domingo, 7 de febrero de 2010

Operational Amplifiers

Operational Amplifiers

•Desired features:
-Differential input
-Infinite voltage gain
-Infinite input resistance
-Zero output resistance
-Switched capacitor
•MOS topologies
-Two stage MOS
-Two stage MOS with cascode
-MOS telescopic-cascode
-MOS folded-cascode
-MOS Active-cascode


•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


•Switched capacitor
-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

•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
-Input resistance
-Output resistance
-Voltage gain

-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



-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

MOS Telescopic Cascode  Op-Amp

•Sometimes just the gain provided by cascode stage is sufficient => drop the second stage in previous topology
-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




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