Operational Amplifiers
OVERVIEW
•Desired features:
-Differential input
-Infinite voltage gain
-Infinite input resistance
-Zero output resistance
•Applications
-Switched capacitor
•MOS topologies
-Two stage MOS
-Two stage MOS with cascode
-MOS telescopic-cascode
-MOS folded-cascode
-MOS Active-cascode
Applications
•Feedback concept
-All op-amp works on the principle of feedback, where feedback network senses output, and develop feedback that will be subtracted from input
•Desired features:
-Differential input
-Infinite voltage gain
-Infinite input resistance
-Zero output resistance
•Applications
-Switched capacitor
•MOS topologies
-Two stage MOS
-Two stage MOS with cascode
-MOS telescopic-cascode
-MOS folded-cascode
-MOS Active-cascode
Applications
•Feedback concept
-All op-amp works on the principle of feedback, where feedback network senses output, and develop feedback that will be subtracted from input
•Inverting amplifier
•Non-inverting amplifier
•Differential amplifier
•Integrator/differentiator
•Integrator/differentiator
•Switched capacitor
-Monolithic integration on CMOS
-Using capacitors instead of resistors
-Dealing with charge
-Suitable for MOS technology
-Monolithic integration on CMOS
-Using capacitors instead of resistors
-Dealing with charge
-Suitable for MOS technology
Switched Capacitor
•An inverting amplifier with capacitive load
-Gain is proportional to capacitor ratios
-No DC path
•An inverting amplifier with capacitive load
-Gain is proportional to capacitor ratios
-No DC path
•Adding switches to fix the problem
-Non-overlapping clocks
-Non-overlapping clocks
•In real life
-The gain of op-amp is finite
-Limited effect from parasitic capacitances
Switched Capacitor Integrator
•Building block for monolithic filters
-Frequency response insensitive to parasitic
-Time constant controlled by capacitance ratios than absolute values
Two Two-Stage MOS Op Stage MOS Op-Amp
•Two stages are:
-Differential input
-Gain output
•Performance:
-Input resistance
-Output resistance
-Voltage gain
-Output resistance
-Voltage gain
•Performance:
-Output swing
-Input offset voltage
•Systematic caused by design as we trade off for other performances
•Random caused by mismatch between pieces where they need to be identical
•Performance:
-CMRR
-Common mode input range
-PSRR (low frequency)
Two-stage MOS Op-Amp with Cascode
•Goal: increasing the voltage gain without frequency degradation
-Using cascode for differential pair to increase unloaded output impedance
-Degrades the input swing
•Goal: increasing the voltage gain without frequency degradation
-Using cascode for differential pair to increase unloaded output impedance
-Degrades the input swing
MOS Telescopic Cascode Op-Amp
•Sometimes just the gain provided by cascode stage is sufficient => drop the second stage in previous topology
•Sometimes just the gain provided by cascode stage is sufficient => drop the second stage in previous topology
•Limitations
-Poor common mode input range
-Limited output swing
-Minimum supply requirements
MOS Folded Cascode Cascode
•Goal: to address the low output swing and limited common mode input range
•Folded since it reverses the signal flow direction
-Poor common mode input range
-Limited output swing
-Minimum supply requirements
MOS Folded Cascode Cascode
•Goal: to address the low output swing and limited common mode input range
•Folded since it reverses the signal flow direction
MOS Folded Cascode
•Schematic of a folded cascode Op-Amp
DAHIANA ALEJANDRA ROSALES HERNÁNDEZ
EES
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