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Simple CMOS Square Wave Generator with Variable Mode Output

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Simple CMOS Square Wave Generator with Variable Mode Output Original scientific paper https://doi.org/10.33180/InfMIDEM2020.104 Journal of Microelectronics, Electronic Components and Materials Vol. 50, No. 1(2020), 35 – 45 Simple CMOS Square Wave Generator with Variable Mode Output Predrag B. Petrović University of Kragujevac, Faculty of Technical Sciences, Čačak, Serbia Abstract: A novel square-wave generator based on a single CCCII (current controlled conveyor), with only two external grounded passive components is proposed in this paper. The circuit provides precise, electronically controllable, voltage or current output square-wave signals. The simulation results using 0.18 mm CMOS parameters and experimental verification confirm the feasibility of the proposed circuit. The proposed generator can operate very well at up to 25 MHz with nonlinearity less than 5%. Keywords: Square-wave generator; CCCII; variable mode output; electronically controllable; simulation; experimental results. Enostaven CMOS generator kvadratnega vala s spremenljivim izhodom Izvleček: Predstavljen je nov generator s kvadratnim valom na osnovi enojnega CCCII (current controlled conveyor) z le dvema zunanjima pasivnima elementoma. Vezje zagodavlja natančen, elektronsko nastavljiv napeotsni ali tokovni izhodni signal kvadratne oblike. Simulacije so izvedene v 0.18 mm CMOS tehnologiji in eksperimentalno preverjene. Generator lahko dobro deluje do 25 MHz, pri čemer je njegova nelinearnost manjša od 5%. Ključne besede: generator kvadratnega vala; CCCII; nastavljiv izhod; eelektronski nadzor; simulacije, eksperiment * Corresponding Author’s e-mail: [email protected] 1 Introduction ports x and y, thereby allowing the design of numerous Square signal generators are widely used in communi- tunable applications [2]. cation, instrumentation, electronic and control systems such as generation of the carrier signal in communica- The proposed generator possesses the following ad- tions or clock signals in electronic systems, or as control vantages: a single active element and grounded pas- signals driving synchronous motors with permanent sive components-based realization; the electronically magnets [1]. They also find their place in various other adjustable period and oscillating condition (OC); op- applications based on processing of analogue signals, erational frequency of up to 25 MHz; and low power they are used for defining the duty cycle of a voltage consumption (1mW). It can be used for the generation controlled oscillator in sensor interfaces, in the opera- of rectangular waveforms and pulse width modulation tion of A/D and D/A converters, signal processing func- waveforms, considering that many applications such as tions, as well as the clock pulse in digital systems. music synthesizers, voltage regulation and power de- livery units need the period adjustment function in the Among various current mode devices, the second waveform generator. Based on HSPICE simulation and generation current conveyor (CCII) is one of the most experimental results, the performance of the proposed versatile building blocks [2]. It is also characterised by square-wave generator is shown, and the obtained a high slew rate, wide bandwidth, and a large dynam- results are fully in line with the conducted theoretical ics range. In CCCII function, Rx (intrinsic resistance) can analysis. be modified by varying the bias current IB, resulting in more precise voltage-following characteristic between 35 P. B. Petrović.; Informacije Midem, Vol. 50, No. 1(2020), 35 – 45 2 Proposed square-wave generator CII based square-wave generator is shown in Fig. 2 (the well-known realization of classic translinear structure circuits of CCCII). In order to generate the current output, it is necessary to add two additional MOS transistors (to The proposed variable mode square-wave generator- form additional z output-dual output CCCII). relaxation oscillator is shown in Fig. 1. The resistor R can be replaced with an active resistor Req at port z, composed of two MOSFET transistors. The parasitic resistances (the transresistance) of ports x is IB approximated as [4] y z v out µ ppW µnnW i RIxB≅+Cgox ≅+mn gmp (2) 12 LLpn1 () CCCII+ z R ix x z i where g denotes the transconductances of the tran- iC + out m sistors M6 (M7) and M4 (M5) (function of bias current IB) C vC - the NMOS and PMOS components of translinear loop. µ is the carrier mobility; COX is the gate capacitance per unit area, respectively. This nonlinear relation (2) im- Figure 1: Variable mode square-wave generator. plies the real limits of the bias currents in the possible practical implementation [4, 5]. Square-wave generator in Fig. 1 consists of only one active component-CCCII; capacitor and one grounded The