Processing of Communication Signal Using Operational Transconductance Amplifier

JOURNAL OF TELECOMMUNICATIONS, VOLUME 1, ISSUE 1, FEBRUARY 2010

118 Processing of Communication Signal Using Operational Transconductance Amplifier

A. Roy, K. Ghosh, S. Mondal and B. N. Ray

Abstract — This paper proposes a signal processing methodology of communication system and realized that circuits using operational transconductance amplifier (OTA). Two important classes of communication circuit, delta modulator and compander have been designed using that procedure. In the first implementation coded pulse modulation system is demonstrated which employ sampling, quantizing and coding to convert analog waveforms to digital signals while the second gives data compression and expansion in digital communication system. The proposed compander circuit is realized with operational transconductance ampliﬁer and diode. Required power supply to operate the circuit is 3.5V. Performance of the circuits realized with OTAs has been demonstrated through SPICE simulation.

Index Terms —Adaptive Delta Modulator, Compander, Delta Modulator, Operational Transconductance Amplifier. —————————— ——————————

1 INTRODUCTION HE objective of this paper is to propose a signal innovative designs implemented with CMOS VLSI tech- T processing methodology for communication system nology and (ii) Reduction of design turn around time. with a network of operational transconductance am- Discrete passive components are major barrier to reduce plifier (OTA). Design procedure of two important class of area overhead in analog VLSI implementation. communication system delta modulation and compander OTA is an excellent current mode device to realize have been demonstrated. A novel technique to convert high frequency resistor-less analog designs. With this analog signal to digital signal is Delta Modulation. The backdrop, we have embarked on design of OTA based RF difficulty of Linear Delta Modulator is removed in Adap- communication circuits. A survey of the literature dealing tive Delta Modulator. In this paper both categories of Del- with OTA based designs depicts following picture. A ta Modulator, realized with OTA have been studied. Data number of researchers have employed OTA as the basic Compression is an essential part in communication sys- building block in the design of non-linear networks. Ed- tem particularly in voice transmission [1]. Data compres- gar Sanchez-Sinencio et al. [2] have reported synthesis of sor is a nonlinear device that compresses the signal ampli- a number of nonlinear circuits with OTA network. The tude. The range of voltage covered by voice signals, from authors have reported two synthesis techniques viz. ra- loud talk to weak talk, is in the order of 1000 to 1. A non- tional approximation functions and piece-wise linear ap- uniform quantizer, characterized by a step size that in- proximation technique. Four-quadrant multiplier and a creases as the separation from the origin of the transfer phase shifter are the two important analog functions used characteristic is increased, satisﬁes this requirement [1]. in data communication system. Bang Sup - Song [3] has Non-uniform quantization can be practically imple- reported the recent development in respect of synthesis of mented with a compressor followed by a uniform quan- these functions with OTA networks. Modulator circuit for tizer. The original signals are restored by the reverse analog and digital communication system (AM, ASK, FM, process of compressor, known as expander. The combina- and FSK) has been synthesized and realized with single tion of a compressor and an expander is called compand- output OTA by Ray et al. [4]. Similar type of digital com- er. A new design technique of low power OTA based munication circuit (i.e., ASK/FSK/PSK/QAM) using compander circuit is introduced in this work. The motiva- multiple output OTA and a number of digitally con- tion to undertake this research stems from the following trolled switches is proposed by Taher et al. [5].Also, Hie- considerations. (i) Growing popularity of analog and tala proposes PSK/GMSK polar transreceiver [6], Rahim mixed signal ICs has provided the impetus to explore et al. present software deﬁned wireless receiver [7]. It is well accepted that the design of the radio frequency (RF) ———————————————— section in a communication integrated circuit (IC) is a • A. Roy is with the Electronics and Telecommunication Department, Ben- challenging problem. gal Engineering and Science University, Shibpur, Howrah-711103, India. In the recent past a few researchers have presented design • K. Ghosh is with the Department of Electronics & Communication Engi- methodology of compander circuit. In [8] Zrilic et.al pro- neering, IERCEM Institute of Information Technology, India. poses a compander circuit based on sigma delta modula- • S. Mondal is with the CST Department Bengal Engineering and Science University, India. tor. That proposed circuit is implemented with fully digi- • B. N. Ray is with the Electronics and Telecommunication Department, tal devices. A new class of compander system is proposed Bengal Engineering and Science University, India. that combines conventional broad band companders with

adaptive ﬁlter is proposed in [9]. Sakran et al [10] present a new scheme based on µ law compander, where opti-

