How it works
An ideal op-amp has five key properties: open-loop voltage gain Aol = ∞, input impedance Zin = ∞, output impedance Zout = 0, bandwidth = ∞, and CMRR = ∞. These lead directly to the two golden rules used in circuit analysis. Rule 1 — virtual short: when negative feedback is applied, the differential input voltage (V+ − V−) ≈ 0, so both input terminals sit at the same voltage. Rule 2 — virtual open: since Zin = ∞, no current flows into either input terminal. Applying both rules to an inverting amplifier with Rf = 10 kΩ and R1 = 1 kΩ: V− = 0 (virtual ground), current through R1 = Vin/1 kΩ = current through Rf, so Vout = −Vin × 10 kΩ/1 kΩ = −10Vin. Gain = −10, entirely set by resistor ratio.
Key points to remember
Five ideal parameters: Aol = ∞, Zin = ∞, Zout = 0, BW = ∞, CMRR = ∞. Real μA741 values are Aol ≈ 200,000, Zin ≈ 2 MΩ, Zout ≈ 75 Ω, GBW = 1 MHz, CMRR ≈ 90 dB — worth comparing in exam answers. The virtual ground concept applies only at the inverting terminal of an inverting amplifier under closed-loop conditions, not in open-loop or comparator circuits. CMRR = 20 log(Ad/Acm); a high CMRR means the op-amp rejects noise common to both inputs, essential in instrumentation. Slew rate (not infinite in real op-amps) limits the maximum output voltage rate: SR = ΔVout/Δt, typically 0.5 V/μs for the 741, which limits full-swing output to below 16 kHz for a ±10 V swing.
Exam tip
Every Anna University paper asks you to apply the virtual ground concept to derive the gain of an inverting amplifier — state "V− = 0 due to virtual ground" explicitly and then use KCL at the inverting terminal to get Vout/Vin = −Rf/R1.