Transistors: SAQ Solutions

Below are the solutions to the Transistors (Parts 1 and 2) Self-Assessment Questions:

Part 1

1. Any two of the following answers are accepted:
   • NPN transistor turns on when the current flows through the base of the transistor, whereas a PNP transistor turns on when there is no current through the base.
   • Current flows from the emitter to collector in the pNP, in NPN, current flows from the collector to the emitter.
   • E and C are composed of a p-type conductor in a PNP, whereas E and C are composed of a n-type conductor.
     Both of these transistor types are made using diodes in different orientations. NPN transistors are more commonly seen and easier to make.
2. E-emitter, B-base, C-collector
   A small current flows through the base to the emitter, while a larger current is generated from the collector to the emitter. This is how the current is amplified.
3. True. Transistors amplify small currents.
   They do this only in the linear region of input base current, as the next question details.
4. The saturation region corresponds to the transistor serving as a closed switch, while the linear region corresponds to the transistor serving an amplifying function.
   In the linear region, the amplified current is proportional to the base current. When the output current reaches a maximum in the saturation region, the emitted current becomes independent of the base current.

Part 2

1. The arrow points to the emitter (where lower voltage is located). This is the direction of positive current flow.
   The arrow always points towards the direction of positive current flow, but this is only towards the emitter in NPN transistors, while the arrow points towards the base in PNP transistors since current flows in the opposite direction.
2. An NPN transistor is used when positive voltage is involved.
   This is a product of the current flow properties as explained in the previous question.
3. Junction becomes shorted – resistance between junctions become very low or close to zero.
   Junction becomes an open circuit – resistance can get very high for the entire part (become infinite).
   Transistor failure is uncommon, but it is extremely dangerous because it can lead to electrical surges. Testing the two different transistor types requires different orientation of the multimeter probes as described in the video.
4. True. Transistors are active components!
   Regardless of transistor type, they amplify an input signal, making them active components.