Question

A CRT has an anode voltage of 1500 V and parallel plates are 1.5 cm long and 5 mm apart. The screen is 50 cm away from centre of the plates. The input voltage is applied to the deflecting plates through amplifier. The input voltage required for a deflection of 4 cm is 2 V (through amplifier). Which of the following are correct?

• A. Gain of amplifier is 60
• B. Gain of amplifier is 40
• C. Voltage applied to deflecting plates is 80 V.
• D. Voltage applied to deflecting plates is 120 V.

Answer 1

Answer: (B), (C)

Voltage applied to deflecting plates is 80 V.

We know,
$\text{Deflection, D}=\frac{L{l}_d{E}_d}{2d{E}_a}$

Voltage applied to deflecting plates,

$E_d=\frac{2d{E}_{a}D}{L{l}_a}=\frac{2\times d\times E_a\times D}{L{l}_d}$

$=\frac{2\times 5\times {10}^{-3}\times 4\times {10}^{-2}\times 1500}{50\times {10}^2\times 1.5\times {10}^{-2}}$

$E_d=\frac{60000\times {10}^{-5}}{75\times {10}^{-4}}=80V$

Input voltage to amplifier = 2 V

We know input voltage required for a deflection of 4 cm

$=\frac{E_d}{Amplifiergain}$

$\text{So, Amplifier gain}=\frac{E_d}{2}=\frac{80}{2}=40$