119 mized value of µ is computed, to reduce peak to peak power ratio (PAPR) and to improve the efficiency of the Proceeding iteratively and assuming zero initial condition, multicarrier modulation technique like OFDM. Also (i.e., mq [− ]1 = 0 and d q ]0[ = 0 we arrive at companding filter is proposed in [11], [12], and [13]. In k [14] Roy et.al presents a generalized synthesis methodol- mq [k] = ∑ d q [m] (4) ogy of including compander, using OTA. The above dis- m=0 cussion depicts that though the attention has been given Equation 4 represents an accumulator (adder). If the out- to design compander circuit using digital components, to put d q[k] is represented by impulses, then the accumula- avoid expensive and bulky analog discrete communica- tor may be realized by an integrator. OTA network of tion circuit, components and higher power supply re- linear delta modulator is shown in Fig. 2. In that figure quirement, or to optimize the value of µ to reduce PAPR. (Fig. 4) the portion of the circuit encircled by dashed lines In this scenario, this work presents design methodology represent OTA realization for the linear delta modulator. of communication (analog and digital) circuits (Delta Along with that portion, circuit outside the dashed line Modulator and Compander) with an array of OTAs and represents continuously variable slope delta modulator active element like semiconductor diode. The simulation (discussed in the next section). Subsequent discussions results of the circuit performance have also been reported. analyze linear delta modulator. Along with that portion, Section 2 and 3 reports the design methodology of OTA circuit outside the dashed line represents continuously based radio frequency non linear communication circuits. variable slope delta modulator (discussed in the next sec- Finally, the simulation results are highlighted in section 4 tion). OTA1 and OTA2 serve the purpose of a de ference to verify the performance of the designed circuits. amplifier leading to a two level quantizer. The output of the quantizer assumes the supply voltage level +V and -V de- pending on whether the input overrides the pre-dicted signal 2 DESIGN OF DELTA MODULATOR or not. The output of the quantizer is sampled at a rate fs= This section deals with design methodology of OTA 2kf m(by the switch S 1in Fig. 2), where f m is the frequency of the based linear and Adaptive Delta Modulator circuit used input sine wave and k is the oversampling factor. The sam- in communication system. pled quantizer output is fed next to the input of an integrator implemented by the OTA3 and capacitor C1. The input to the 2.1 Delta Modulator Delta modulation (DM) may be viewed as two-level (1- bit) quantizer. A block diagram of DM encoder is shown in Fig. 1. From this figure, we obtain mq [k] =m q [k − ]1 + d q [k] (1) Hence, mq[k − ]1 =mq [k − ]2 + dq[k − ]1 (2) Substitution of Eqn. 1 in Eqn. 2 yields mq[k − ]1 =mq [k − ]2 + dq[k − ]1 (3)

Fig. 2. OTA realization of Adaptive Delta Modulator.

integrator is given by the following equation. si (t) = V ∑an p(t − nT s ) (5) where V is the maximum output level of the quantizer and an is either +1 or -1, and T s=1 / f s. The pulse shape p(t) is given by Ts p(t) = u(t) − u(t − ) (6) 2 where u(t) is Heaviside step function. Assuming ideal OTA,

Fig. 1. Block diagram of Delta Modulator. the output of the integrator is given by

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essentially constant for wide range of input power levels. If the step size of the quantizer increases non- uniformly then the dynamic range of the quantizer is improved. By using a device called compressor the desired form of the non-uniform quantization can be achieved. Characteristic of a compressor can be mathematically represented by logarithmic function [15]. Figure 3 shows OTA realization of compressor. Analysis of OTA-Compressor circuit is as follows. At the output node of OTA2 of that circuit, characteristic of a compressor can be expressed by logarithmic

ln( 1+ µ.vi (t)) Fig. 3. OTA realization of Compressor. v0 (t) = (9) ln( 1+ µ) g m3 Ts where vi (t) and vo (t) are the magnitude of the input and s0 (t) = ∑ an p(t − nT s ) − ρ(t − nT s − ) (7) C1 2 output signal respectively and µ is a parameter that is se- Where ρ(t) is a ramp function and Ts=1/ fs . lected to give the desired compress characteristic. If it is as- The step size δ of the staircase waveform is given by sumed ln( 1+ µ) = D , then the equation (1) can be written

