Question E6B11:
What is used to control the attenuation of RF signals by a PIN diode?  

Choices:  
A: Forward DC bias current  
B: A variable RF reference voltage  
C: Reverse voltage larger than the RF signal  
D: Capacitance of an RF coupling capacitor    

Correct Answer is  A: Forward DC bias current  

To control RF attenuation with a PIN diode, you vary its forward DC bias current. In a PIN diode, forward bias injects charge into the intrinsic region, making its RF resistance drop; reverse bias removes charge, making it high. At RF, the diode behaves like a current-controlled resistor.

Why A is correct (Forward DC bias current):
- A PIN diode’s RF resistance is set by the charge stored in its intrinsic region, which depends on the forward DC current $I_F$.
- More $I_F$ → more carriers → lower RF resistance → less attenuation in a series element (or more shunt loss in a shunt element).
- Rule of thumb: the RF resistance varies roughly inversely with current:
  $$R_{\text{RF}} \propto \frac{1}{I_F}$$
  A common approximation (device-dependent) is:
  $$R_{\text{RF}} \approx \frac{K}{I_F}$$
  where $K$ is specified by the diode/frequency. For a small-signal diode, the small-signal resistance trend is similar to $r_d \approx \frac{V_T}{I}$.

Why the other choices are incorrect:
- B: A variable RF reference voltage does not control a PIN diode; it is a DC current-controlled device. RF is what you’re attenuating, not what sets the resistance.
- C: Reverse voltage larger than the RF signal just ensures the diode is “off” (high isolation). It does not provide smooth, controllable attenuation; the controllable attenuation is achieved by forward bias current.
- D: The capacitance of an RF coupling capacitor is not the control mechanism; coupling capacitors block DC while passing RF and do not set the diode’s attenuation.

Useful rules of thumb:
- Think “PIN diode = current-controlled RF resistor.”
- Series PIN element: increase $I_F$ → lower $R_{\text{RF}}$ → lower insertion loss.
- Shunt PIN element: increase $I_F$ → lower shunt resistance → higher attenuation.