Question E6E04:
Which is the most common input and output impedance of MMICs?  

Choices:  
A: 50 ohms  
B: 300 ohms  
C: 450 ohms  
D: 75 ohms    

Correct Answer is  A: 50 ohms  

To most MMIC designers and users, the “default” RF system impedance is 50 Ω. MMICs are therefore designed to present approximately 50 Ω at both input and output so they can be directly connected to common RF components, coax, and test gear without extra matching.

Why 50 ohms is correct:
- RF systems, coax cables (e.g., RG-58/RG-213), and test equipment (network analyzers, signal generators) are standardized to $Z_0 = 50\ \Omega$.
- Most MMIC datasheets specify S-parameters in a 50 Ω environment, and many MMICs include on-chip matching to 50 Ω for broadband use.
- Using 50 Ω simplifies design and minimizes reflections when interconnected.

Why the other choices are incorrect:
- 75 ohms (D): Common in CATV/video and some receive-only systems for lower loss, but not the most common impedance for MMICs used in general RF/microwave and amateur applications.
- 300 ohms (B): Balanced twin-lead antenna feed; not used for MMIC I/O.
- 450 ohms (C): Ladder line; also a balanced high-impedance transmission line, not used for MMIC ports.

Useful formulas and rules of thumb:
- Maximum power transfer occurs when source and load impedances are matched: $Z_S = Z_L = 50\ \Omega$.
- Reflection coefficient: $$\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}$$
- VSWR: $$\text{VSWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|}$$
- Return loss: $$\text{RL} = -20 \log_{10} |\Gamma|\ \text{dB}$$
- Rule of thumb: If an RF IC or S-parameter file doesn’t say otherwise, assume 50 Ω ports.