Exam-style question
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Why is the output of a summing amplifier inverted relative to the input signals?.
- A.Because the op-amp has a negative feedback loop
- B.Because the non-inverting input is grounded
- C.Because the summing node is at the inverting input
- D.Because the input resistors are larger than the feedback resistor
Model answer
What a good answer should say
- Because the summing node is at the inverting input
Explanation
Why this works
Formula: Vout = -(Rf/R1)V1 -(Rf/R2)V2 ... Substitution: Rf, R1, R2, V1, V2.
Working: The negative sign arises because the summing node is at the inverting input, causing phase inversion. Answer: Because the summing node is at the inverting input.
Units: none. Conclusion: The negative sign is due to the inverting input.
Common mistake
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