode distortion and better noise immunity. Table 1 compares several recently reported solutions of generators [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. We focused our overview on electronically controllable solutions only. There are two ways of understanding the number of active devices. Table 1 includes solutions based on discrete active elements and also active elements involving several (typically two) active subparts, in Table 1 noted in separated columns.
Circuit presented in [2] allows differential triangle and square wave outputs. However, circuit requires two active elements based on composition of two subparts used in frame of the voltage differencing buffered/inverted amplifiers (VDBA/VDIBA). Therefore, solution presented here seems to be simpler. Chung et al. [3] utilizes three OTAs (controllable by bias currents), two grounded resistors and capacitor. A number of active elements (considered also subparts of our device) in [3] is also higher than in our case. Very similar solution to [3] was also proposed by Siripruchyanun et al. [4]. An approach presented by Kumbun et al. [5] is based on two so-called multiple-output through transconductance amplifiers (MO-CTTAs) and grounded capacitor. These MO-CCTAs are complemented by current follower with two inputs having special features (Iinp+=−Iinp−) and output transconductance section. Therefore, our solution is less complex than realization in [5]. Silapan et al. [6] and Sristakul et al. [7] also proposed interesting solutions (similar to [5]), where two multiple-output current controlled current differencing amplifiers (MO-CCCDTAs) and grounded capacitor were used. The CDTA device is also based on two counterparts (current differencing unit and transconductance amplifier [1]). A very interesting solution was introduced by Chien [8]. His generator utilizes two differential voltage current conveyors (DVCCs), four passive elements and it is controllable by passive elements and by DC control voltage (electronic control was elaborated very precisely). Generator utilizing two current conveyors was presented by Janecek et al. [9]. It has also capability of electronic control but requires four passive elements and also additional diodes, however control of duty cycle was not verified. However, circuit offers differential square wave output. Circuit discussed in [10] requires three commercially available devices (so-called diamond transistor and two voltage controllable amplifiers) and knowledge of dynamics of used active elements (integrator and comparator). Quite simple solution, introduced in [11], was also designed by using of the active element that combines internal subsections (adjustable current amplifier). Unfortunately, generator does not have capability of current and differential square wave output.
Discussed solutions in Table 1 have some drawbacks:
- 1)
too many passive elements [8], [9];
- 2)
too many active elements [3], [4], [10];
- 3)
not proposed/designed duty cycle control [5], [6], [7], [9];
- 4)
differential square wave output not available [3], [4], [5], [6], [7], [8], [10], [11];
- 5)
current and voltage square wave output not available simultaneously [3], [11].
We can see (Table 1) that many drawbacks of recently reported types are removed in our solution based on so-called Z-Copy Controlled Gain Voltage Differencing Current Conveyor (ZC-CG-VDCC). Simple VDCC was firstly reported in [1]. So-called differential difference current conveyor (DDCC) [1], [12], [13] or DVCC [1], [8] implements very similar types of voltage and current transfer relations. However, no electronic control of basic CMOS structure is directly possible, as will be discussed in the following section. Despite the fact that internal structure of presented ZC-CG-VDCC is very complex, the solutions of generators discussed in [5], [6], [7] also require two active elements, which already combine at least two elementary building sub-blocks. Therefore, generator utilizing only one ZC-CG-VDCC seems to be simpler and more flexible from the controllable features point of view. Our solution offers similar benefits as some of the previously reported circuits, but in addition it allows current output of square wave signal as well, which (after transformation to voltage through resistor) can be useful for differential (symmetrical) purposes.
Organization of this work is following: Section 1 explains the reasons for presented development and features (and comparison) of several important hitherto published solutions. Basic behavior of the proposed active device is explained in Section 2. Section 3 introduces and investigates design of a generator based on ZC-CG-VDCC in detail, including graphical simulation results (step-by-step for better understanding of principle) and simple experimental test. Concluding remarks and overall summarization of obtained results are given in Section 4.
Section snippets
Z-copy controlled gain voltage differencing current conveyor (ZC-CG-VDCC)
ZC-CG-VDCC is multi-terminal active device with three types of electronic control (three adjustable parameters of the active element are available and controllable mutually independently). Behavior of the model of ZC-CG-VDCC is shown in Fig. 1. In fact, there are two functional parts: operational transconductance amplifier (OTA) [1], [14] followed by electronically controllable current conveyor of second generation (ECCII) [1], [15], [16], [17], [18], [19]. The OTA has typical adjustable
Adjustable functional generator
Proposed ZC-CG-VDCC is also applicable in triangle and square wave (functional) generator by very simply way. The circuit is shown in Fig. 3 and it provides also current output of square wave signal. OTA part of the VDCC serves as Schmitt comparator [3], [28] with hysteresis (parameters of the hysteresis window of the comparator are adjustable by transconductance gm and Iset_gm) and ECCII part forms loss-less integrator with electronically adjustable time constant set by B.
A detailed
Pulse width modulator as exemplary application of the designed generator
Functional generators providing triangular and square wave outputs are applicable in many areas of signal processing, for example as sources of clock timing, in pulse width modulators (PWM), in serial sigma-delta converters, etc. We proposed the following very simple pulse width modulator as very typical example of utilization of triangular wave output of the generator. Two ZC-CG-VDCCs are sufficient to provide simple PWM modulator, proposed solution is shown in Fig. 18. PWM generators are very
Conclusion
The details of designed generator are summarized in Table 4. Our solution is useful for voltage- and current-mode signal processing, where square wave input signal is required. Simulations confirmed analytical presumptions and expectations. Note that functions of internal sections of ZC-CG-VDCC can be interchanged (integrator is created by OTA and comparator by ECCII section, see Fig. 1b) in order to obtain another solution of the generator. Simple exemplary application of ZC-CG-VDCC in
Acknowledgment
Research described in the paper was supported by Czech Science Foundation project under GP14-24186P and by internal grant no. FEKT-S-14-2281 and project Electronic-biomedical co-operation ELBIC M00176. The support of the project CZ.1.07/2.3.00/20.0007 WICOMT, financed from the operational program Education for Competitiveness, is gratefully acknowledged. The described research was performed in laboratories supported by the SIX project; the registration number CZ.1.05/2.1.00/03.0072, the
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