222

lElCE TRANS. ELECTRON.,

VOL.E91-C,

NO.5 MAY 2008

806

I LETTER

A 90 dB 1.32 mW 1.2 V 0.13 mm2 Two-Stage Variable Gain Amplifier in 0.18 pm CMOS Quoc-Hoang

DUONGta),

Jeong-Seon

SUMMARY An all CMOS variable gain amplifier (VGA) which features wide dB-linear gain range per stage (45 dB), low power consumption (1.32 mW), small chip size (0.13 mm2), and low supply voltage (1.2 V) is described. The dB-linear range is extended by reducing the supply voltage of the conventional V-to-I converter. The two-stage VGA implemented in 0.18 J1m CMOS offers 90 dB of gain variation,

3 dB bandwidth of greater than 21 MHz, and maximin input IP3 and PI dB, respectively, of -5/-42 and -12/-50dBm. key words: variable gain amplifier (VGA), automatic gain control (AGC), amplifier, analog circuit, transceiver

1.

Introduction

Variable gain amplifiers (VGA) are important blocks to accommodate the large dynamic range of signals [I], [2]. VGAs require dB-linear gain variation which can be satisfied with components of the exponential 1- V characteristic. Due to the square-law 1- V characteristic, CMOS transistors cannot directly be applied for the dB-linear VGAs. Instead, dB-linear performance is achieved by adopting the circuits that approximate the exponential equations [I], [2]. In this work, the conventional VGA cell based on the well-known pseudo-exponential equation y=!(x)=(I+x)

(I -

(I) x)

is adopted as the basis for the proposed VGA design [2]. The conventional VGA that adopts (I) offers 15 dB of gain variation with linearity error of less than ±0.5 dB [2]. Unfortunately, there are many applications that require a wide dynamic gain range; for example, code-division multiple access (COMA) systems need 80 dB of the gain variation. To cover such a wide dynamic gain range, up to 6 VGAs that adopt (I) must be cascaded in the COMA system, leading to high power dissipation, large chip area, high noise figure, poor linearity, and high cost. The low-power VGA is essential to reduce the overall power dissipation of the system. Moreover, a VGA with a wider dynamic gain range is one of the efficient solutions to decrease the number of the Manuscript received March 12, 2007. Manuscript revised May 21, 2007. tThe authors are with the School of Engineering, RFME Laboratory, Information and Communications University, 119-Munjiro, Yuseong-gu, Daejeon, Republic of Korea, 305-714 Korea. ttThe authors are with Samsung company, Suwon, Korea. a) E-mail: [email protected] DOl: \O.1093jieteleje9I-c.5.806

Copyright

LEEt, Sang-Hyun

MINtt, Joong-Jin KIMtt, Nonmembers, and Sang-Gug LEEt, Member

required VGA stages, leading to reduction of the power dissipation, chip area, and cost. Consequently, the VGA with a wide dynamic gain range and low power dissipation is critically desired for low-power applications. Lowering the supply voltage is also an effective way of reducing power consumption and it is the technology trend. However, many of the existing CMOS analogue building blocks, designed to operate with higher supply voltages, will lose a significant amount of operating range and need to be reconsidered. In this letter, the disadvantage at low-voltage operations is adopted as a mean to widen the dB-linear gain range of the VGA drastically. The low supply voltage leads to low power consumption and the wider decibel-linear gain range reduces the number of the required VGAs such that the power consumption and the chip size are reduced significantly. The circuit design detail is given in the following section. 2.

Newly Proposed VGA

Figure I shows a block diagram of the two-stage VGA which includes a V-to-I converter, two VGA cells, and a buffer. In Fig. I, the VGA cell adopts (I) for dB-linear gain variation and the V-to-I converter generates currents to control the gain of the VGA cell. The buffer is added for measurement purposes only. Unlike conventional VGAs based on (I), the proposed VGA adopts the V-to-I converter with distorted V-I characteristic due to supply voltage reduction to extend the dBlinear gain variation drastically. The schematic of the V-to-I converter and VGA cell are shown in Figs. 2 and 3, respec-

v •• v•.

v,

Fig.l

Block diagram of the proposed two-stage VGA.

© 2008 The Institute of Electronics, Information and Communication Engineers

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LETTER D---1 V.M,M. f-

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Fig. 4 Simulation for VDD = 1.2 and 1.8 V for (a) -It vs. to-I converter and (b) gain vs. VCTRL of the proposed VGA, and 0.7 V for VDD = 1.8 and 1.2 V, respectively).

VCTRL (VB/AS

of V= 0.9

Schematic of the proposed V-to- I converter.

cml" V~to·l converter

From the ( D CTRL, Variable

Fig. 3

gain amplifier

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circuit

feedback --.{

Schematic of the VGA cell [I].

tively, where all transistors are biased in saturation region. In the V-to-I converter shown in Fig. 2, the voltages V'.2 are where VB/AS is the DC given as V,,2 = VB/AS ± (I/2)VcTRL, bias and VCTRL the differential voltage to control the gain of the VGA cell. Then, the currents II and h in Fig. 2 can be given by = -I, = (1m - 1m)

=

(V)

(a)

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12

at VDD~ I 8V

VCTRL variable

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V3 - VTH)

