Question 3
- State two applications of:
- Fig. 2 is a schematic diagram of a tetrode valve.
- State two advantages of semiconductor devices over vacuum tubes:
(i) thermionic emission;
(ii) photo emission.
Name the parts labelled X and Y.
The expected responses were:
(a) (i) Applications of thermionic emission
- Vacuum tubes
- CRT (in television sets and oscilloscope)
- X – rays
- Fluorescent tubes
(ii) Applications of photo emission
- Photocells
- Solar cells
- Burglar alarms
- Television cameras
- Automatic door switching
- Automatic light switching devices
- Fibre optic communication
(b) X – screen grid
Y – control grid
(c) Advantages of semiconductor devices over vacuum tubes
- Much smaller size
- No/less heat production
- Longer life span
- Faster response time
- Lower power consumption
It was reported that quite a few candidates had problems recalling the applications of thermionic emission and photo emission in part (a). Part (b)is essentially a ‘recall’ type question, for the question on pentode, so that X represented the screen grid, and Y, the control grid. The Chief Examiner reported that many candidates listed these correctly, quite a few mixed them (one for the other) and still many could not correctly identify the grids. Performance in the question (c) was rated rather poor.