Question 6A155

The screen grid in a vacuum tube, typically found in tetrodes and pentodes, primarily serves to electrostatically shield the control grid from the plate. This shielding effect has two key benefits: 1. **Reduced Grid-to-Plate Capacitance (A):** By placing an additional grid at a fixed positive potential between the control grid and the plate, the screen grid significantly reduces the interelectrode capacitance between the control grid and the plate (Cgp). This greatly minimizes unwanted feedback, which can cause oscillation in RF amplifiers, making neutralization unnecessary except at very high frequencies where even residual capacitance becomes significant. 2. **Higher Amplification (B):** With the screen grid isolating the control grid from plate voltage variations, the "Miller effect" capacitance is substantially reduced. This allows the tube to exhibit a much higher plate resistance and therefore a higher amplification factor ($\mu$), leading to significantly greater voltage gain compared to triodes, which suffer from substantial Cgp and Miller effect. Therefore, both A and B are correct, making C the comprehensive answer.