ECE Lab #7: Operational Amplifiers: Part 2

Department of Electrical and Computer Engineering

Spring 2025 (Note: Acknowledgment: The lab was derived from Simple Op Amps, For ADALM2000 by Doug Mercer)

Overview

The purpose of Lab 7 is to:

• Implement and Experiment with Op-Amp Circuits:
• Perform experiments involving basic operational amplifier (op-amp) circuits using negative feedback.

• Become familiar with non-inverting amplifiers and inverting amplifiers.

• Understand the Characteristics of Op-Amps:

• Learn about the defining properties of op-amps, including high input resistance, low output resistance, and large differential gain.
• Understand how these characteristics make op-amps nearly ideal amplifiers and versatile building blocks in circuit applications.

1. Prelab Assignment

1.1 Reflective AI Exercise: Op-Amp Amplifier Configurations

Objective: Demonstrate understanding of how the same two Golden Rules produce two different amplifier configurations, and when each configuration is the right choice.

Part 1: Exploration

Example prompts are provided below. You may use them, adapt them, or write your own at the same level of specificity.

Focus Area 1: Two Configurations, Two Golden Rules

"I am an electrical engineering student preparing for a lab on op-amp amplifier circuits. Without deriving any formulas, can you explain what the two Golden Rules of an ideal op-amp force the circuit to do in an inverting amplifier configuration? Focus on what is happening physically at the inverting input terminal."

Follow up with:

"Now apply the same Golden Rules to the non-inverting configuration. What is physically different at the input compared to the inverting case? Given that both configurations follow from identical rules, what accounts for the difference in how each one is used in practice?"

Focus Area 2: Gain-Bandwidth Product

"I am using a real op-amp with a gain-bandwidth product of 1 MHz. Can you explain in physical terms what gain-bandwidth product means and why it exists as a constraint? If I configure this op-amp for a closed-loop gain of 10, what is the highest frequency at which it will still behave as expected?"

Follow up with:

"If I increase the closed-loop gain to 100 using the same op-amp, what happens to the usable bandwidth? Describe what I would see on an oscilloscope if I drove this amplifier with a signal at a frequency above that limit."

After completing both focus areas, connect them: the inverting and non-inverting configurations both have a closed-loop gain set by external resistors. Does the gain-bandwidth product constrain both configurations equally, or does the choice of configuration affect how that limit is encountered in practice?

Part 2: The Self-Test

Open Gemini and write your own quiz prompt targeting these two concepts. Your questions must involve explaining in physical terms, using the Golden Rules, why the two amplifier configurations behave differently, and predicting what happens to a real amplifier's output when the signal frequency or gain pushes against the gain-bandwidth product. Your prompt must explicitly instruct the AI not to ask questions that require algebraic derivations.

Apply the meta-prompt from A Mind Worth Questioning to evaluate and strengthen your draft, then run the quiz. Submit your original draft, the AI's critique, your revised prompt, and the full quiz transcript via the course submission app.

Part 3: Formal Reflection (150--250 words)

Your written synthesis must address all three of the following points:

• The Link -- How the same two Golden Rules produce two configurations with different input behavior, and what that difference means for how each is used in practice.
• The Technical "Why" -- Correct use of terms such as virtual ground, virtual short, closed-loop gain, or gain-bandwidth product.
• The Lab Application -- A specific Lab 7 measurement (either a gain deviation at high frequency or an output saturation event) that you can now explain using one of the real-device limits you explored.

Prelab Deliverable #1

Upload up to two screenshots capturing your Self-Test prompt-craft work (original draft prompt, the AI's critique, your revised prompt, and the quiz transcript) via the course submission app. Your name must be visible in each image before uploading.

Prelab Deliverable #2

Submit your formal written reflection (150--250 words, continuous prose) addressing all three points: The Link, The Technical "Why", and The Lab Application. Submit via the course submission app.

2. Lab Procedure: Non-Inverting Amplifier

2.1 Background

The non-inverting amplifier configuration is shown in Figure 1. Like the unity-gain buffer (which is a special type of non-inverting amplifier, the feedback resistor is 0 Ohms), this circuit has the (usually) desirable property of high input resistance, so it is useful for buffering non-ideal sources:

Figure 1: Non-inverting amplifier with gain of 2. The 1 kΩ feedback and 1 kΩ ground resistors set the closed-loop gain to $1 + R_f/R_1 = 2$. The 10 kΩ resistor is the output load.

