Question E7B06:
What is a characteristic of a grounded-grid amplifier?  

Choices:  
A: High power gain  
B: Low input impedance  
C: High electrostatic damage protection  
D: Low bandwidth    

Correct Answer is  B: Low input impedance  

Correct answer: B — Low input impedance

Why B is correct:
- In a grounded-grid amplifier, the grid is at RF ground and the drive is applied to the cathode. The input “looks into” the cathode and is dominated by the tube’s transconductance.
- A useful rule of thumb is:
  $$R_{in} \approx \frac{1}{g_m}$$
  For typical triodes with $g_m \approx 5$–20 mA/V, this gives $R_{in} \approx 50$–200 Ω, which is considered low and conveniently close to 50 Ω for coax-driven stages.

Why the others are incorrect:
- A: High power gain
  - Grounded-grid stages usually have only moderate gain (often around 6–12 dB), lower than grounded-cathode configurations. High gain is not their hallmark; stability and simplicity are.
- C: High electrostatic damage protection
  - ESD concerns are mainly for solid-state devices. Vacuum tubes are inherently robust, and grounding the grid does not confer a special ESD advantage relevant to exam concepts.
- D: Low bandwidth
  - Grounded-grid amplifiers are typically broadband. With the grid at RF ground, Miller effect is minimized, improving stability and bandwidth.

Useful formulas / rules of thumb:
- Input impedance (cathode-driven):
  $$R_{in} \approx \frac{1}{g_m} \quad\text{(in parallel with stray capacitances)}$$
- Drive current relation:
  $$I_{drive} \approx g_m \cdot V_k$$
  where $V_k$ is the RF cathode-drive voltage.
- Practical note: The relatively low $R_{in}$ makes matching to 50 Ω sources straightforward, a common design advantage of grounded-grid stages.