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A Low-voltage Wide-band Current-mode Automatic Gain Control (AGC) Kriangkrai Sooksood1 and Montree Siripruchyanun2 1, 2 Department of Teacher Training in Electrical Engineering, Faculty of Technical Education, King Mongkut’s Institute of Technology North Bangkok, Bangkok, 10800, THAILAND Email: [email protected], [email protected] ABSTRACT In this paper, a current-mode automatic gain control (AGC) is presented. Due to operation in current-mode, the proposed circuit provides a wide frequency response, a low supply voltage, low power consumption and electronic controllability. Thus, it is very suitable for use in portable and battery-powered equipment such as a hearing aid instrument and a wireless radio device. The proposed AGC consists of current controlled exponential amplifier, current-mode precision rectifier, current-mode low pass filter and current-mode integrator, which can be fabricated into Integrated Circuit (IC) form. The performances of the proposed circuit are explored through HSPICE simulation program using BSIM3v3 model from MOSIS, they demonstrate good agreement to the theoretical anticipation. The proposed circuit works at ±1.5 V supply voltage, power consumption is merely 712 µW. Keywords: AGC, CMOS, Low-voltage circuit 1. INTRODUCTION As well known, AGCs play a very important role in modern hearing aid devices and communication systems [1-3]. An AGC is a closed-loop system that automatically adjusts the voltage gain such that the output voltage stays within a desired range. Many published literatures have proposed different methods to design the AGC circuits [4-6]. However, as investigated, all above AGCs work in voltage-mode. As a result, there are some restrictions in such: high supply voltage and power consumption, narrow frequency response, narrow dynamic range, complicated circuit details and absence of electronic control. In the last decade, there has been much effort to reduce the supply voltage of analogue CMOS systems. This is due to the command for portable and batterypowered equipment. Since a low-voltage operating circuit becomes necessary, the current–mode technique is ideally suited for this proposes. Actually, a circuit using the current-mode technique has many other advantages, which can be found in many literatures [7-8]. The purpose of this article is to present a realization of an AGC circuit functioning in current-mode. The proposed circuit comprises exponential current controllable amplifier, precision rectifier, low-pass filter and integrator. All circuits can be realized using CMOS technology. The simulation results through HSPICE using 0.5 μm CMOS technology are achieved to confirm that the realized circuit can provide wide dynamic range, low supply voltage, and wide bandwidth including low power consumption. In addition, the gain of AGC can be controlled by the reference current. Consequently, the proposed circuit is very appropriate for further fabricating into Integrated Circuit (IC) form to employ in portable electronic equipment such as hearing aid instrument and wireless communication device. 2. PRINCIPLE AND CIRCUIT DESCRIPTION 2.1 The Principle of the Proposed AGC The proposed AGC scheme can be found in Fig. 1. It consists of the exponential current amplifier whose gain can be adjusted by an input bias current. The currentmode precision rectifier is employed to convert the output sinusoidal current to a positive full wave ( I A ). Subsequently, this signal will be applied to current-mode first-order low pass filter to change it to a DC current level ( I F ). After that, the DC current is subtracted to a reference current, the remaining current is employed as input current of integrator ( I E ). The output current of integrator is used as exponentially control current ( I B ) of the gain of the current amplifier. I in I out Current Controlled Exponential Amplifier IB Current-mode Integrator − I REF IE ∑ +I F Current-mode Low Pass Filter IA Current-mode Precision Rectifier Fig.1: The principle of proposed AGC 2.2 Current-mode Exponential Control Current Amplifier The current amplifier using in the AGC system must be exponentially controlled to maintain AGC loop settling time which is independent of signal levels of M 6M 7 M1 VDD M 16 M 10 M 18 IB IX M3 M2 I out + I X + I in M4 M 5M 8 M 9 M 11 I out − M 13 I X − I in M 14M 15 M 12 M 17 VSS Fig.2: Current-mode exponential control current amplifier input current [9]. Fig. 2 shows the current-mode exponential control current amplifier where the output current ( I out ) can be exponentially controlled using I B , modified from [10]. From Fig. 2, the output current; I X generates exponential function of I B , thus the output current equation can be shown as full-wave signal consists of DC fundamentals, a currentmode low-pass filter is required. Fig. 4 shows the firstorder current-mode low-pass filter, which is modified from using bipolar version [12]. When I 4 = I 5 , the transconductance gains of M26 and M27 ( g m ) are equaled and matched. The transfer function of the filter is given by IB I out = I out + − I out − = I in e KG . 2 IF ( s) (1) IA (s) Where K is a transconductance parameter, considered as all matched PMOS and NMOS. G equals VDD − VSS − VTP − VTN . From Eqn. (1), we can found that 2 C1 I A = I out . (2) VDD M 23 M 24 M 25 IA I1 M 22 M 19 I out I3 C1 s +1 gm I4 I5 IA IB Since the output current of the current amplifier must be changed to full-wave signal, consequently, a currentmode precision full-wave rectifier is required. Fig. 3 illustrates the circuit, improved from [11]. The current source I 3 , which equals to I 2 , is used to eliminate a DC current offset of the output current I A . Hence, I A can be found as 1 (3) . VDD the controlled gain is e KG . 