*Question E4B03:
Which S parameter is equivalent to forward gain?*  

*Correct Answer is  C: S21*  

### AI Response: (may contain errors)  

The correct answer to the question about which S parameter is equivalent to forward gain is **C: S21**.

### Explanation of the Correct Answer (S21):
- **S21** represents the forward transmission coefficient in a two-port network. It quantifies how much of the input signal at port 1 is transmitted to port 2. In other words, it measures the gain from port 1 to port 2, which is why it is referred to as forward gain.

### Why the Other Choices are Incorrect:
- **A: S11**: This parameter represents the input reflection coefficient at port 1. It measures how much of the signal is reflected back into port 1 rather than being transmitted. It does not indicate gain.
  
- **B: S12**: This parameter represents the reverse transmission coefficient, measuring how much of the signal input at port 2 is transmitted to port 1. It is not the forward gain but rather the gain in the reverse direction.
  
- **D: S22**: This parameter represents the output reflection coefficient at port 2. Similar to S11, it measures how much of the signal is reflected back into port 2 and does not provide information about forward gain.

### Useful Formulas and Rules of Thumb:
- The S-parameters are often used in the context of RF and microwave engineering to describe the behavior of linear electrical networks. The key relationships are:
  - **Forward Gain**: $S21$ (input at port 1 to output at port 2)
  - **Reverse Gain**: $S12$ (input at port 2 to output at port 1)
  - **Reflection Coefficients**: $S11$ and $S22$ (how much signal is reflected back at ports 1 and 2, respectively)

Understanding these parameters is crucial for analyzing and designing RF circuits and systems.