voltage output vout (Fig. 1) has two possible satu- resistor (which can be electronically controlled). The ration levels; VSAT+=-VSAT-=VDD, and vout can be expressed circuits employ a Schmitt trigger connection with a only either as VSAT+ or as VSAT-, because the proposed con- grounded capacitor. The oscillation frequency depends figuration possesses high positive feedback. At steady- strongly on the nonlinear behaviour of the CCCII and stage operation, we can assume that vout switches from the value of the x terminal resistance. The terminal rela- VSAT- to VSAT+. The voltage across the capacitor C (vC) that tionships of CCCII in an ideal situation can be described is charged from the lower threshold voltage (VTH-) to as [2] VSAT+ and can be expressed as − t iv==vi+=Ri i (1) RCx (3) yx0; yxxz; x vVCT=−()HS−+VeAT +VSAT + These equations, however, represent the CCCII in the At the end of first half-period, the capacitor voltage re- linear operating region, when current at x port and tained its upper threshold voltage (VTH+). On the basis of voltages at z and x ports are limited [3]. In fact, CCCII is a the internal structure of CCCII and its terminal relation- nonlinear component and outside of the linear region ships we can conclude that V and V can be given by the currents at x port and the voltages at z and x ports TH- TH+ are saturated. A detailed structure of the proposed CC- RR VV=−xxVV=− (4) TH ++1;RRSATTHS−−1 AT VDD M M M 1 M 2 M3 12 M13 15 The interval at which the capacitor is charged-T1, can be derived from (3) as M M VVTH −+− SAT R 6 7 2 (5) TR1 ==xxCRln C ln −1 z z VV− R y x v i TH ++SATx I B out out M M C R 4 5 Analogously, the interval in which the capacitor dis- charged can be expressed as M M M 8 M9 10 M11 14 V SS VVTH +−− SAT 2R TR21==xxCRln CTln −=1 (6) Figure 2: Schematic view of the CMOS square-wave VVTH −−− SATxR generator based on CCCII. 36 P. B. Petrović.; Informacije Midem, Vol. 50, No. 1(2020), 35 – 45 From (5) and (6), bias current IB can be used for control- frequency fo can be given by ling the period of oscillation, owing to the existing de- pendency between the value of the transresistance of 11 fo == ports x (defined with equation (2)) and thus the dura- (7) TT12+ R tion of the intervals T and T . Additional controllability 2lRCx n12 − 1 2 Rx would be achieved by installation of an active resistor Req at port z of CCCII, or an active C. For oscillation, the The voltage limits of x terminal are dominant, in order oscillator must fulfil two Barkhausen conditions: the to describe the nonlinear behaviour of the proposed loop gain must be slightly greater than unity; the loop circuits. In this case the capacitor C can be assumed phase shift must be 0 or 360 degrees [6]. The proposed to be charged with the dependent voltage source circuits (relaxation oscillator) basically possess a certain vx=vy+ixRx through terminal resistance, controlled by form of amplifiers with positive feedback (forming cir- the bias current. In the context of the functional de- cuits with two threshold voltage levels- Schmitt trigger pendency expressed in this way, the operation of the comparator). In this way a portion of its output was fed proposed relaxation oscillator can be analysed on the back re-generatively to the input, and one of the out- basis of the current-controlled resistive elements (CC- put transistors is driven to saturation (ON state) and the CII) driving plot (DP) in conjunction with the capacitor other to cut-off (OFF state). In order for the proposed C-as seen by the capacitor at node (port) x. In the pro- generator circuit to be able to generate a square wave posed relaxation oscillator, Fig.1, the linear capacitor C output signal (voltage/current) – a oscillation condition is connected at port x, e.g. to a current-controlled resis- (OC), it is necessary to set the value of capacitor C, the tor described by functional relation (2), which is not bi- time constant that defines the rate of change on the x jective. This approach can be found in well-known texts port of the CCCII, as well as the trigger thresholds lev- pertaining to the problem of nonlinear system analysis els. Namely, it is necessary that the trigger threshold, [7-10] as original Chua vintage, because any two-termi- which depends directly on the value of Rx, be reached nal resistive device is characterized by its driving-point fast enough (a function of time constant), in order to (DP) characteristic [8]. For the proposed circuits, Fig. 1, change the condition at the output of the generator port x possesses such a characteristic. Fig. 3 shows the circuit at all. Also, from (5) and (6) it is obvious that driving-point (DP) characteristic of CCCII port x (volt- 2R>Rx must be satisfied as one of OC. From these facts age vs. current) depending on the applied bias current it is clear that there is control of the oscillation condi- IB.
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