Op-Amp Configurations: Six Circuits, Two Golden Rules

Learning Objectives

Key Terms & Concepts

🖥 Tinkercad Setup Notes for Op-Amp Circuits

Fourth lab, same toolkit — with one new chip and one supply trick you already know:

• The op-amp: search "op amp" in the parts panel — Tinkercad's part is the classic LM741 in an 8-pin DIP. Straddle it across the breadboard trench like any IC. Hover every pin and record the pinout before wiring: pin 2 = inverting (−), pin 3 = non-inverting (+), pin 6 = output, pin 7 = V+, pin 4 = V−. Pins 1/5 (offset null) and 8 (NC) stay unconnected in this lab.
• Dual supply, familiar trick: the 741 wants voltage above and below ground. Exactly like the center-tap emulation in the Rectifier lab, wire two 9 V sources in series and ground the midpoint: the outer ends become +9 V and −9 V rails. Use the breadboard's top rails for +9/−9 and reserve a column group for the 0 V midpoint reference.
• Test signals: a potentiometer across +9/−9 with its wiper as V_in gives you a smooth, adjustable DC input for the follower, amps, and comparator. The function generator supplies the sine/square/triangle inputs — its output is referenced to your 0 V midpoint.
• Instruments: multimeters (V mode) for DC gain measurements; two oscilloscopes (input + output, 5 ms/div at 100 Hz) for the waveform configurations, exactly as in the Rectifier lab.
• Sanity anchor: after every rebuild, before trusting any measurement, verify pin 7 reads +9 V and pin 4 reads −9 V relative to your midpoint. Nine out of ten "broken op-amp" reports are supply-pin wiring.

🔧 Virtual Parts List

📐 The Six Circuits You Will Build

One op-amp, six personalities — the only thing that changes is the feedback network. All schematics assume the ±9 V rails and 0 V midpoint from Part 1 (supply pins omitted for clarity, but they are always connected: pin 7 → +9 V, pin 4 → −9 V).

Expected Results at a Glance

🛠 Step-by-Step Procedure

Suggested split: Session 1 = Parts 1–4 (DC configurations). Session 2 = Parts 5–6 (waveform configurations) + analysis.

Part 1 — Dual Supply & Voltage Follower (≈25 min)

1. Create the workspace. New circuit named "LastName – Op-Amp Lab." Place a small breadboard.
2. Build the ±9 V rails. Two 9 V supplies in series: Supply A's − terminal to Supply B's + terminal — that junction is your 0 V midpoint. Wire A's + to the top red rail (+9 V), B's − to the top black rail (−9 V), and the midpoint to a dedicated column group. Verify with a multimeter: +9.0 and −9.0 relative to midpoint. (Same trick as the Rectifier lab's center tap.)
3. Seat and power the 741. Place the op-amp across the trench. Hover-verify all pins, then wire pin 7 → +9 V and pin 4 → −9 V. Re-verify both supply pins with the meter before continuing — make this your ritual for every rebuild.
4. Adjustable DC input. 10 kΩ potentiometer across +9 V and −9 V; its wiper is V_in, swinging anywhere between the rails. Multimeter M1 from wiper to 0 V.
5. Wire the follower. Wiper → pin 3 (+). One wire from pin 6 (output) back to pin 2 (−). Multimeter M2 from pin 6 to 0 V.
6. Test unity gain. Simulate. Sweep the pot and record five (V_in, V_out) pairs across the range in Data Table 1 — including near +9 V and −9 V, where the output stalls at the ±7.5 V saturation limits while the input keeps going. That stall is your first rail sighting.
7. Answer before moving on. Gain = 1 — so what did the circuit accomplish? (Hint: how much current did the pot's wiper have to supply?) One sentence in your report.

Part 2 — Inverting Amplifier (≈20 min)

1. Rewire per Figure 2. Remove the follower wire. Wiper → R_in (10 kΩ) → pin 2. R_f (100 kΩ) from pin 6 back to pin 2. Pin 3 → 0 V midpoint.
2. Predict. A = −100k/10k = −10. Fill in the predicted column of Data Table 2 for inputs +0.25 V, +0.50 V, −0.50 V.
3. Measure the gain. For each input, set the pot (watch M1), record V_out, compute measured gain. Signs matter — record them.
4. Probe the virtual ground. Move a voltmeter to pin 2 while the amp is working: ≈0 V, with no wire to ground. Golden Rule #2, measured. Record it.
5. Find the clip point. Slowly raise V_in until V_out stops at −7.5 V. Record the input voltage where clipping began and check it against 7.5 V ÷ 10.
6. Change the gain. Swap R_f to 47 kΩ (A = −4.7), re-measure one point, and confirm the ratio rules. Restore 100 kΩ.

