Chapter 2

Ideal Op Amp Characteristics

• Infinite input impedance
• Infinite open-loop gain for the differential signal
• Zero gain for the common mode signal
  • You can easily change an differential amplifier into a common-mode amplifier by grounding one of the inputs
• Zero output impedance
• Infinite bandwidth
  • To allow for infinite gain regardless of the frequency? Instantaneous feedback?

Op Amp Nodal Analysis

• No current flows into the + or - terminals
• If negative feedback is present (and no positive feedback), then $V_{+}=V_{-}$
• Write nodal equations at $V_{+}$ and $V_{-}$, but not at $V_o$
  • There is a voltage source inside the op amp. Writing a nodal equation at a voltage source adds an extra equation and an extra variable. You gain no ground.