Materials

• 2 1 kΩ resistors
• 1 10 kΩ resistor
• 1 LMC662 dual op-amp
• 2 0.1 μF capacitors

2.2 Hardware Setup

• Use the same +5 and -5 Volt connections to power as in Lab #6.
• With the power turned off, modify your unity gain amplifier circuit of Lab #6 as shown in Figure 1. Start with a feedback resistor of R = 1 kΩ.

IMPORTANT

Leave the power supplies off. Get your setup checked off by TA or lab assistant before proceeding to the next step.

• Turn on the power supplies and observe the current draw to be sure there are no accidental shorts.

2.3 Procedure

• Apply a 1 volt amplitude peak-to-peak, 1 kHz sine wave at the input, and display both input and output on the scope. Measure the voltage gain of this circuit, and compare to the theory discussed in class. Export a screenshot of the waveforms. An example is shown in Figure 2.

Lab Deliverable #1

Screenshot of the oscilloscope display showing the input and output waveforms of the non-inverting amplifier with $R_2 = 1\,\text{k}\Omega$. Submit the image via the course submission app. Your name must be visible in the image before uploading.

Lab Deliverable #2

Report the measured gain with $R_2 = 1\,\text{k}\Omega$ and compare it to the theoretical value.

Figure 2: Example oscilloscope waveforms for the non-inverting amplifier. The orange Channel 1 output has twice the amplitude of the purple Channel 2 input with no phase shift, confirming a closed-loop gain of 2.

• Increase the feedback resistor from 1 kΩ to about 5 kΩ. What is the measured gain now?

Lab Deliverable #3

Report the measured gain with $R_2 \approx 5\,\text{k}\Omega$ and compare it to the theoretical value.

• Increase the feedback resistance further until the onset of clipping, that is, until the peaks of the output signal begin to be flattened due to output saturation. Record the value of resistance where this happens.

Lab Deliverable #4

Record the feedback resistance at which the output begins to saturate (peaks begin to clip).

• Now increase the feedback resistance to 100 kΩ.

Lab Deliverable #5

Screenshot of the oscilloscope display showing the output waveform with the feedback resistor at 100 kΩ. Submit the image. Your name must be visible in the image before uploading.

Lab Deliverable #6

For a feedback resistance of 100 kΩ, work on paper: (1) sketch the input and output waveforms; (2) calculate the theoretical gain; (3) calculate the minimum input signal amplitude needed to keep the output below 5 V. Photograph your completed paper and submit the image via the course submission app. Your name must be visible in the photo.

Lab Deliverable #7

Set the waveform generator to the minimum amplitude you calculated in Lab Deliverable #6. Describe what you observe at the output. Submit via the course submission app.

The last step underscores an important consideration for high-gain amplifiers. High-gain necessarily implies a large output for a small input level. Sometimes this can lead to inadvertent saturation due to the amplification of some low-level noise or interference, for example the amplification of stray 60 Hz signals from power-lines that can sometimes be picked up. Amplifiers will amplify any signals at the input terminals, whether you want it or not.

3. Lab Procedure: Inverting Amplifier

3.1 Background

Figure 3 shows the conventional inverting amplifier configuration with a 10 kΩ "load" resistor at the output.

Figure 3: Inverting amplifier configuration with gain of $-R_f/R_{\text{in}} = -4.7\text{ k}\Omega / 1\text{ k}\Omega = -4.7$. The 10 kΩ resistor is the output load.

3.2 Hardware Setup

• Shut off the power supply before assembling a new circuit.
• Assemble the inverting amplifier circuit shown in Figure 3 using 4.7 kΩ feedback resistor.
• Cut and bend the resistor leads as needed to keep them flat against the board surface, and use the shortest jumper wires for each connection. Remember, the breadboard gives you a lot of flexibility.

IMPORTANT

Leave the power supplies off. Get your setup checked off by TA or lab assistant before proceeding to the next step.