Part 3 — Non-Inverting Amplifier (≈15 min)

1. Rewire per Figure 3. Wiper → pin 3. R_f (100 kΩ) pin 6 → pin 2; R_in (10 kΩ) pin 2 → 0 V.
2. Predict, then measure. A = 1 + 100k/10k = +11. Test +0.25 V and +0.50 V; record predicted vs measured in Data Table 2. No sign flip this time.
3. The follower connection. Thought check for the report: set R_f = 0 (a wire) and R_in = ∞ (removed) in the gain formula. What circuit does the non-inverting amp become?

Part 4 — Comparator (≈15 min)

1. Remove ALL feedback. Take R_f out entirely — nothing connects pin 6 to pin 2. Pot wiper → pin 2 as V_ref; set it near +2 V. A second voltage (reuse the other pot terminal wiring or a supply tap through a divider) → pin 3 as V_in. Simplest: move the function generator in as V_in, sine, 100 Hz, ±4 V.
2. Output indicators. Two LED + 220 Ω branches from pin 6 to 0 V, opposite polarities (Figure 4): green lights on +7.5 V, red on −7.5 V.
3. Observe the snap. Simulate with the scope on pin 6: the sine input emerges as a square wave — the output only ever visits the two rails, switching the instant the sine crosses V_ref. Record the duty cycle at V_ref = 0 V and V_ref = +2 V in Data Table 3. Notice: same chip as Part 2, opposite personality — the only difference is feedback.

Part 5 — Integrator (≈20 min)

1. Wire Figure 5. Function generator (square, 100 Hz, ±1 V) → R = 10 kΩ → pin 2. Feedback: C = 0.1 µF from pin 6 to pin 2, with 100 kΩ in parallel. Pin 3 → 0 V. Scopes on input and output, 5 ms/div.
2. Predict the shape and size. RC = 1 ms. Each half-cycle (5 ms) integrates a constant ±1 V: slope = 1 V/1 ms = 1000 V/s for 5 ms → ~5 V swing. Sketch your predicted output before running.
3. Run and record. Square in, triangle out — measure the output V_pp and note the inversion (output ramps DOWN while input is HIGH). Record in Data Table 3.
4. Frequency experiment. Double the generator to 200 Hz and re-measure V_pp: half the integration time → half the amplitude. Record and explain.

Part 6 — Differentiator (≈20 min)

1. Wire Figure 6. Generator (triangle, 100 Hz, ±1 V) → 1 kΩ → C = 0.1 µF → pin 2. Feedback: R = 10 kΩ, pin 6 → pin 2. Pin 3 → 0 V.
2. Predict. The ±1 V triangle swings 2 V over each 5 ms half-period: slope = ±400 V/s. V_out = −RC·slope = −(1 ms)(±400 V/s) = ∓0.4 V. Predicted: a ±0.4 V square wave, inverted relative to the slope.
3. Run and record. Triangle in, square out. Measure the output levels against your prediction; note any rounding at the transitions (the 1 kΩ stability resistor at work). Record in Data Table 3.
4. The spike test. Switch the generator to a square input for a moment: the differentiator answers each edge with a sharp spike and rests at zero between them — the slope of a flat line is zero. Sketch what you see, then restore the triangle.
5. Wrap up. Complete the Analysis Questions and Knowledge Check, then Print/Save your report with the data tables filled in.

📋 Data Tables

Type readings directly into the tables — they persist when you print this page.

Data Table 1 — Voltage Follower (Part 1)

Data Table 2 — Amplifier Gains (Parts 2–3)

Data Table 3 — Comparator & Waveform Circuits (Parts 4–6)

📈 Interactive Op-Amp Waveform Playground

One virtual 741 on ±9 V rails (saturation ≈ ±7.5 V), six selectable personalities. The gray trace is the input; the green trace is the output. Push the gain or amplitude until the output flattens against the rails — clipping is easier to recognize once you've caused it on purpose.

✍ Analysis Questions

Answer in complete sentences in your lab report.

1. Derive the inverting amplifier's gain formula from the two golden rules, in three steps or fewer, using your Part 2 component values.
2. The follower has a gain of exactly 1. Describe a concrete scenario (sensor, audio, measurement) where inserting a follower fixes a real problem, and name the property it exploits.
3. In Part 2 you measured ≈0 V at pin 2 with no ground wire attached to it. Explain the mechanism: what is the op-amp's output physically doing to hold that node at zero?
4. Predict each result and justify: (a) the inverting amp's R_f fails open; (b) the comparator accidentally gets a feedback resistor from output to pin 2.
5. Using your Part 5 data: show the calculation predicting the integrator's output V_pp at 100 Hz, and explain why doubling the frequency halved it.
6. The differentiator answered a square wave with spikes and a flat zero between them. Explain both features using V_out = −RC·dV_in/dt.
7. Design exercise: choose R_in and R_f from standard values (1 k, 4.7 k, 10 k, 22 k, 47 k, 100 k) for an inverting amp with a gain as close as possible to −22, and state the largest input peak it can handle on ±9 V rails without clipping.

Key Facts Reference Box

Interactive Knowledge Check