design and implementation of cmos rail to-rail operational amplifiers
TRANSCRIPT
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WELCOME
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DESIGN AND IMPLEMENTATION OF CMOS RAIL-TO-RAIL
OPERATIONAL AMPLIFIERS
Grace AbrahamRoll No: 01VLSI & ES
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3CONTENTS
Dept. of ECE09/08/2015
• INTRODUCTION
• RAIL-TO-RAIL INPUT STAGE
• RAIL-RAIL OUTPUT STAGE
• SCHEMATIC DESIGN
• LAYOUT IMPLEMENTATION
• SIMULATION RESULTS
• CONCLUSION
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INTRODUCTION
• Rail-to-rail operational amplifiers allow signals to swing from the negative supply rail to positive supply rail• Input common-mode voltage specifies the range of input for normal operation• Output voltage swing is defined as the range of max. negative to positive peak output voltage• Rail-to-rail high swing op-amps have highly linear response when used as a voltage follower• Rail-to-rail operation is based on op-amp topology
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• Rail-to-rail input stage allows the op-amp to operate with input voltages near the voltage supply rails • The output stage minimizes the voltage drops at the output to achieve high swing
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RAIL-TO-RAIL INPUT STAGE
• Input stages
• PMOS transistors in input terminal allows i/p voltage to operate near the negative supply rail• NMOS differential i/p pairs operate near the positive rail • Parallel-connected PMOS and NMOS differential stage achieves operating mode at both rails• At least one of the differential i/ps is still active at either rail
PMOS differential input pairs or NMOS differential input pairs
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1. Complementary Differential Pair With Active Load
• Effect of active load is to realize a single-ended output for both NMOS & PMOS pairs• Each output is then connected as inputs to a push-pull inverter stage• Output of NMOS pair biases the PMOS in push-pull inverter and PMOS pair biases the NMOS • Push pull inverter stage integrates the o/ps of previous stage into single output• This circuit uses less transistors and less biasing network• With an addition of an o/p stage after push pull inverter lead to an issue of op-amp stability and compensation
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2. Complementary Differential Pair With Cascode Load
• Folded cascode op-amp with a PMOS differential pair and NMOS cascode load• Folded cascode op-amp with a NMOS differential pair and NMOS cascode load• More transistors compared to input stage at active load • Topology is more complex but offer more flexibility in design• Topology has fewer stages making op-amp compensation for stability manageable
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RAIL-TO-RAIL OUTPUT STAGE
• Output stage minimizes voltage drops at the output branch • Allows the output to reach the rails• Output stage increases the op-amp’s voltage gain
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1. Push – pull Inverter Output Stage 12
• Also known as complementary common-source output stage• Uses a common-source NMOS & PMOS connected at the drain• Push-pull inverter output stage due to behavior of topology with each device conducting for alternate half-cycles at the input• Uses 2 transistors which are at saturation• Bias is the o/p voltage from previous stage• No current bias network• Current in o/p branch depend on size of transistor• Limitation in the design of transistor sizes
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1. Common Source Output Stage 14
• Composed of a common source amplifier with a current driver• A common source PMOS with an NMOS current mirror load or a common source NMOS with a PMOS current mirror load• Current source is mirrored to provide current bias• Supply current consumption of output is more controllable compared to push-pull inverter
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SCHEMATIC DESIGN
• Initial sizes and bias are set and using MOS current equation in saturation• Corresponding W/L ratio’s are calculated depending on current flow in different branches• Design requires a maximum supply current of 250μA, input offset voltage less than 1mV and unity gain bandwidth greater than 100KHz
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1. Complimentary differential pair with active load with common source o/p stage
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• Transistors are grouped into pairs and implemented with equal ratios to reduce mismatches• To minimize current consumption, currents are generated using only 1 current bias network• To minimize area of amplifier, resistor for biasing is realized using a properly-sized diode-connected transistor• Two main considerations in design
• Compensation is necessary because multiple stages of op-amp make circuit unstable
Operating point Two identical diode-connected transistors are used to generate the required current
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• Lead compensation to compensate instability of opamp• Mirror-pole compensation to stabilize circuit
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Dept. of ECE
192. Complimentary differential pair with cascode load with push pull o/p stage
• A folded cascode op-amp with PMOS differential pair & NMOS cascode load is used• Widths of differential pairs are increased to increase the transconductance of i/p transistors and improve the gain• Widths of voltage bias & upper current mirror PMOS are decreased• Widths of cascode load & lower current mirror PMOS are increased• Bias voltages are adjusted to put the transistors at edge of saturation• Two main considerations in design
Operating point Supply current of o/p stage - without using current bias network
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• Lead compensation to compensate instability of opamp• This circuit has fewer stages compared to other one
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LAYOUT IMPLEMENTATION
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• Comdiff-Act topologies have larger areas since larger compensating capacitors are used
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SIMULATION RESULTS
• Op-amps are implemented in 0.25μm CMOS process
• Simulated USING cadence design system software
• Op-amps with push-pull inverter output stage have higher gain
• OVSR and ICMR of op-amps with CS o/p stage is far from
supply rails
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• Input offset voltage of op-amp with CS o/p stage is larger than those with push pull inverter o/p stage• Complementary differential pair with active loads results in lower phase margin
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CONCLUSION
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• Rail-to-rail input and output stages are necessary for rail-to-rail input and output operation• A standard design methodology is formulated for different topologies• Main design consideration is stability issue since passive components for compensation n/w consumes a large amount of chip space• Input stage : Complementary differential pair with cascode load
• Output stage : Push-pull inverter
Less stability issues
Maximum gain Does not limit the output voltage swing range of op-amp
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REFERENCE
• Design and Implementation of CMOS rail-to-rail operational amplifiers – Michal Angelo G Lorenzo, Maria Theresa A Gusad, John Richard E. Hizon
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THANK YOU