PHY070 · Electronics · College
Op-amp inverting gain
Objective: To measure the gain of an inverting op-amp amplifier and compare it with −R_f/R_in.
🧰 Apparatus
- Op-amp IC (741 or TL071)
- ±15 V (or ±12 V) dual DC supply
- Resistors for R_in and R_f
- Signal generator
- Dual-channel oscilloscope
📐 Formulas and SI units
A_v = −R_f / R_in
V_out = A_v V_in (limited to about ±13 V on ±15 V rails)
💡 Theory
An op-amp with negative feedback keeps its two inputs at almost the same voltage. In the inverting circuit the − input is a 'virtual earth', so the input current V_in/R_in all flows through R_f. Hence V_out = −(R_f/R_in)V_in. The output cannot go beyond the supply rails, where it saturates.
🪜 Procedure
- Build the inverting amplifier on the breadboard with the dual supply.
- Feed a small sine wave into R_in.
- View input and output on the scope.
- Measure the peak values and calculate the gain.
- Change R_f and repeat; increase V_in until the output clips.
📋 Observations Simulated values
✅ Result and conclusion
R_f = 100 kΩ and R_in = 10 kΩ give a gain of −10, so 0.2 V in gives −2 V out (inverted).
🔬 In a real lab
Real op-amps have limited bandwidth and slew rate, so at high frequency the gain falls and the output shape distorts.
⚠️ Safety
Low-voltage dual DC supply only; check the IC pin numbers and polarity before switching on.
❓ Quick check
1. The minus sign in the gain means the output is…
2. R_f = 47 kΩ, R_in = 4.7 kΩ. Gain?
3. If the output tries to exceed the supply it…
🏋️ Practice
EasyGain −20, V_in = 0.1 V. V_out?
Answer: −2 V
MediumR_in = 2 kΩ, gain −50. R_f?
Answer: 100 kΩ
ChallengeV_out for gain −100 and V_in = 0.5 V on ±15 V rails?
Answer: Saturates at about −13 V