Analog Integrated Circuits

Basic Operational Amplifiers

18 questions By Tony R. Kuphaldt

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Question 10 of 18

Predict how the operation of this operational amplifier circuit will be affected as a result of the following faults. Specifically, identify whether the output voltage (V_out) will move in a positive direction (closer to the V rail) or in a negative direction (closer to ground). Consider each fault independently (i.e. one at a time, no multiple faults):

Transistor Q₅ fails shorted (collector-to-emitter):

Transistor Q₆ fails shorted (collector-to-emitter):

Resistor R₁ fails open:

Current source I₂ fails shorted:

For each of these conditions, explain why the resulting effects will occur.

Reveal answer

Transistor Q₅ fails shorted (collector-to-emitter): V_out goes negative.

Transistor Q₆ fails shorted (collector-to-emitter): V_out goes positive.

Resistor R₁ fails open: V_out goes negative.

Current source I₂ fails shorted: V_out goes negative.

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.
Question 11 of 18

Predict how the operation of this operational amplifier circuit will be affected as a result of the following faults. Specifically, identify whether the output voltage (V_out) will move in a positive direction (closer to the V rail) or in a negative direction (closer to the -V rail). Consider each fault independently (i.e. one at a time, no multiple faults):

Diode D₁ fails open:

Resistor R₁ fails shorted:

Transistor Q₂ fails shorted (drain-to-source):

Transistor Q₅ fails shorted (collector-to-emitter):

Resistor R₂ fails open:

Current source I₂ fails open:

For each of these conditions, explain why the resulting effects will occur.

Reveal answer

Diode D₁ fails open: V_out goes positive.

Resistor R₁ fails shorted: V_out goes negative.

Transistor Q₂ fails shorted (drain-to-source): V_out goes negative.

Transistor Q₅ fails shorted (collector-to-emitter): V_out goes positive.

Resistor R₂ fails open: V_out goes negative.

Current source I₂ fails open: V_out goes negative.

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.
Question 12 of 18

Ideally, what should the output voltage of an op-amp do if the non-inverting voltage is greater (more positive) than the inverting voltage?

Reveal answer

In this condition, the output of the op-amp should saturate positive ( V), as if a direct connection were made inside the op-amp between the output terminal and the V power supply terminal:

Notes:
Determining which “way” the output of an op-amp drives under different input voltage conditions is confusing to many students. Discuss this with them, and ask them to present any principles or analogies they use to remember “which way is which.”