Understanding Integrated Circuit Amplifiers and Design Philosophy
Integrated Circuit (IC) amplifiers are essential components in modern electronics, consisting of miniaturized active and passive devices on a semiconductor substrate. This article explores the building blocks, design philosophy, terminology, device variety, and biasing of ICs, emphasizing the use of transistors over resistors for efficiency. It delves into IC technology like CMOS and bipolar, showcasing the design principles behind IC biasing using current sources and mirrors.
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BUILDING BLOCKS OF INTEGRATED- CIRCUIT AMPLIFIERS INTEGRATED-CIRCUIT AMPLIFIERS
WHAT IS IC? INTEGRATED CIRCUIT (IC), ALSO CALLED MICROELECTRONIC CIRCUIT, MICROCHIP, OR CHIP, AN ASSEMBLY OF ELECTRONIC COMPONENTS, FABRICATED AS A SINGLE UNIT, IN WHICH MINIATURIZED ACTIVE AND PASSIVE DEVICES (E.G., TRANSISTORS , DIODES, CAPACITORS AND RESISTORS) AND THEIR INTERCONNECTIONS ARE BUILT UP ON A THIN SUBSTRATE OF SEMICONDUCTOR MATERIAL (TYPICALLY SILICON).
TERMINOLOGY 1-RESISTORS:TO MINIMIZE THE CHIP AREA, LARGE AND EVEN MODERATE-SIZE RESISTORS ARE TO BE AVOIDED. ON THE OTHER HAND, TRANSISTORS CAN BE MADE SMALL AND CHEAPLY, AND THE DESIGNER IS ENCOURAGED TO USE TRANSISTORS IN PREFERENCE TO RESISTORS WHEREVER POSSIBLE. 2- CAPACITORS :SMALL-SIZE CAPACITORS, IN THE PICOFARAD AND FRACTION-OF-A-PICOFARAD RANGE, ARE EASY TO FABRICATE IN IC MOS TECHNOLOGY. 3- POWER SUPPLIES: TO PACK A LARGE NUMBER OF DEVICES ON THE SAME IC CHIP, AND THUS REDUCE SYSTEM COST , POWER SUPPLIES ARE LIMITED TO 1 V OR SO.
4-DEVICE VARIETY: THE IC DESIGNER HAS THE FREEDOM TO SPECIFY THE DEVICE DIMENSIONS AND TO UTILIZE DEVICE MATCHING AND ARRAYS OF DEVICES HAVING DIMENSIONS WITH SPECIFIED RATIOS. 5- BIPOLAR TECHNOLOGY: BJTS ARE STILL USED IN SPECIAL ANALOG APPLICATIONS 6- CMOS TECHNOLOGY: CURRENTLY THE VAST MAJORITY OF ANALOG INTEGRATED CIRCUITS ARE DESIGNED USING CMOS TECHNOLOGY.
8.2 :IC BIASINGCURRENT SOURCES, CURRENT MIRRORS, AND CURRENT-STEERING CIRCUITS BIASING IN INTEGRATED-CIRCUIT DESIGN IS BASED ON THE USE OF CONSTANT- CURRENT SOURCES. ON AN IC CHIP WITH A NUMBER OF AMPLIFIER STAGES, A CONSTANT DC CURRENT (CALLED A REFERENCE CURRENT). IS GENERATED AT ONE LOCATION AND IS THEN REPLICATED AT VARIOUS OTHER LOCATIONS FOR BIASING THE VARIOUS AMPLIFIER STAGES THROUGH A PROCESS KNOWN AS CURRENT STEERING.
8.2.1 THE BASIC MOSFET CURRENT SOURCE ID1 = 1 /2 K( N) (W/ L )1* (VGS VTN)^ 2 :WHERE WE HAVE NEGLECTED CHANNEL-LENGTH MODULATION. THE DRAIN CURRENT OF Q1 IS SUPPLIED BY VDD THROUGH RESISTOR R, WHICH IN MOST CASES WOULD BE OUTSIDE THE IC CHIP. SINCE THE GATE CURRENTS ARE ZERO, ID1 = IREF = (VDD VGS) /R WHERE THE CURRENT THROUGH R IS CONSIDERED TO BE THE REFERENCE CURRENT OF THE CURRENT SOURCE AND IS DENOTED IREF. NOW CONSIDER TRANSISTOR Q2: IT HAS THE SAME VGS AS Q1; THUS, IF WE ASSUME THAT IT IS OPERATING IN SATURATION, ITS DRAIN CURRENT, IO=ID2 = 1 /2 K( N) (W/ L )2* (VGS VTN)^ 2
* WHERE WE HAVE NEGLECTED CHANNEL-LENGTH MODULATION ENABLE US TO RELATE THE OUTPUT CURRENT IO TO THE REFERENCE CURRENT IREF AS FOLLOWS: IO /IREF = (W/L)2/ (W/L)1 THIS CALLED THE CURRENT GAIN OR CURRENT TRANSFER RATIO OF THE CURRENT MIRROR THIS IS A SIMPLE AND ATTRACTIVE RELATIONSHIP: THE SPECIAL CONNECTION OF Q1 AND Q2 PROVIDES AN OUTPUT CURRENT IO THAT IS RELATED TO THE REFERENCE CURRENT IREF \
EFFECT OF VO ON IO VO VGS VTN OR, EQUIVALENTLY, VO VOV RO VO/ IO = RO2 = VA2/ IO VA IS PROPORTIONAL TO THE TRANSISTOR CHANNEL LENGTH; THUS, TO OBTAIN HIGH OUTPUT-RESISTANCE VALUES, CURRENT SOURCES ARE USUALLY DESIGNED USING TRANSISTORS WITH RELATIVELY LONG CHANNELS. FINALLY, NOTE THAT WE CAN EXPRESS THE CURRENT IO IO =( (W/L)2/ (W/L)1 )*IREF (1+ (VO VGS/) VA2))
