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Summer and Subtractor OpAmp Circuits
25 questions By Tony R. Kuphaldt
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Question 19 of 25
Explain what common-mode rejection ratio means for a differential amplifier, and give a formula for calculating it.
Reveal answerCommon-mode rejection ratio compares an amplifier’s differential voltage gain to its common-mode voltage gain. Ideally, CMRR is infinite.
CMRR = 20 log ( Adiff(ratio) ACM(ratio)) The fundamental mechanism causing a common-mode signal to make it through to the output of a differential amplifier is a change in input offset voltage resulting from shifts in bias caused by that common-mode voltage. So, sometimes you may see CMRR defined as such:
CMRR = 20 log ( ∆Vin(common) ∆Voffset) Notes:An application that really shows the value of a high CMRR is differential signal transmission, as shown in question #02519. For those students not grasping the significance of CMRR, this would be a good example circuit to show them.
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Question 20 of 25
Predict how the operation of this passive averager network will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):
- Resistor R1 fails open:
- Solder bridge (short) across resistor R1:
- Resistor R2 fails open:
- Solder bridge (short) across resistor R2:
- Resistor R3 fails open:
- Solder bridge (short) across resistor R3:
For each of these conditions, explain why the resulting effects will occur.
Reveal answer- Resistor R1 fails open: Vavg becomes the average of V2 and V3 only.
- Solder bridge (short) across resistor R1: Vavg becomes exactly equal to V1.
- Resistor R2 fails open: Vavg becomes the average of V1 and V3 only.
- Solder bridge (short) across resistor R2: Vavg becomes exactly equal to V2.
- Resistor R3 fails open: Vavg becomes the average of V1 and V2 only.
- Solder bridge (short) across resistor R3: Vavg becomes exactly equal to V3.
Notes:The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
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Question 21 of 25
Predict how the operation of this summer circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):
- Resistor R1 fails open:
- Solder bridge (short) across resistor R3:
- Resistor R4 fails open:
- Resistor R5 fails open:
- Solder bridge (short) across resistor R5:
- Resistor R6 fails open:
For each of these conditions, explain why the resulting effects will occur.
Reveal answer- Resistor R1 fails open: Vout becomes equal to 4/3 the sum of voltages V2, V3, and V4.
- Solder bridge (short) across resistor R3: Vout becomes equal to 4 times V3.
- Resistor R4 fails open: Vout becomes equal to 4/3 the sum of voltages V1, V2, and V3.
- Resistor R5 fails open: Circuit operates as an averager, not a summer.
- Solder bridge (short) across resistor R5: Vout saturates in a positive direction.
- Resistor R6 fails open: Vout saturates in a positive direction.
Notes:The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
