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Open-Loop OpAmp Circuits
19 questions By Tony R. Kuphaldt
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Question 4 of 19
Determine the output voltage polarity of this op-amp (with reference to ground), given the following input conditions:
Reveal answerIn these illustrations, I have likened the op-amp’s action to that of a single-pole, double-throw switch, showing the “connection” made between power supply terminals and the output 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.”
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Question 5 of 19
Although the following symbol is generally interpreted as an operational amplifier (“op-amp”), it may also be used to represent a comparator:
What is the difference between a comparator such as the model LM319, and a true operational amplifier such as the model LM324? Are the two devices interchangeable, or is there any significant difference despite the exact same schematic symbols? Explain your answer.
Reveal answerComparators are designed for open-loop operation only (no feedback), while operational amplifiers are designed to perform well with feedback. For many simple applications, though, a true op-amp does a reasonable job as a comparator.
Notes:The answer to this question invokes a couple of terms your students may not be familiar with yet: “open-loop” and “feedback”. Discuss these terms with your students, asking them first if they were able to arrive at definitions for them.
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Question 6 of 19
In this circuit, a solar cell converts light into voltage for the opamp to “read” on its non-inverting input. The opamp’s inverting input connects to the wiper of a potentiometer. Under what conditions does the LED energize?
Reveal answerThe LED energizes under bright-light conditions, de-energizing when the light decreases below the threshold set by the potentiometer.
Follow-up question: determine what would have to be changed in this circuit to make the LED turn on when the solar cell becomes dark.
Notes:There is more than one way to accomplish the task posed by the follow-up question. Be sure to ask your students for their ideas on how to reverse the LED’s operation!
