An op amp multiplies a voltage by a number you choose with two resistors. You design an inverting amplifier with a gain of −4.7, build it with a classic 741 on ±15 V, push it until it clips, then test the non-inverting transimpedance amplifier on your project board that turns USB charging current into a voltage. Finally you solder the parts for next week.
An op amp has two inputs, − (inverting) and + (non-inverting), and makes Vout ≈ A × (V+ − V−) with A around a million. With a feedback resistor from the output back to the − input, any difference ΔV makes the output move in the direction that shrinks ΔV. It keeps adjusting until ΔV ≈ 0. Two rules then solve almost every op-amp circuit: (1) V+ = V−, (2) no current flows into the inputs. Move the input and watch the output chase it.
Tap a pin. The dimple marks pin 1; pins count counter-clockwise from there (seen from the top).
For the inverting amplifier AV = −Rf/Rin. You need standard 5% values (the E24 table times a power of ten) between 1 kΩ and 1 MΩ. Pick Rin and the page finds the nearest standard Rf.
Why stay between 1 kΩ and 1 MΩ: below 1 kΩ the op amp has to supply too much current; above ~10 MΩ noise, board leakage and interference take over.
The 741 runs from +15 V (supply channel 2, + to pin 7) and −15 V (channel 3, − to pin 4), both referenced to the supply’s green ground. Pin 3 (+) goes to ground. The generator sends a 1 kHz triangle through Rin into pin 2; Rf runs from pin 6 back to pin 2. CH1 watches the input, CH2 the output. Raise the generator amplitude until the output flattens: those flat spots are the op amp’s limits.
Same as Labs 4 and 5: the generator assumes a 50 Ω load, but Rin is kilohms, so the real voltage is about twice the setting. Always measure the input on CH1.
The gain is negative. When the input goes up, the output goes down by 4.7 times as much. Report the gain as −Vout,rms/Vin,rms, adding the minus sign yourself because RMS values are always positive.
An op amp cannot output more than its supply allows. A 741 on ±15 V tops out around +14 V and −13 to −14 V. Above that it leaves its linear region and the output simply flattens.
Both supply channels should show CV and a couple of milliamps. CC means something is shorted or miswired: turn it off and check pins 4 and 7.
USB charging current flows through R11 = 0.07 Ω, making a tiny voltage Vin = 0.07 × I. The non-inverting amplifier U2 with R12 = 150 kΩ and R13 = 4.75 kΩ multiplies it by 1 + R12/R13 = 32.58 and puts the result on test point XTP. Overall, XTP = 2.281 Ω × I: current in, voltage out, hence trans-impedance. Drag the current like the supply’s current limit in steps 37–43.
The same 150 kΩ, 4.75 kΩ and 100 nF you used as a high-pass filter in Lab 5 become a low-pass filter here, because the capacitor now sits across the feedback resistor. At DC the capacitor is open and the gain is 32.58. At high frequency it shorts R12 and the gain falls toward 1.
All on the component (front) side. First cut R1–R5 off flush with the board: the display goes there later.