Op-amp Assumptions

Assumptions that are always good:

• The input terminals require no input current: $I_{in+}=I_{in-}=0$

  • Equivalently, $R_{in}\to \infty$ o.c

• Output terminal can supply any amount of current: $I_{out}$ can go to $\infty$

  • Equivalently, $R_{out}=0$, $V_{out}$ won’t drop under loading

• Open-loop gain is infinite: $A_0\to \infty$

  • $V_{in+}$ and $V_{in-}$ are so close they’re effectively equal

• Output terminal can be any voltage as determined by the open-loop gain.

  • Note that this still needs to stay in between the supply voltage rails.

If a negative closed-loop feedback is present:

• We set $V_{+}$
• Op-amp makes $V_{-}=V_{+}$
  • This equivalency is more accurate as $A_0$ increases
• In reality, op-amp stops at $V_{out}=A_0(V_{+}-V_{-})$
  • Since there’s a very small difference between the two, $A_0$ is very big
• General solution procedure for closed-loop negative feedback:
  1. Find $V_{+}$ (might be a function of $V_{in}$)
  2. Find equation for $V_{-}$
  3. Rearrange to get $V_{out}$ as a function of input