Question E7G01:
What is the typical output impedance of an op-amp?  

Choices:  
A: Very low  
B: Very high  
C: 100 ohms  
D: 10,000 ohms    

Correct Answer is  A: Very low  

To understand op-amp output impedance, remember the ideal op-amp model:
- Ideal: input impedance is infinite, output impedance is zero.
- Real devices approximate this, especially when negative feedback is used.

Why “Very low” is correct:
- An op-amp’s effective (closed-loop) output impedance is driven very low by negative feedback.
- Rule of thumb: with feedback, $Z_{out}$ is typically well under 1 Ω at low frequencies, limited by the output stage and loop gain.

Why the other choices are incorrect:
- Very high: That describes the input impedance, not the output. High output impedance would make it a poor voltage source.
- 100 ohms: Too high for a typical op-amp output with feedback; this would cause large load-dependent voltage drops.
- 10,000 ohms: Characteristic of input impedance, not output; far from typical op-amp output behavior.

Useful formulas and rules:
- Ideal op-amp properties:
  - $Z_{in} \to \infty$
  - $Z_{out} \to 0$
- Negative feedback reduces output impedance:
  $$Z_{out,cl} \approx \frac{Z_{out,ol}}{1 + A\beta}$$
  where:
  - $Z_{out,ol}$ = open-loop output impedance (often tens of ohms),
  - $A$ = open-loop gain,
  - $\beta$ = feedback factor,
  - $A\beta$ (loop gain) is large (e.g., $10^3$ to $10^5$), making $Z_{out,cl}$ very low.