Operational Amplifiers (Op‑Amps) — A/L Physics

What must I remember about Operational Amplifiers (Op‑Amps)?

• Op-Amp Characteristics: High-gain direct-coupled differential voltage amplifier with two inputs: Inverting input (-) and Non-inverting input (+); Output voltage Vout = A_ol (V+ - V-).
• Ideal Op-Amp Properties: Infinite open-loop voltage gain (A_ol = ∞); Infinite input impedance (R_in = ∞, hence input currents I+ = I- = 0); Zero output impedance (R_out = 0); Infinite bandwidth and slew rate; Zero offset voltage.
• Golden Rules under Negative Feedback: 1) Virtual Short / Virtual Earth: Because A_ol = ∞, differential input voltage must be zero: V+ = V-; 2) Zero Input Current: I+ = I- = 0.
• Standard Linear Op-Amp Configurations: Inverting Amplifier: Vout = - (Rf / Rin) * Vin; Non-Inverting Amplifier: Vout = (1 + Rf / Rin) * Vin; Voltage Follower (Buffer): Vout = Vin; Summing Amplifier: Vout = - Rf [ (V1/R1) + (V2/R2) + (V3/R3) ]; Difference Amplifier: Vout = (Rf / Rin)(V2 - V1).

Test yourself (1 minute)

1. Op-Amp Comparator (Open-Loop)

   Without feedback, op-amp compares V+ and V-: If V+ > V-, output saturates at positive rail +Vsat (≈ +Vcc - 1.5 V); If V+ < V-, output saturates at negative rail -Vsat; used in light-activated relays, temperature controllers, and ADC converters.

2. Virtual Ground concept in Inverting Amplifier

   Since non-inverting terminal is tied to 0 V (ground), negative feedback drives inverting node to 0 V (V- = 0 V); this node acts as a 'Virtual Ground' that sinks current without being physically wired to ground.

3. Output Saturation Limits

   Output voltage cannot exceed supply rails: -Vcc ≤ Vout ≤ +Vcc.

Practise Operational Amplifiers (Op‑Amps) questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 9: Electronics

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Unit 9.6 · Year 2 · Term 2