VCTRL

(2)

where ID',D2, V3, and VTH are the drain currents, the source and the threshold voltages, respectively, of transistors M7-8. In Fig. 2, the currents Ic, = I, + 10 and IC2 = 12 + 10 are used as gain control signals to the VGA cells, where 10 is the bias current. The currents IC',2 from the V-to-I converter are mirrored to transistors M32,33 of the VGA cell. The VGA cell shown in Fig. 3 is a differential amplifier with diode-connected loads [I]. Since the amplifier offers high gain, the common-mode feedback circuit (CMFB) is needed to stabilize the output DC voltage for biasing the following stages [I]. Assuming that (WI L)M26,29 = (WI L)M27,28, the gain of the proposed two-stage VGA shown in Fig. I can be given by [I]

= ( gm,M27,28 gm,M26,29

r

Ic,

IC2

(3)

where gm,M26,29 and gm,M27.28 are transconductances of the corresponding transistors. From (3), the gain of the proposed VGA is equivalent to (I), where x = 1,110, and I, is a function of VCTRL as given in (2). Consequently, the gain of the VGA can be controlled by varying VCTRL. Figure 4(a) shows plots of -I, in (2) as a function of VCTRL for the supply voltage VDD of 1.8 and 1.2 V, respectively. As shown in Fig.4(a) by the solid line, the 1"VcTRL relation diverges from the linear behavior as VDD reduces from 1.8 to 1.2 V. The divergence of the 1,- VCTRL characteristic is caused by the early entry into the linear-mode operation of one of the transistors, M7 or M8, while the other cuts off. Additionally, the divergence of the 1" VCTRL characteristic is also caused by a small output resistance of the current sources, hias, 1" and 12, as VDD is reduced. For the linear IJ - VCTRL relation given in (2) and shown in Fig. 4(a) by the dashed line, Fig. 4(b) shows the plot of (3) (dashed line); Au as a function of VCTRL. As can be seen by the dashed line in Fig. 4(b), the VGA offers only 30 dB of gain variation with linearity error of less than ±0.5 dB and the gain deviates from the dB-linear behavior as IVCTRLI increases, To extend the dB-linear gain range of (3), the nonlinear 1,- VCTRL relation as shown in Fig.4(a) by the solid line is adopted. From Fig. 4(a), as VCTRL increases from 0 to 0.6 V, the numerator and denominator of (3) as a function of VCTRL are respectively increased and decreased so that the gain as given in (3) increases. Consequently, the dashed line in Fig. 4(b) is shifted to the solid line, leading to extension of the dB-linear gain range. Similarly, as VCTRL reduces from o to -0.6 V, the numerator and denominator of (3) are respectively decreased and increased so that (3) reduces from the dashed line to the solid line, leading to the extension of the dB-linear gain range. Consequently, it can be seen that the divergence from the dB-linear characteristic of the conventional VGA is compensated by introducing distortion in the V-to-I converter through the supply voltage reduction. With the proper amount of distortion in the V-to-I converter, the VGA can be corrected to nearly ideal dB-linear perfor-

IEICE TRANS. ELECTRON .. VOL.E91-C.

NO.5 MAY 200S

808

mance, leading to the extension of the dB-linear gain range. The interesting aspect of the proposed VGA is that the reduction in supply voltage, which tends to degrade circuit performance in general, leads to an extension of the dBlinear gain range. Therefore, the proposed VGA is suitable for low-voltage and low-power applications. Since the larger amount of dB-linear range reduces the number of VGA stages, the proposed VGA offers advantages in power dissipation, chip area, and cost. 3.

Measurement

Results

The proposed VGA is optimized for a supply voltage of 1.2 V while dissipating 1.1 mA (excluding buffer) based on a 0.18 11m CMOS technology. Figure 5 shows the measured

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Fig. 5

gain versus VCTRL. In agreement with expectations, the proposed VGA shows 90 dB (-45 - 45 dB) of gain variation over a wide range of VCTRL (-0.4 - 0.4 V), which is a significant improvement compared to conventional VGAs that adopt (I) [2]. In Fig. 5, the proposed VGA offers about 85 dB of dB-linear range with linearity error of less than ± I dB. The measured 3 dB bandwidth is greater than 21 MHz, and the maximin input IP3 and PI dB are -5/-42 and -12/-50 dBm, respectively. Figure 6 shows the microphotograph of the tested chip where the active area occupies 0.13 mm2. The proposed VGA described in this work represents a significant performance improvement in CMOS-based VGA designs. The key improvements are derived from a huge extension of the dB-linear gain range by lowering the supply voltage. The low supply voltage leads to low power consumption. The wider decibel-linear gain range reduces the number of VGAs such that the power consumption and the chip size are reduced significantly. Compared to all previously-reported CMOS-based VGAs [I], [2], the proposed VGA offers lowest power, smallest size (lowest cost), and high performance solutions for the integrated lowvoltage differential CMOS VGAs.

VGA gain and linearity error vs. VCTRL.

Conclusions

A wide dynamic range VGA by lowering the supply voltage has been presented. The wide gain range and the lower supply voltage lead to low-power consumption and small chip size. The proposed VGA can be used in many low-voltage and low-power applications, such as medical equipments, telecommunications systems, hearing aids, disk drives, etc. Acknowledgements This work was supported in part by MOST-KOSEF (Intelligent Radio Engineering Center) under the SRC/ERC Program. References

Fig. 6

Microphotograph

of the tested chip.

[I] Q.-H. Duong, Q. Le, c.-w. Kim, and S.-G. Lee, "A 95-dB linear lowpower variable gain amplifier," IEEE Trans. Circuits Syst .. vo1.53. no.8, pp.I648-1657. Aug. 2006. [2] c.w. Mangelsdorf, "A variable gain CMOS amplifier with exponential gain control," VLSI Circuits Dig. Tech. Papers, pp.146-149, 2000.