How to Calculate Op-Amp Output Voltage in a Lab Report
Quick answer: to find an op-amp’s output voltage, work out the closed-loop gain from the resistors (−R2/R1 for inverting, 1 + Rf/R1 for non-inverting), multiply by the input peak, keep the sign, then check the answer against the supply rails. If it exceeds what the rails allow, the output clips and your lab report should say so.
That is the whole calculation, but a lab report is more than one line of arithmetic. Markers want the working shown, the waveforms sketched and the simulation compared honestly with the theory.
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What an op-amp lab usually asks for
A typical lab has three circuits: an inverting amplifier, a non-inverting amplifier and a multistage arrangement. Each has a 741, dual supplies (often ±12 V), and a 1 V peak, 10 kHz sine input. You calculate the output, sketch input and output, then simulate in a tool such as NI Multisim and show schematics and scope screenshots.
A full worked extract is available as a sample, for reference only: op-amp inverting, non-inverting and multistage lab sample. You can also browse more engineering assignment samples.
The formulas you need
| Circuit | Gain | Phase | Notes |
|---|---|---|---|
| Inverting | −R2/R1 (feedback over input resistor) | 180° | Inverting input is a virtual earth |
| Non-inverting | 1 + Rf/R1 | 0° | Gain is never below 1 |
| Voltage follower | 1 | 0° | Special case with Rf = 0 |
| Inverting summer | Vout = −Rf × (V1/R1 + V2/R2 + …) | 180° | Each input scaled separately |
| Cascaded stages | Multiply stage gains | Count the inversions | Two inversions give a net 0° |
Worked examples (all values illustrative)
Example 1: inverting stage
Take R1 = 1 kΩ, R2 = 10 kΩ and a 1 V peak input. Gain = −10 kΩ / 1 kΩ = −10, so the output is 10 V peak and inverted. Write it as Vout = −10 sin(ωt) V.
Example 2: non-inverting stage
With the same resistors, gain = 1 + 10 / 1 = 11, so the ideal peak is 11 V, in phase with the input. If there is an extra resistor from the output to ground, treat it as a load. It does not change the ideal gain, so say that in your report rather than silently ignoring it.
Example 3: two stages feeding a summer
Suppose stage A has gain −4 and stage B has gain −2, with 1 V peak inputs. They output −4 V and −2 V. A summing stage with 3 kΩ and 10 kΩ input resistors and 10 kΩ feedback gives Vout = −10 × (−4/3 − 2/10) = +15.33 V peak. Notice two things: the signs flip twice, and the result is above any ±12 V rail. These are illustrative numbers, so use your own sheet’s values and follow the same sequence.
Check the answer against reality
This is the step students skip. An op-amp output cannot exceed its supplies, and a 741 falls short of them by a margin. The Texas Instruments uA741 datasheet gives guaranteed output swing for ±15 V supplies only (a minimum of ±10 V into 2 kΩ and ±12 V into 10 kΩ). It has no figure for ±12 V supplies, so if you assume a clipping level, label it as an assumption.
The second check is slew rate. The datasheet gives 0.5 V/µs at unity gain under specific test conditions. A sine of peak Vp at frequency f changes at up to 2πfVp. For 10 V peak at 10 kHz that is 2π × 10,000 × 10 ≈ 628,319 V/s, about 0.63 V/µs. That is above 0.5 V/µs, so a 10 V output at 10 kHz is likely to look slightly distorted. Treat it as indicative, since your simulation model decides the exact outcome.
Want this kind of working checked? Send your brief through report writing help, or ask for homework helper support on the calculation steps.
Sketching the waveforms
- Choose one time axis. At 10 kHz the period is 100 µs, so mark 0, 25, 50, 75 and 100 µs.
- Draw the input first, peak at 1 V, with a clear volts scale.
- Draw the output on the same axis. For an inverting stage, it starts going negative when the input goes positive.
- Mark peak values on both traces and label the 180° shift or the in-phase relationship.
- If the ideal peak is above the limit, draw the clipped version as a second trace and label both.
Multisim tips
- Connect pins 4 and 7 of the 741 to the supplies. A missing supply connection is the commonest reason for a flat output.
