Question E7G12:
What is an operational amplifier?  

Choices:  
A: A high-gain, direct-coupled differential amplifier with very high input impedance and very low output impedance  
B: A digital audio amplifier whose characteristics are determined by components external to the amplifier  
C: An amplifier used to increase the average output of frequency modulated amateur signals to the legal limit  
D: A RF amplifier used in the UHF and microwave regions    

Correct Answer is  A: A high-gain, direct-coupled differential amplifier with very high input impedance and very low output impedance  

Correct answer: A — A high-gain, direct-coupled differential amplifier with very high input impedance and very low output impedance

Why A is correct:
- An operational amplifier (op-amp) is defined as a high-gain differential amplifier that is direct-coupled (handles DC), with ideally infinite input impedance and near-zero output impedance.
- It amplifies the difference between two inputs and, with negative feedback, its behavior (gain, filtering, etc.) is set by external components.

Why the others are incorrect:
- B: Describes a “digital audio amplifier” (e.g., Class D). While op-amp circuits use external components to set characteristics, an op-amp is not inherently “digital” nor specifically an “audio” amplifier.
- C: Not the purpose of an op-amp. FM transmitter power control is done with RF power amplifiers; op-amps are low-power signal-level devices.
- D: UHF/microwave RF amplifiers are specialized RF devices. Typical op-amps are low-to-mid frequency; “op-amp” is not synonymous with “RF amplifier.”

Useful facts and formulas:
- Ideal op-amp assumptions:
  - Open-loop gain: $A_{OL} \to \infty$
  - Input impedance: $R_{in} \to \infty$
  - Output impedance: $R_{out} \to 0$
  - With negative feedback: $V_+ \approx V_-$ (virtual short)
- Closed-loop gain with feedback factor $\beta$:
  $$A_{CL} = \frac{A_{OL}}{1 + A_{OL}\beta} \approx \frac{1}{\beta} \quad \text{when } A_{OL} \gg 1$$
- Common configurations:
  - Inverting: $$V_{out} = -\frac{R_f}{R_{in}} V_{in}$$
  - Non-inverting: $$V_{out} = \left(1 + \frac{R_f}{R_g}\right) V_{in}$$
- Rule of thumb: Gain–bandwidth product
  $$A_{CL} \cdot f_{BW} \approx \text{GBW (constant for a given op-amp)}$$
- Practical limits: finite slew rate, input bias currents, and output swing limited by supply rails.