D.v0 (t ) g m3Ts as, e −1 = µ.vi (t) (10) δ = (8) 2C1 A semiconductor diode is characterized by the equation V .q nKT 2.3 Continuously Variables slope Delta Modulator I = I s (e − )1 (11) A linear delta modulator requires a large over sampling factor 'k’ (from 8 to 16) for proper operation. By operating the mod- where I s =reverse saturation current, n=emission coeffi- ulator in an adaptive mode which in it in the granular region cient, K is the Boltzman constant (K=1.381x10 -23 ), T is the the over-sampling factor can be reduced down to 4 to 8 thus absolute temperature in degree Kelvin, and q is the decreasing the output bit rate. A continuously variable charge of an electron (q =1.6x10 -19 ). The symbol ‘n’ is uni- slope delta modula tor adapts the step size in a continuous ty for germanium and approximately 2 for silicon [16]. fashion both in the granular and overload regions. Fig. 2 (in- Therefore exponential behavior of equation (10) conceived cluding linear delta modulator encircled by dashed lines) vividly as real by semiconductor p-n junction diode equa- represents continuously variable slope delta modulator. The tion [(3)] and an OTA. Fig. 3 displays the OTA based com- front end of the circuit (encircled by dashed line) is an integra- pressor circuit. Applying K.C.L at the node ‘x’ in the Fig. 3 tor followed by a rectifier and a low pass filter (LPF). When we get, the delta modulator is overloaded the input to the integrator V0ut g will be a sequence of all positive or all negative pulses. The nKT Vin .g m = (e − )1 I s magnitude of the output of the integrator in this case will be (12) higher. The integrator output is rectified and filtered by LPF. The output of the low pass filter increases the transconductance g m3 of the integrator of the linear delta modulator block. qV out Vin .g m As a result step size (δ) increases because step size (δ) directly e nKT = 1+ ln[ 1+ ] (13) varies with g m3 (equation 8). When the modulator hunts I s around a flat portion of the input signal the output of the Taking natural logarithm on both side of the equation (11) switch will be alternate in polarity (i.e. its magnitude changes produces between positive and negative values alternatively). As a re- q.Vout Vin .g m sult, transconductance of OTA3 (g m3) will assume a low value. = ln[ 1+ ] (14) Consequently, from equation (8) it can be concluded that step nKT I s size (δ) decreases. Thus Fig. 4 realizes continuously variable Thus the output of the compressor is stated as, slope delta modulator increases the step size in the overload nKT Vin .g m region and decreases Vout = [ln( + )]1 (15) q I s 3 COMPANDER KT At room temperature 300 K the value of =26mV and For transmission of speech signal using waveform coding q techniques the same quantizer has to accommodate input assuming n=2, output of the compressor can be written as, signals with widely varying power level. The range of Vin g m voltages covered by voice signals, from loud talk to weak −3 ln[ 1+ talk, is in the order of 1000 to 1. For accept able voice trans- Vout 26 ×10 Vin .g m I s = ln[ 1+ ] = (16) mission signal-to-quantization noise ratio should remain x x I s x ×19 23.

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tion (15). Again applying K.C.L at node ‘a’, we get

Vi .g m1 = Vi′g m3 (20)

Substituting values of Vi′ from equation (18) in equation I s (17), and putting µ = we get g m3

gm1 Vi 38. 46. 1 gm3 V0 = [e − ]1 (21) µ

4 SIMULATION RESULTS AND DISCUSSION Fig. 6. Output characteristic of simulated Expander. To confirm the practical validity of the proposed circuits, shown in Fig 2, 3 and 4 are simulated using CADENCE g m SPICTRE cad tool. A CMOS OTA structure with dimension By comparing eqn (9) and (15) it can be stated that is of the channel length (L) and width (W) of the NMOS and I s PMOS are 0.3µm and 0.4µm respectively. Parasitic output equivalent to µ and x ×19 23. = ln[ 1+ µ]. Thus the capacitance of the OTA is 2.1 fF. A simple CMOS linear value of µ can tuned with the transconductance of OTA OTA with deferential input was proposed by Szczepaski [13]. Such an OTA with g m varying 70 to 120 µA/V is employed in our design application of Delta Modulator. Figure 7 shows the input and output signal of the delta modultor. Frequency of the input signal is 10 MHz and step size is 250 mv. Step size can be tuned by varying OTA3 and C1 in Fig. 4. The output of adaptive delta modulator is depicted in Fig. 8. Like delta modulator the input signal frequency is 10 MHZ and the sampling frequency is 90 MHZ. It is clear from Fig. 8 that step size varies from 0.4 volt to 0.9 volt. Fig. 5 depicts the simulated output characteristic of compressor. As the practical value of µ of equation (1) is approximately 255 the range of 10µA to 50µA [10], thus the transconductance of the OTA is tuned from 1mA/V to 50mA/V.

