2023 A/L Physics Past Paper · Question 07 (English Medium)
Where a transistor works as a switch
When transistors are used in digital circuits they operate in the
Answer and explanation
Answer: (5) saturation and cutoff regions.
What the question asks
We want: the regions in which a transistor works when it is used in a digital circuit.
A transistor in a digital circuit is used as a switch.
Relevant theory
A transistor in the common emitter configuration can be used as a switch. Only the cut- off and saturation states are made use of for the switching purpose.
Reading the options
For example, think of a switch on the wall.
It has two positions, OFF and ON.
A transistor used as a switch also works in two states. The quote names them: the cut-off state and the saturation state.
So the answer must have both of these.
Any other region is not used for switching.
Test each option against the quote.
(1) Active region: not one of the two states for switching.
(2) Breakdown region: not one of the two states for switching.
(3) Linear region: not one of the two states for switching.
(4) Saturation region: only one of the two. Cut-off is left out.
(5) Saturation and cutoff regions: both of the two states. This matches.
Only option (5) matches.
Why the other options fail
The quote names the cut-off and saturation states for switching. The active region is neither of them.
The quote names the cut-off and saturation states for switching. The breakdown region is neither of them.
The quote names the cut-off and saturation states for switching. The linear region is neither of them.
The quote names two states for switching, cut-off and saturation. This option has saturation only.
Conclusion
In digital circuits a transistor is used as a switch, and it works in the saturation and cut-off regions.
The answer is (5).
A digital circuit uses the transistor as a switch.
Remember the two states of a switch, cut-off and saturation. Any option that names only one of them, or another region, is not the answer.
Exam tip