Question E5B12:
What is admittance?  

Choices:  
A: The inverse of impedance  
B: The term for the gain of a field effect transistor  
C: The inverse of reactance  
D: The term for the on-impedance of a field effect transistor    

Correct Answer is  A: The inverse of impedance  

To understand admittance:

- Admittance is the reciprocal of impedance.
- It tells how easily a circuit allows AC to flow.
- Units: siemens (S), also called mhos.

Why A is correct:
- By definition,
  $$Y = \frac{1}{Z}$$
  where $Z = R + jX$ is impedance and $Y = G + jB$ is admittance.
- $G$ is conductance (real part), $B$ is susceptance (imag part).

Why the others are incorrect:
- B: “Gain of a field-effect transistor” is not admittance; gain is dimensionless, while admittance has units of siemens.
- C: “Inverse of reactance” describes susceptance, not admittance. Susceptance $B$ is the imaginary part of admittance. For a pure reactance $X$,
  $$Y = \frac{1}{jX} = -j\frac{1}{X} \quad \Rightarrow \quad B = -\frac{1}{X}$$
  but admittance includes both $G$ and $B$, not just $1/X$.
- D: “On-impedance of a FET” is a device parameter (often $R_{\text{DS(on)}}$), not admittance.

Useful formulas and tips:
- Reciprocal relationship:
  $$Y = \frac{1}{Z}, \quad Z = \frac{1}{Y}$$
- Decomposition:
  $$Z = R + jX, \quad Y = G + jB$$
- For pure components:
  - Resistor $R$: $Y = \frac{1}{R}$ (purely real).
  - Inductor $X_L = +\omega L$: $B_L = -\frac{1}{X_L}$ (negative).
  - Capacitor $X_C = -\frac{1}{\omega C}$: $B_C = -\frac{1}{X_C}$ (positive).
- Parallel circuits add admittances: $Y_{\text{total}} = \sum Y_i$. Series circuits add impedances: $Z_{\text{total}} = \sum Z_i$.