P. Piot, PHYS 375 –Spring 2008Op. Feedback and Op. Amps• Operational Feedback– Current versus voltage feedback• Op Amp– Properties – Common Op-Amp-based setups• Op-Amps.P. Piot, PHYS 375 –Spring 2008Type of feedback• Convention for voltage and current feedback:P. Piot, PHYS 375 –Spring 2008Operational Voltage feedback• Input impedance of A0 very large, no current can flow into the amplifier• Input must cancel the feedback• Input resistor sets Vin• Input current must all go through the feedback resistorfoutinRViα−=• Amplifier gain is•SoP. Piot, PHYS 375 –Spring 2008Operational Current feedback• Resistor is used to sample the output currentP. Piot, PHYS 375 –Spring 2008Operational Amplifier (Op. Amp.)Transistors (we will learn about these later)P. Piot, PHYS 375 –Spring 2008Ideal Operational Amplifier• The ideal op-amp is characterized by 7 properties– Knowledge of these properties is sufficient to design and analyze a large number of useful circuits• Basic op-amp properties– Infinite open-loop voltage gain– Infinite input impedance– Zero output impedance– Zero noise contribution– Zero DC output offset– Infinite bandwidth– Differential inputs that “stick together”P. Piot, PHYS 375 –Spring 2008Ideal Op. Amp. PropertiesP. Piot, PHYS 375 –Spring 2008Ideal Op. Amp. PropertiesP. Piot, PHYS 375 –Spring 2008Ideal Op. Amp. PropertiesP. Piot, PHYS 375 –Spring 2008Ideal Op. Amp. PropertiesP. Piot, PHYS 375 –Spring 2008Op. Amps Rules: The Golden Rules••This golden rules obscure what is really going on but they This golden rules obscure what is really going on but they greatly greatly simplify the analysis of circuit based on Op. Amps. simplify the analysis of circuit based on Op. Amps. • (I) The output attempts to do whatever is necessary to make the voltage difference between the two inputs zero.• (II) The inputs draw no current1. (+) is grounded so Rule I implies (-) is also grounded,2. So the voltage across R2is Voutand the voltage across R1is Vin3. So rule II implies:Vout/R2=-Vin/R1Or gain is A=-R2/R1P. Piot, PHYS 375 –Spring 2008Examples of Op. Amp. applicationsP. Piot, PHYS 375 –Spring 2008Examples of Op. Amp. applicationsP. Piot, PHYS 375 –Spring 2008Examples of Op. Amp. applicationsP. Piot, PHYS 375 –Spring 2008Examples of Op. Amp. applicationsP. Piot, PHYS 375 –Spring 2008Examples of Op. Amp.