TABLE 1: VALUES OF

gm, Is and µ

gm Is µ Fig. 4. OTA realization of Compander. 10 mA/V 28.5µA 350 i.e. g m . Inverse process of compressor is known as expander. Thus expander output can be expressed as vi (t ) v0 (t) = e (17) vi (t) is the compressed signal and input of the expander and v0 (t) is the output of the expander. Fig. 4 depicts the OTA based expander circuit. At node ‘b’ in Fig. 6 applying KCL we get, Vi′q KT g m2V0 = (e − )1 I s (18)

Hence V0 g m2 ln[ 1+ ] I s Fig. 5. Output characteristic of simulated Compressor. Vi′ = (19) 38 .46 9 mA/V 39.5 µA 230 Equation (18) is valid at room temperature 300K (ref. equa- 8 MA/V 39.5 µA 204

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A ramp signal, which varies from 0V to 2.5V during the time [3] B. S. Song, “Cmos rf circuits for data communication applica- interval 0 to 1ms, is applied at the input of the compressor. tions,” IEEE journal of solid state circuits , Vol - Sc - 21, No -2. Output characteristic curves depicts for three different val- April, PP - 350-377., 1992. ues of g m, I S, and µ as shown in the Table 1. Fig. 5 shows the [4] A. Roy, S. Mandal, B. Ray, and P. Ghosh, “Synthesis of cmos ota simulated output of the expander. It can be shown that the based communication circuit,” 5 th International Conference on simulated result validates the theory of the compressor dis- Electrical and Computer Engineering , ICECE 2008, 20-22 Dec. cussed in Section 3. Dhaka, Bangladesh, pp. 107-112 , 2008. [5] M. Taher and Abuelma’atti, “New ask/fsk/psk/qam wave generator using multiple output oper- 5 CONCLUSION ational transconductance amplifiers,” IEEE Tran on Circuit and The potential of the use of current mode device, OTA, for System -I Fundamental Theory and Application , Vol. 48 , No. 4, pp. RF circuit has been discussed. From considerations of cell 487-489 December, 2001. based design of non linear circuit, OTA is preferred for its [6] A. W. Hietala, “A quad band 8 psk / gmsk polar transreciever,” simplicity. Though three types of communication circuits IEEE Journal of Solid State Circuits , Vol. 41 , No. 5, PP - 1131 - are presented in this paper, using OTA as basic building 1141 May, 2006. [7] R. Bagheri, “An 800mhz - 6ghz software defined wireless recei- verin 90 nm cmos,” IEEE Journal of Solid State Circuits , Vol. 41, No. 12, pp. 2860 - 2879 December, 2006. [8] D. G. . Zrilic and U. J. Dole, “A digital square-law compander,” World Automation Congress 2004 Proceeding, Vol. 16, 28 June - 1st July, 2004, pp. 449-454 , 2004. [9] U. Z. Martin Holters, Florian Keiler and J. Peissig, “Compander systems with adaptive preemphasis/deemphasis using linear predic- tion,” Signal Processing System Design and Implemntation, IEEE Workshop, on 2-4 Nov. pp. 386-390 , 2005. [10] M. S. H. Sakran and A. A. Elazm, “A new peak-to-average power reduction technique in the ofdm system using -law compander,” IEEE Symposium on Wireless Communication System, ISWCS08, 21-24 Oct., pp. 386-390 , 2008. [11] E. M. D. Andreas G. Katsiamis, Konstantinos N. Glaros, “In- Fig. 7. Output characteristic of Delta Modulator (DM). sights and advances on the design of cmos sinh companding filters,” IEEE TRANS- ACTIONS ON CIRCUITS AND SYSTEMS–I: REGULAR PAPERS, VOL. 55, NO. 9, OCTOBER, pp. 2539-2550 , 2008. [12] A. Lopez-Martin and A. Carlosena, “1.5 v cmos companding filter,” ELECTRONICS LETTERS 24th October, Vol. 38 No. 22, pp.1346-1348 , 2002. [13] Y. Y. Tao Jiang and Y.-H. Song, “Exponential companding technique for papr reduction in ofdm systems,” IEEE TRANS- ACTIONS ON BROAD- CASTING, VOL. 51, NO. 2, JUNE, pp. 244-248 , 2005. [14] A. Roy, S. Mondal, B. Ray, and P. Ghosh, “Synthesis of CMOS OTA based communication circuit,” 5 th International Confe- rence on Electrical and Computer Engineering, ICECE 2008, 20- 22 Dec. Dhaka, Banladesh, pp. 107-112,2008. [15] J. G. Proakis, Digital Communication, Second Edition . Internation- al Edition: McGraw-Hill Book Company, 1989. [16] Millman and Halkias, Electronic Devices and Circuits . Interna- Fig. 8. Output characteristic of Adaptic Delta Modulator (ADM). tional Student Edition: McGRAW-HILL KOGAKUSHA Ltd., 1967. block and with the cell library concept other types of nonlinear circuits can easily be designed. .

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