- Use the AC voltage source and set the amplitude as peak voltage.
- Use separate volts-per-division settings for each scope channel so a 1 V input is still readable beside a 10 V output.
- Take three screenshots per circuit: the schematic, the measured output value and the two waveforms together.
- Run a lower-amplitude case to prove that clipping, not a wiring fault, causes the flat tops.
NI describes Multisim as SPICE-based, which is why it shows limits your hand calculation ignores.
Comparing calculated and simulated results
| Circuit | Calculated | Simulated | Percentage difference | Likely reason |
|---|---|---|---|---|
| Inverting | 10 V pk | Your reading | (sim − calc) / calc × 100 | Slew rate, rail limit |
| Non-inverting | 11 V pk | Your reading | As above | Clipping |
| Multistage | Your calculation | Your reading | As above | Clipping, tolerance |
Leave the simulated column to your own results. Explain each difference in a sentence or two, with the physical cause.
Common mistakes
- Forgetting inversion. A negative gain means a 180° shift. State both magnitude and phase.
- Ignoring saturation. An answer of 15 V on a ±12 V supply is not a result. Say it clips.
- Peak versus RMS. A “1 V” source may mean peak or RMS. RMS = peak ÷ √2 for a sine. Check what the brief says and keep it consistent.
- Mixing up the gain formulas. The non-inverting gain has the “+1”.
- Screenshots without labels. Add units, channel names and a caption.
- Quoting datasheet values without conditions. Always mention the supply voltage and load.
Where students get stuck
Most of the trouble is in the discussion section, not the calculation. Students get 10 V on paper and 8 V on screen and panic. That gap is not an error; it is the point of the lab. The marks usually go to the student who explains it using rails, slew rate and bandwidth.
Second, the unreadable resistor values on a scanned sheet. If you cannot read one, say so, state the value you used and label it illustrative or assumed. For a larger project, final year project help covers circuit design and write-up at greater depth.
Prefer to see how an expert approaches circuit coursework? Read the profile of Ahmad Firdaus, who completed a Bachelor’s degree in Electrical Engineering from Universiti Teknologi Malaysia.
Ready to get your lab report checked? University assignment help is available if you want a structured review before you submit.
FAQs
How do I calculate the output voltage of an inverting op-amp?
Divide the feedback resistor by the input resistor and put a minus sign in front. Multiply that gain by the input voltage. With 10 kΩ and 1 kΩ the gain is −10, so 1 V in gives −10 V.
Why is the non-inverting gain 1 + Rf/R1?
The feedback forms a voltage divider that holds the inverting input equal to the non-inverting input. The output must therefore be larger than the input by the ratio of the total resistance to R1, which is 1 + Rf/R1.
What happens if the calculated output is bigger than the supply?
The output saturates. It flattens near the rail, so a sine wave gets flat tops. Report the ideal value, the limited value, and the reason.
Do I need to include the 741 pinout in my report?
A short pin table helps, especially for the schematic. Pin 2 is the inverting input, pin 3 non-inverting, pin 4 V−, pin 6 output, pin 7 V+. Pins 1 and 5 are offset null and pin 8 is not connected.
Is 1 V peak the same as 1 V RMS?
No. For a sine wave, RMS is the peak divided by √2, about 0.707 of the peak. Use whichever the brief specifies, and label every value.
Why does my simulated output differ from my calculation?
The ideal formula assumes an infinite-gain, infinite-speed op-amp with unlimited swing. The simulation model has finite slew rate, bandwidth and output swing. Explain which one applies, with numbers.
If you would like to talk through your own lab, start with our do my assignment service page, or see the research paper help page for longer written work.
References
National Instruments. (n.d.). Multisim. NI. https://www.ni.com/en-my/shop/product/multisim.html
Sedra, A. S., Smith, K. C., Carusone, T. C., & Gaudet, V. (2020). Microelectronic circuits (International 8th ed.). Oxford University Press.
Texas Instruments. (n.d.). uA741 general-purpose operational amplifier [Data sheet]. https://www.ti.com/lit/ds/symlink/ua741.pdf
