Assume the op amp is powered by 10V and now vout 10V Obviously vin must be less

# Assume the op amp is powered by 10v and now vout 10v

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Assume the op-amp is powered by ± 10V, and now v out = +10V. Obviously, v in must be less than v A : What happens if v in moves just above 10 R 1 /( R 1 + R 2 )? Clearly, v out falls to – 10V because of comparator action. Therefore, v A drops to –10 R 1 /( R 1 + R 2 ), and v in must be greater than v A :
C.K. Tse: Operational Amplifiers 14 Examples (where Golden rules do not work) Schmitt Trigger We have a situation similar to hysteresis.
C.K. Tse: Operational Amplifiers 15 Examples (where Golden rules do not work) Schmitt Trigger What are the upper and lower trip points?
C.K. Tse: Operational Amplifiers 16 Practical considerations Finite input currents Very small currents are in fact needed to bias the op-amp input stage. Circuits that have no DC path to inputs won’t work! None of these works! v o + v i C v o + v i C x x
C.K. Tse: Operational Amplifiers 17 Practical considerations Offset in integrator The op-amp integrator is very easily saturated if there is a small lack of symmetry in the input signals. This is because the error gets integrated quickly and the output will soon move towards the maximum voltage. + v i C C + In practice we need a discharge path to prevent saturation. Usually R has to be big enough, so that the discharge rate becomes insignificantly slow compared to the signal frequency. R
C.K. Tse: Operational Amplifiers 18 Applications Current source + R I o LOAD v R We see that v R is fixed by the voltage divider. The op-amp will make sure that the voltage across R is also equal to v R , which is fixed! Therefore the current flowing down R must be which is the load current. Thus, this circuit provide a constant current source for the load. Note: the load is floating for this case!
C.K. Tse: Operational Amplifiers 19 Applications Current source for grounded load + R I o LOAD v R Again v R is fixed by the voltage divider. The op-amp will make sure that the voltage at the lower end of R is also equal to v R , which is fixed!

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