*Question E7G12:
What is an operational amplifier?*  

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

### AI Response: (may contain errors)  

An operational amplifier, often called an op-amp, is a fundamental building block in analog electronics. Here's why the correct answer is A:

### Correct Answer: A
- **High Gain**: Op-amps have a very high open-loop gain, which means they can amplify a small input voltage difference to a much larger output voltage.
- **Direct-Coupled**: They can amplify DC signals as well as AC signals because they are direct-coupled, meaning there are no capacitors in the signal path that would block DC.
- **Differential Amplifier**: Op-amps amplify the difference between two input voltages, which is why they are called differential amplifiers.
- **High Input Impedance**: This means they draw very little current from the input source, minimizing the loading effect on the previous stage.
- **Low Output Impedance**: This allows the op-amp to drive heavy loads without significant loss of output voltage.

### Why Other Choices Are Incorrect:
- **Choice B**: Describes a digital audio amplifier, which is not the same as an operational amplifier. Op-amps are analog devices, not digital.
- **Choice C**: Refers to an amplifier for frequency modulated signals, which is not specific to the function or characteristics of an op-amp.
- **Choice D**: Describes an RF amplifier used in UHF and microwave regions, which is a specialized type of amplifier, not a general-purpose op-amp.

### Useful Formulas:
- **Voltage Gain**: The open-loop voltage gain of an op-amp is typically very high, often exceeding $10^5$.
- **Input and Output Impedance**: 
  - Input impedance ($Z_{in}$) is typically in the range of megaohms ($M\Omega$).
  - Output impedance ($Z_{out}$) is typically very low, often less than 100 ohms.

Understanding these characteristics helps in recognizing the versatility and importance of op-amps in various electronic circuits.