*Question G5A10:
Which of the following devices can be used for impedance matching at radio frequencies?*  

*Correct Answer is  D: All these choices are correct*  

### AI Response: (may contain errors)  

Impedance matching is crucial in radio frequency systems to ensure maximum power transfer and minimize reflections. Let's look at each option:

1. **Transformer**:  
   - Transformers can be used to match impedances by adjusting the turns ratio. The impedance transformation is given by:  
     $$Z_{\text{out}} = Z_{\text{in}} \left(\frac{N_{\text{secondary}}}{N_{\text{primary}}}\right)^2$$  
   - This is effective for matching impedances in RF circuits, especially at lower frequencies.

2. **Pi-network**:  
   - A Pi-network consists of two capacitors and an inductor arranged in a "π" shape. It is commonly used in RF amplifiers and transmitters for impedance matching.  
   - By adjusting the component values, you can match a wide range of impedances.

3. **Length of transmission line**:  
   - A specific length of transmission line can be used as an impedance transformer, known as a "quarter-wave transformer."  
   - The characteristic impedance of the line and its length determine the transformation:  
     $$Z_{\text{in}} = \frac{Z_0^2}{Z_{\text{load}}}$$  
   - This is particularly useful for matching impedances at specific frequencies.

**Correct Answer: D: All these choices are correct**

- Each of these devices can be used for impedance matching at radio frequencies, making option D the correct choice.
- The other choices (A, B, and C) are not incorrect; they are simply individual examples of devices that can perform impedance matching.