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Voltage/Current Converter OpAmp Circuits
9 questions By Tony R. Kuphaldt
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Question 1 of 9
Don’t just sit there! Build something!! Learning to mathematically analyze circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.
You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:
- Carefully measure and record all component values prior to circuit construction.
- Draw the schematic diagram for the circuit to be analyzed.
- Carefully build this circuit on a breadboard or other convenient medium.
- Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
- Mathematically analyze the circuit, solving for all voltage and current values.
- Carefully measure all voltages and currents, to verify the accuracy of your analysis.
- If there are any substantial errors (greater than a few percent), carefully check your circuit’s construction against the diagram, then carefully re-calculate the values and re-measure.
Avoid using the model 741 op-amp, unless you want to challenge your circuit design skills. There are more versatile op-amp models commonly available for the beginner. I recommend the LM324 for DC and low-frequency AC circuits, and the TL082 for AC projects involving audio or higher frequencies.
As usual, avoid very high and very low resistor values, to avoid measurement errors caused by meter “loading”. I recommend resistor values between 1 kΩ and 100 kΩ.
One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.
Reveal answerLet the electrons themselves give you the answers to your own “practice problems”!
Notes:It has been my experience that students require much practice with circuit analysis to become proficient. To this end, instructors usually provide their students with lots of practice problems to work through, and provide answers for students to check their work against. While this approach makes students proficient in circuit theory, it fails to fully educate them.
Students don’t just need mathematical practice. They also need real, hands-on practice building circuits and using test equipment. So, I suggest the following alternative approach: students should build their own “practice problems” with real components, and try to mathematically predict the various voltage and current values. This way, the mathematical theory “comes alive,” and students gain practical proficiency they wouldn’t gain merely by solving equations.
Another reason for following this method of practice is to teach students scientific method: the process of testing a hypothesis (in this case, mathematical predictions) by performing a real experiment. Students will also develop real troubleshooting skills as they occasionally make circuit construction errors.
Spend a few moments of time with your class to review some of the “rules” for building circuits before they begin. Discuss these issues with your students in the same Socratic manner you would normally discuss the worksheet questions, rather than simply telling them what they should and should not do. I never cease to be amazed at how poorly students grasp instructions when presented in a typical lecture (instructor monologue) format!
A note to those instructors who may complain about the “wasted” time required to have students build real circuits instead of just mathematically analyzing theoretical circuits:
What is the purpose of students taking your course?
If your students will be working with real circuits, then they should learn on real circuits whenever possible. If your goal is to educate theoretical physicists, then stick with abstract analysis, by all means! But most of us plan for our students to do something in the real world with the education we give them. The “wasted” time spent building real circuits will pay huge dividends when it comes time for them to apply their knowledge to practical problems.
Furthermore, having students build their own practice problems teaches them how to perform primary research, thus empowering them to continue their electrical/electronics education autonomously.
In most sciences, realistic experiments are much more difficult and expensive to set up than electrical circuits. Nuclear physics, biology, geology, and chemistry professors would just love to be able to have their students apply advanced mathematics to real experiments posing no safety hazard and costing less than a textbook. They can’t, but you can. Exploit the convenience inherent to your science, and get those students of yours practicing their math on lots of real circuits!
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Question 2 of 9
Calculate the current through resistor R2 in this opamp circuit for several different values of R2:
R2 IR2
1 kΩ
2 kΩ
3 kΩ
4 kΩ
5 kΩ
6 kΩ
For each value of R2, what is it that establishes the amount of current through it? Do you see any practical value for a circuit such as this?Reveal answer
R2 IR2
1 kΩ 3 mA
2 kΩ 3 mA
3 kΩ 3 mA
4 kΩ 3 mA
5 kΩ 3 mA
6 kΩ 3 mA
This circuit acts like a current mirror, except much more precise.Follow-up question: what factor(s) limit the greatest resistance value of R2 that the operational amplifier may sustain 3 milliamps of current through?
Notes:Besides reviewing the purpose of a current mirror circuit, this question draws students’ attention to the current-regulating capabilities of an operational amplifier by having them analyze it as though it were simply a non-inverting voltage amplifier circuit.
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Question 3 of 9
Explain how the operational amplifier maintains a constant current through the load:
Write an equation solving for the regulated load current, given any relevant variables shown in the schematic diagram (R1, VZ, Vsupply, AV(OL), etc.).
Reveal answerIload = VZ R2Follow-up question: is the transistor sourcing current to the load, or sinking current from it?
Challenge question #1: modify the given equation to more precisely predict load current, taking the β of the transistor into account.
Challenge question #2: modify the location of the load in this circuit so that the given equation does precisely predict load current, rather than closely approximate load current.
Notes:This is a good example of how operational amplifiers may greatly improve the functions of discrete-component circuits. In this case, the opamp performs the function of a current mirror circuit, and does so with greater precision and reliability than a simple current mirror ever could.
It should be noted that the equation provided in the answer does not directly predict the current through the load, rather it predicts current through resistor R2. This is equal to load current only if the transistor’s base current is zero, which of course it cannot be. The real equation for predicting load current will be a bit more complex than what is given in the answer, and I leave it for your students to derive.