• After check-off, turn on the power supplies and observe the current draw to be sure there are no accidental shorts.

3.3 Procedure

• Adjust the waveform generator to produce a 1 volt amplitude peak-to-peak, 1 kHz sine wave at the input ($\text{V}_{\text{in}}$), and display both the input and output voltages on the oscilloscope.

An example of the expected waveforms is shown in Figure 4.

Lab Deliverable #8

Screenshot of the oscilloscope display showing the input and output waveforms of the inverting amplifier. Submit the image. Your name must be visible in the image before uploading.

Lab Deliverable #9

Record the measured voltage gain of the inverting amplifier ($R_f = 4.7\,\text{k}\Omega$) and compare it to the theoretical value. Submit via the course submission app.

Figure 4: Example oscilloscope waveforms for the inverting amplifier. The orange Channel 1 output is approximately 4.7 times larger in amplitude than the purple Channel 2 input and is inverted by 180°, confirming the expected gain of $-4.7$.

3.4 Output Saturation

Procedure

• Now change the feedback resistor in Figure 3 from 4.7 kΩ to 10 kΩ. You do not have to switch off the power supply.
• Measure the gain now.

Lab Deliverable #10

Record the measured gain with $R_f = 10\,\text{k}\Omega$ and compare it to the theoretical value. Submit via the course submission app.

• Slowly increase the amplitude of the input signal towards 2 volts until the output saturates.

Lab Deliverable #11

Screenshot of the oscilloscope display showing the saturated output waveform. Before submitting, annotate the image (using any tool, or by writing on a printed copy) with the measured positive and negative clipping voltages — these numbers must be clearly readable in your submission. These values characterise the actual output voltage swing of the LMC662 under your load conditions and are needed for the post-lab analysis. Submit the annotated image via the course submission app. Your name must be visible in the image before uploading.

4. Lab Procedure: Summing Inverting Amplifier

The circuit of Figure 5 is a basic inverting amplifier with an additional input, called a "summing" amplifier. Using superposition we can show that $\text{V}_{\text{out}}$ is a linear sum of $\text{V}_{\text{in1}}$ and $\text{V}_{\text{in2}}$, each with their own unique gain or scale factor. In this experiment you will add two signals. One is a sinusoidal signal ($W1$) with no DC offset, and the second is a DC value ($W_2$). The output signal should be the result of the input signal $W1$ and the DC offset ($W_2$).

Figure 5: Inverting summing amplifier configuration. Signal W1 is amplified by $-R_f/R_1 = -4.7$ and signal W2 by $-R_f/R_2 = -1$, giving $V_{\text{out}} = -4.7\,V_{\text{in1}} - V_{\text{in2}}$. The 10 kΩ resistor is the output load.

4.1 Hardware Setup

• With the power turned off, modify your inverting amplifier circuit as shown in Figure 5. Check your connection to make sure it is connected correctly.
• Turn on the power supplies and observe the current draw to be sure there are no accidental shorts.

4.2 Procedure

• Use the first waveform generator (W1) to apply a sine wave $\text{V}_{\text{in1}}$, and the second waveform generator (W2) for $\text{V}_{\text{in2}}$ to apply a DC voltage.

• Apply a 1 volt amplitude peak-to-peak sine wave with no DC offset for $\text{V}_{\text{in1}}$ and 1 volt DC for $\text{V}_{\text{in2}}$.

• Observe and record the input/output waveforms on the oscilloscope screen. When used in this way, such a circuit could be called a level shifter.

Lab Deliverable #12

Screenshot of the oscilloscope display showing the input and output waveforms of the level shifter. Submit the image. Your name must be visible in the image before uploading.

• Adjust the DC offset of waveform generator W1 ($\text{V}_{\text{in1}}$) until $\text{V}_{\text{out}}$ has zero DC component.

Lab Deliverable #13

Estimate the required DC offset on W1 by observing the input waveform on the oscilloscope (note: it is not $V_{\text{in2}}$). Explain what you observe and why it makes sense. Submit via the course submission app.

