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.

DC imperfections

DC Imperfections

• Bias currents are the average dc currents flowing into the op amp input terminals. They can be caused by the signal source, feedback resistors, etc.
  • The bias current is the average of the dc currents. $I_{B}={\frac {I_{B+}+I_{B-}}{2}}$
• Offset current is the difference between the bias currents. $I_{off}=I_{B+}-I_{B-}\,$
• Offset voltage occurs when the output voltage is nonzero for zero input voltage.

Canceling bias currents

• Because bias currents flow have equal magnitude and direction, it is possible to negate their effects.
  • The orientation of the offset voltage and the direction of the offset current are unknown, thus it is not possible to correct for these parameters with a circuit design.