2.3 Current-mode Precision Rectifier = M 26 IF M 27 VSS Fig.4: Current-mode low-pass filter 2.5 Current-mode Integrator It was stated that it is desirable to keep the DC loop gain as large as possible in order to maintain a constant output level [1]. One method is to employ an integrator, since an ideal integrator has infinite gain at DC, the steady-state output amplitude will not change in response to slowly varying changes in the input amplitude. Fig. 5 shows the current-mode integrator, developed from the current-mode differential integrator [13]. When the current source I 6 , which is equal to I 7 , equals to 2 I and I 8 is set to K i I . Consequently, the output current ( I E ) can be shown as IE ( s) = Ki g m IE ( s) C2 s (4) I2 M 20 M 21 VSS Fig.3: Current-mode precision rectifier 2.4 Current-mode Low-pass Filter The resulting output current of the precision rectifier must be filtered to achieve a DC current level. Because a where K i is a positive real number and all matched transistors are considered. From the block diagram of the proposed AGC in Fig. 1, the remaining part is a current differential circuit. It is not required, because the current sources can be directly connected. This is an advantage of current-mode circuit over voltage-mode version. VDD I6 I7 I8 IE M 29 M 28 M 30 C2 M 31 M 33 M 32 IB M 34 VSS Fig.5: Current-mode integrator Table 1: Transistor Aspect Ratios Transistor M1-M18 M19, M20, M21, M23, M24 M22 M25 M26- M29 M30- M33 M34 W/L (µm/µm) 30/3 4/1 1/4 16/16 10/3 10/5 10/5 (M=5) Fig.7: Output current when input current is 25 µAp-p and decreased to 15 µAp-p with delay 200 ms. 3. SIMULATION RESULTS AND DISCUSSIONS 24 22 Iout/μΑ (dB) The proposed AGC system was simulated by HSPICE simulation program through BSIM3v3 model of 0.5 µm CMOS technology obtained from MOSIS. It was operated by ±1.5 V supply voltages and the bias currents are following: I1 = 3 µA, I 2 = I 3 = 5 µA, I 4 = I 5 = 20 µA, I 6 = I 7 = 40 µA and I8 = 100 µA. The aspect ratios of all transistors are listed in Table 1, where capacitor C1 and C2 are 1 μF and 100 nF, respectively. The feedback current I B is also amplified with 5 times gain and K i = 5 before feeding to the current amplifier. The responses of AGC, for 500 Hz sinusoidal input current with 15 μAp-p and changed up to 25 μAp-p with delay 200 ms and 25 μAp-p and changed down to 15 μAp-p with delay 200 ms, are shown in Fig. 6 and 7, respectively, with I REF = 5 μA. It is readily seen that the AGC system provides a constant output current when input current is changed, however some settling times are required. 20 18 16 0 20 40 60 80 100 120 140 160 Iin (μAp-p) Fig.8: The current transfer characteristic 60 50 Iout (μAp-p) 40 30 20 10 0 5 10 15 20 IREF (μA) Fig.9: Output current against current reference variations Fig.6: Output current when input current is 15 µAp-p and increased to 25 µAp-p with delay 200 ms. The current transfer characteristic of the AGC system is illustrated in Fig. 8, which was achieved with 500 Hz sinusoidal input current and I REF = 5 μA. It is clearly seen that AGC provides the output current almost constant for input current varied between 12 to 150 μAp-p. This means that the AGC input dynamic range when no more than 1 dB normalized output current acceptable is 22 dB. In addition, the behaviour of the AGC system when I REF changed was also investigated in Fig. 9. It illustrates output current when I REF was varied from 5 to 20 μA with I in = 60 μAp-p of 500 Hz sinusoidal signal. The result shows that AGC output current is proportional to the current reference ( I REF ). Fig. 10 shows the frequency response of the AGC system for 20 μAp-p input current and I REF = 5 μA. It is obviously seen that the proposed AGC can operate over a wide range of frequencies, the -3 dB bandwidth of the proposed AGC system is about 110 MHz. [2] [3] [4] [5] [6] [7] [8] Fig.10: Frequency response of the proposed AGC [9] 4. CONCLUSION A wide-band current-mode AGC system has been proposed in this paper. The proposed AGC system composes of exponential control current amplifier, precision rectifier, low-pass filter and integrator. The proposed AGC circuit provides a wide frequency response, a low supply voltage and low power consumption because all circuits operate in current-mode. In addition, the closed-loop gain can be controlled by the reference current. The simulation results confirm the performances of the proposed AGC. They show the dynamic range of 22 dB, controllability of gain, -3 dB bandwidth of approximately 110 MHz, and power consumption of 712 µW. With the above features, the proposed AGC is very suitable for further fabricating into IC form, where the large-value capacitor ( C1 ) can be an external element, for use in portable electronic equipments. Our future work is to develop the circuit to obtain lower settling time and lower power consumption. 5. REFERENCES [1] J. R. Smith, Modern Communication Circuits, 2nd ed., McGraw-Hill, Singapore, 1998, ch. 5. [10] [11] [12] [13] K. M. Abdelfattah and A. M. Soliman, “Variable gain amplifier based on a new approximation method to realize the exponential function,” IEEE Transaction on Circuits and Systems, Vol. 49, No.9, pp.1348-1354, 2002. W. Serdijn, A Vander Woerd, J. 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