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



Switched Capacitor

•An inverting amplifier with capacitive load
-Gain is proportional to capacitor ratios
-No DC path

•Inverting amplifier













•Non-inverting amplifier













•Differential amplifier





•Integrator/differentiator











•Switched capacitor
-Monolithic integration on CMOS
-Using capacitors instead of resistors
-Dealing with charge
-Suitable for MOS technology





•Adding switches to fix the problem
-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







•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)
















TwoTwo--stage MOS Opstage MOS Op--Amp with Amp with CascodeCascod

•Goal: increasing the voltage gain without frequency degradation
-Using cascode for differential pair to increase unloaded output impedance
-Degrades the input swing
















MOS Telescopic MOS Telescopic Cascode Cascode Op Op-Amp

•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









MOS Folded Cascode Cascode

•Schematic of a folded cascode Op-Amp