Question
Which S parameter is equivalent to forward gain?
Answer Options
- A) S11
- B) S12
- C) S21
- D) S22
Correct Answer: C
Explanation
S-parameters (Scattering parameters) are a method used to characterize how RF energy propagates through linear electrical networks, particularly at high frequencies where conventional impedance parameters become impractical. A common two-port device (like an amplifier or filter) has four S-parameters, denoted as S_{ij}, where ‘i’ is the output port and ‘j’ is the input port.
The parameter S_{21} represents the signal transmission from Port 1 (input) to Port 2 (output). This is exactly the definition of forward gain (or transmission coefficient). Specifically, |S_{21}| is the magnitude of the power gain (or loss) in the forward direction, making it the most fundamental measure of a filter or amplifier’s performance.
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