ECE1352F – Analog Circuit Design I

1 ECE1352F Analog Circuit Design I University of Toronto ECE1352F Analog Circuit Design I Term Paper: cmos Variable Gain Amplifier (VGA) Student No. : 981668460 Prepared by : Raymond S. P. Tam Submitted to : Prof. Phang Date : November 15, 2002 ECE1352F Analog Circuit Design I University of Toronto Table of Contents INTRODUCTION .. 1 SECTION I: TWO MAIN APPLICATIONS OF VGAs .. 4 1. Magnetic Data Storage Systems .. 4 2. 5 SECTION II: BASIC Design REQUIREMENTS OF VGAs .. 6 1. Gain Varying 6 2. Exponential Input-to-Gain Variation .. 7 3. High Dynamic 9 SECTION III: EVOLUTION OF cmos 10 1. Gilbert Gain Cell [6].. 10 2. Opamp with Voltage-Controlled Attenuator [10].. 12 3. Parasitic Bipolar Junction Transistors in cmos [5].. 13 4. State-of-Art RF cmos VGA .. 15 Psuedo-Exponential Voltage Generator [4].

2 15 VGA Cell [6].. 17 About .. 19 SECTION IV: FUTURE DIRECTIONS OF 20 1. New Approximation Methods to Realize Exponential Function .. 20 2. Digitized Gain Control Algorithm .. 21 3. Higher Bandwidth Product .. 21 CONCLUSION .. 23 24 ECE1352F Analog Circuit Design I University of Toronto 1 cmos Variable Gain Amplifier (VGA) by Raymond Tam Abstract The basic background of variable gain amplifiers (VGAs) and their recent developments in RF communication systems are discussed in this paper. Several common Circuit topologies are studied. The focus is on cmos techonologies for high integration processes such as System-on-Chip (SOC). Finally, it can be concluded that the future main research areas for cmos VGAs are (1) finding new approximations to the exponential relationship between gain and controlling signal, (2) digitalized gain control algorithm, and (3) higher bandwidth product.

3 INTRODUCTION Over the past few years, there have been dramatic developments in the fields of wireless communication due to the increasing demand for information exchange. As a result, this has become one of the major driving forces for Radio-frequency (RF) IC engineers to incorporate designs with wide dynamic range, high integration at low cost as well as minimal power consumption. One of the many Design challenges faced by communication engineers is the maximum range of data transfer due to unexpected signal fading from obstacles, atmospheric effects, Such consideration, in turn, has led to the Design of robust and adaptive automatic gain control (AGC) employed in the front end of a typical radio receiver. The primary function of AGC is to maintain a constant signal level at the output, regardless of the signal variations at the receiver input.

4 ECE1352F Analog Circuit Design I University of Toronto 2 The major component of AGC is a variable gain amplifier (VGA) whose gain can be dynamically varied by a feedback control signal as shown in figure 1: Thus, a VGA is an indispensable function block for all radio communication systems. A good Design ( large dynamic range for output gain) of VGA is a major factor to ease the Design of AGC systems. In some applications, a transmit VGA in mobile handsets is also needed in CDMA systems to regulate each mobile unit transmit power so equal power from each user is received at the base station for optimal system capacity. Moreover, in the era of mixed-mode integrated circuits, the Design of Analog circuits should be done in the presence of noisy digital circuits.

5 This is because digital circuits always become the bottleneck for technology optimization, instead of Analog circuits. This has always been the main reason why Analog integrated circuits with wide dynamic range have gained so much attention in recent years, particularly in low voltage-low power applications In recognizing the importance of VGAs, this paper introduce readers to the background of VGAs as well as to their recent development from the perspective of RF communication systems. Moreover, special attention will be paid to cmos VGAs since cmos technologies ECE1352F Analog Circuit Design I University of Toronto 3provide high integration solution for system-on-chip (SOC) processes. A few practical cmos VGA circuits will be presented and analyzed for comparison.

6 A list of references will be included at the end of this paper for those who could be interested in a more in-depth study of the subject. ECE1352F Analog Circuit Design I University of Toronto 4 SECTION I: TWO MAIN APPLICATIONS OF VGAs This section provides a general overview of the two major applications of VGAs over the past 15 years, namely magnetic data storage systems and telecommunications. 1. Magnetic Data Storage Systems Before fields of communication had taken off in mid 90s, the major application of variable gain amplifier was for magnetic data storage purposes in 1980s. Disk drive subsystems operating at data rates close to 20 to 40 Mbps in large drives constitute a large market for video speed Analog and digital ICs. VGAs were used to stablize the average amplitude of the read-back pulses, which consists of different combinations of heads and magnetic media and might normally vary by a factor of 30dB with, to a reference value before they were sent to peak-detector (PD) for best signal detection [2].

7 Finally, AGC was employed to set the gain of VGA by sensing the average pulse amplitudes in an envelope detector. The 3 major Design criteria for such VGA were a controllable gain range of 30dB, a linear phase versus frequency ( , flat group delay) characteristic, and low noise figure to prevent SNR degradation [2]. Figure 2 presents a common cmos topology of a VGA for such purpose. In this topology, the input differential pair (M1) is capable of providing a variable gain by changing the resistance of its load transistors (M3) with a dynamic bias current Ic2. The amplified signal of the input stage is then further magnified by the following differential pair (M2). It should also be note that in the case of large gain variation, the bias current for the input stage, Ic1, also serves as a control for the overall gain of the two stages.

8 The rest of the Circuit helps to provide an additional gain stage of fixed amplification as well as widening the bandwidth of the overall circuits by means of neutralization capacitor compensation. ECE1352F Analog Circuit Design I University of Toronto 5 Figure 2: Circuit Diagram of cmos VGA for Magnetic Data Storage Systems[2] 2. Telecommunication Due to the rapid growing trend of RF communication systems in recent years, the need for high selectivity and good control of the output signaling level is also one of the many focuses in any communication systems. An AGC system is thus almost found in every transceiver Design of wireless systems due to unpredictable incoming signal amplitudes from any possible directions. The need for VGA is undoubtedly essential in these systems for normalizing the ECE1352F Analog Circuit Design I University of Toronto 6signal amplitude for further digital signal processing in the receiver.

9 The dynamic range and bandwidth of the VGA required range from 50dB to 100dB and 50 MHz to 200 MHz, respectively, depending on particular applications. Several Circuit configurations used in this area will be discussed in the section III. It should be brought to attention that telecommunication has become the major driving force for the development of high performance VGAs nowadays. SECTION II: BASIC Design REQUIREMENTS OF VGAs Prior to discussing the advancement of variable gain amplifier (VGA) by studying several typical Circuit topologies, it would be beneficial to develop readers with the basic Design requirements of VGA. This helps to understand the limitations and Design challenges often faced by IC Circuit designers. 1. Gain Varying Methods Traditionally, variations in the gain of a VGA, ( cmos process), can be obtained by either varying the transconductance of a MOS device operating in saturation or by varying the channel resistance of a MOS device operated in the linear region [3].

10 The former method requires a varying bias current but the effect is usually not satisfactory because of the square-root relationship between the transconductance of a MOS device with its bias current as shown: Gain gm = sqrt (2*KP*(W/L)*Ibias) For example, a 30dB change in gain implies a large change in bias current in the range of 1x 1000x. This large current variation presents a major drawback in designing high output dynamic range VGAs due to the tremendous power consumption. Moreover, according to [4], the region within which MOS device operates in the saturation region (in a differential pair input ECE1352F Analog Circuit Design I University of Toronto 7stage) is usually not enough for large output variation in low power operation due to the voltage headroom,Veff = Vgs-Vt, required.