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Summer and Subtractor OpAmp Circuits
25 questions By Tony R. Kuphaldt
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Question 10 of 25
Identify some of the distinguishing characteristics of inverting and non-inverting summer circuits. How may you identify which is which, and how may you determine the proper resistor values to make each one work as it should?
Reveal answerI won’t directly answer the questions here, but I will give some hints. A non-inverting summer circuit is composed of a passive voltage averager circuit coupled to a non-inverting voltage amplifier with a voltage gain equal to the number of inputs on the averager. An inverting summer circuit is composed of a passive current summer node coupled to a current-to-voltage converter.
Notes:This question is designed to spur discussion amongst your students, exchanging ideas about each circuit’s defining characteristics. Having students explore each circuit type on their own, reaching their own conclusions about how to differentiate the two, is a far more effective way of making them understand the differences than simply telling them outright.
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Question 11 of 25
Complete the table of values for this opamp circuit, calculating the output voltage for each combination of input voltages shown:
V1 V2 Vout
0 V 0 V
1 V 0 V
0 V 1 V
2 V 1.5 V
3.4 V 1.2 V
-2 V 4 V
5 V 5 V
-3 V -3 V
What pattern do you notice in the data? What mathematical relationship is there between the two input voltages and the output voltage?
Reveal answer
V1 V2 Vout
0 V 0 V 0 V
1 V 0 V -1 V
0 V 1 V 1 V
2 V 1.5 V -0.5 V
3.4 V 1.2 V -2.2 V
-2 V 4 V 6 V
5 V 5 V 0 V
-3 V -3 V 0 V
Notes:Thought it may be tedious to calculate the output voltage for each set of input voltages, working through all the voltage drops and currents in the opamp circuit one at a time, it shows students how they may be able to discern the function of an opamp circuit merely by applying basic laws of electricity (Ohm’s Law, KVL, and KCL) and the “golden assumptions” of negative feedback opamp circuits (no input currents, zero differential input voltage).
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Question 12 of 25
How does the operation of this difference amplifier circuit compare with the resistor values given (2R = twice the resistance of R), versus its operation with all resistor values equal?
Describe what approach or technique you used to derive your answer, and also explain how your conclusion for this circuit might be generalized for all difference amplifier circuits.
Reveal answerIt is very important that you develop the skill of “exploring” a circuit configuration to see what it will do, rather than having to be told what it does (either by your instructor or by a book). All you need to have is a solid knowledge of basic electrical principles (Ohm’s Law, Kirchhoff’s Voltage and Current Laws) and know how opamps behave when configured for negative feedback.
As for a generalized conclusion:
Notes:It is easy for you (the instructor) to show how and why this circuit acts as it does. The point of this question, however, is to get students to take the initiative to explore the circuit on their own. It is simple enough for any student to set up some hypothetical test conditions (a thought experiment) to analyze what this circuit will do, that the only thing holding them back from doing so is attitude, not aptitude.
This is something I have noticed over years of teaching: so many students who are more than capable of doing the math and applying well-understood electrical rules refuse to do so on their own, because years of educational tradition has indoctrinated them to wait for the instructor’s lead rather than explore a concept on their own.
