Understanding Op-Amps

The Swiss Army knife of analog electronics - amplifiers, filters, oscillators, and more.

The Operational Amplifier (Op-Amp) is a high-gain DC amplifier with differential inputs that forms the foundation of modern analog electronics. Originally developed for analog computers to perform mathematical "operations" (hence the name), op-amps are now used in virtually every analog circuit - from audio equipment to precision instrumentation. With nearly infinite gain and high input impedance, an op-amp becomes a "perfect" building block whose behavior is determined entirely by external feedback components.

Objectives

  • Understand ideal op-amp characteristics and the two "Golden Rules"
  • Recognize real op-amp limitations (offset voltage, bias current, slew rate, bandwidth)
  • Design and analyze inverting and non-inverting amplifiers
  • Build summing amplifiers, difference amplifiers, and instrumentation amplifiers
  • Create integrators and differentiators for signal processing
  • Design active filters (low-pass, high-pass, band-pass)
  • Use op-amps as comparators and oscillators
  • Select appropriate op-amps for specific applications
  • Troubleshoot common op-amp circuit problems
  • Salvage and identify op-amps from electronic equipment

Key Takeaways

  • Op-amps have very high gain; behavior is determined by feedback network
  • Golden Rules: inputs draw no current; negative feedback makes inputs equal
  • Inverting: Av = -Rf/Rin, input impedance = Rin
  • Non-inverting: Av = 1 + Rf/R1, very high input impedance
  • Real op-amps have offset voltage, limited bandwidth, and finite slew rate
  • GBW product: Gain × Bandwidth = constant
  • Active filters provide gain and buffering; no inductors needed
  • Comparators use positive feedback or open-loop operation
  • Always use bypass capacitors on power supply pins
  • Select op-amp based on application: precision, audio, high-speed, low-power

Practice Problems

Problem 1: Design an inverting amplifier with gain of -5 and Zin ≥ 10kΩ.

Problem 2: A non-inverting amplifier has R1=1kΩ and Rf=9kΩ. What is the gain?

Problem 3: An op-amp has GBW=5MHz. What is bandwidth at gain of 100?

Problem 4: Design summing amplifier for Vout = -(V1 + 2×V2) with Rf=20kΩ.