• Reset the offset of waveform generator W1 to zero and apply a 2 volt amplitude peak-to-peak sine wave for $\text{V}_{\text{in1}}$. Increase the offset voltage of waveform generator W2, $\text{V}_{\text{in2}}$ slowly to 4V.

Lab Deliverable #14

Record the DC voltage of $V_{\text{out}}$ when $V_{\text{in2}}$ reaches 4 V. Explain what you observe. Submit via the course submission app.

Self-Verification Checklist

Before leaving the lab, verify that you have collected all the necessary information to complete your post-lab report:

• 1: Screenshot of non-inverting amplifier waveforms ($R_2 = 1\,\text{k}\Omega$).
• 2: Measured gain with $R_2 = 1\,\text{k}\Omega$ and comparison to theory.
• 3: Measured gain with $R_2 \approx 5\,\text{k}\Omega$ and comparison to theory.
• 4: Feedback resistance at onset of output saturation.
• 5: Screenshot of output waveform with 100 kΩ feedback resistor.
• 6: Paper work: waveform sketch, theoretical gain calculation, and minimum input amplitude calculation (100 kΩ feedback).
• 7: Description of output observed at the calculated minimum input amplitude.
• 8: Screenshot of inverting amplifier waveforms ($R_f = 4.7\,\text{k}\Omega$).
• 9: Measured gain ($R_f = 4.7\,\text{k}\Omega$) and comparison to theory.
• 10: Measured gain ($R_f = 10\,\text{k}\Omega$) and comparison to theory.
• 11: Annotated screenshot of saturated output — clipping voltages (positive and negative) must be readable in the image.
• 12: Screenshot of level shifter input and output waveforms.
• 13: Estimated DC offset on W1 and explanation.
• 14: Recorded DC output voltage and explanation.

5. Post-Lab Analysis Report

5.1 Quantitative Analysis

Post-Lab Deliverable #1

Using the clipping voltages you annotated in Lab Deliverable #11, answer the following in a short paragraph: (1) Calculate and state the voltage drop from the positive supply rail (+5 V) to the positive clipping level, and from the negative supply rail (−5 V) to the negative clipping level. (2) Are the two drops approximately equal, or does one rail clip earlier? (3) What does this tell you about whether the LMC662 reaches its supply rails — and why does that matter when choosing an op-amp for a real application? Submit via the course submission app.

5.2 Discussion Questions

Post-Lab Deliverable #2

Write a structured response with clearly labelled items matching the sections below. Aim for 200–300 words total.

(1) State the positive and negative clipping voltages from Lab Deliverable #11 (in volts).

(2) For your inverting amplifier with $R_f = 10\,\text{k}\Omega$ and $R_\text{in} = 1\,\text{k}\Omega$, calculate the maximum peak input amplitude that would keep the output just below clipping. Show the calculation and state the result in volts.

(3) Explain in physical terms why a higher closed-loop gain reduces the allowable input amplitude before saturation occurs. Connect your explanation to the specific resistance value at which saturation began in Lab Deliverable #4.

(4) Look up the gain-bandwidth product of the LMC662 from its datasheet. State the value (in Hz) and include the datasheet source or URL.

(5) Using that GBP, calculate the −3 dB bandwidth for: (i) your non-inverting amplifier with $R_f = R_1 = 1\,\text{k}\Omega$; and (ii) your inverting amplifier with $R_f = 4.7\,\text{k}\Omega$, $R_\text{in} = 1\,\text{k}\Omega$. Show each calculation and state the result in Hz.

(6) At the 1 kHz signal frequency used throughout this lab, was the GBP a limiting factor for either configuration? State yes or no for each and give a one-sentence justification.

(7) At 1 kHz with your measured gains, which constraint — output saturation or GBP — was the binding limit on amplifier performance? Then describe one specific change to the circuit or input signal that would make GBP the dominant constraint instead, and explain why.

You are encouraged to use an AI assistant to help structure your analysis or clarify concepts. Ask it to explain, check your reasoning, or suggest a framework; then apply that framework to your own data. The analysis you submit must be your own work: use AI as a thinking partner, not as a substitute for your own conclusions.

IMPORTANT

Submit your completed work via the course submission app. All plots, images, data tables, and calculations must be clearly labeled and referenced in your post-lab report.

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