Opamps: DC Imperfections, Bandwidth Response withBandwidth Response withFinite Gain, Slew RateEE122D. W. ParentD. W. ParentThe effect of a finite open loop Gain, A b l dldcan be easily modeled•For large Gains the effect disappearsFor large Gains the eff ect disappears.Define Large?Define Large?•For large Gains the effect disappearsFor large Gains the eff ect disappears.10 is not large.Define Large?Define Large?•For large Gains the effect disappearsFor large Gains the eff ect disappears.100 is not large.1+R2/R1<<A is large (A is at least 100 times 1+R2/R1)1+R2/R1<<A is large (A is at least 100 times 1+R2/R1)Gain dependence on finite A for non‐lfinverting amplifiersOPAMPS have a built in offset that will ff h l faffect the voltage swing of your circuit.•InLtspiceyou have to add a voltage source toIn Ltspiceyou have to add a voltage source to the positive terminal of the opamp.Set to zero totest to maketest to make sure circuit works.Set VOS to the data sheet value (60uV)Set VOS to the data sheet value (60uV)•InLtspiceyou have to add a voltage source toIn Ltspiceyou have to add a voltage source to the positive terminal of the opamp.Not much noticeable change.What if you have multipleopamps?What if you have multiple opamps?This can cause a reduction in the loutput voltage swing.Numerical ExampleNumerical ExampleThe Gain of the VOS signal is like a DC lfinput to a non‐inverting amplifierVOS_GAIN=(1+R2/R1)The text uses a voltage simple to l hd fsimulate method of measuring VOSTo see an LTSPICE part’s Data Sheet pright click on the part. The i hi fprogram gives you a choice of visiting the Ltspice website or gpopening up a sample file for using hthe part.To reduce eliminate the effects of DC offsets, capacitive couple the stages of anopampcapacitive couple the stages of an opamptogether. Open loop gain (A) has a frequency dddependence.ft is when the gain=1Note: This only works in simulation!This system as a single dominate pole systemAIt i f it l ti t tω+=1)(0bsAsAIt arises from internal compensation to prevent oscillations. If you want to go faster, get an opamp with out compensation and provide it yourself. These are very conservative.ωωω+=1)(0bjAjAy y231RtdB+=ωωωωωω×=>>)(0 bbbAjA121RR+ωωωω==)(0 btAjAUsed for inverting and non‐inverting!πω×=2ttfOn data sheets this is called the unity gain band width.This is a low pass filter?This is a low pass filter?Yes!Yes!Maximize Frequency by breaking gain up into stages.
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