Circuit configurations
Good to know
Ideal op-amp assumptions
- Infinite input impedance (no input current)
- Zero output impedance
- Infinite open-loop gain
- Infinite bandwidth
- Output limited by supply rails (Vs+, Vs−)
Virtual ground rule
In a closed-loop configuration with negative feedback, the op-amp drives its output until V+ ≈ V−. This "virtual short" is the key to analysing inverting and summing circuits.
Gain sign convention
- Positive gain — output in phase with input
- Negative gain — output inverted (180° phase shift)
- Gain < 1 is possible (voltage follower = gain 1)
- Gain is set by external resistors, not the op-amp itself
Common op-amp ICs
- LM741 — classic general purpose, ±15 V
- LM358 — dual, single supply, 3–32 V
- TL071 / TL081 — JFET input, low noise
- MCP6001 — rail-to-rail, 1.8–6 V (MCU-friendly)
- LM393 — comparator (open-collector output)