ASSIGNMENT Answer the following question. a) With reference to a p-n-p transistor, explain briefly what is meant by the term transistor action and why a bipolar junction transistor is so named. b)...

ASSIGNMENT

Solution: 1(a) The various rules are:
Rule1: p.q Rule2: r.s
Rule3: (p ⊕ q).(r + s) Rule4: IF s.q = 1 THEN f = 0.
The truth table for the given set of rules is as tabulated below:
Temp.
Sensor 1
Temp.
Sensor 2
Motion
Detector
Sound
Detector

Output

Note
p q r s f
0 0 0 0 0
0 0 0 1 0
0 0 1 0 0
0 0 1 1 1 Motion and sound
0 1 0 0 0
0 1 0 1 0 Temp and sound BUT excluded
0 1 1 0 1 Temp and motion
0 1 1 1 0 Motion and sound BUT excluded
1 0 0 0 0
1 0 0 1 1 Temp and sound
1 0 1 0 1 Temp and motion
1 0 1 1 1 Motion and Sound
1 1 0 0 1 Both temp
1 1 0 1 0 Both temp BUT excluded
1 1 1 0 1 Both temp
1 1 1 1 0 Both temp BUT excluded
Solution: 1(b) The Boolean expression for the above truth table is as expressed below:
f = p’q’rs + p’qr’s + p’qrs’ + pq’r’s + pq’rs’ + pq’rs + pqr’s’ + pqrs’
Solution:1(c) Let us draw the K-MAP for the above function:
f= qrs’ + pqs’ + q’rs + pq’s + prs’
Solution: 1(d) Either one of the following diagrams is fine.


Solution: 2(a) For p-n-p transistor as shown in fig. 2a(i), the emitter is relatively heavily doped with
holes. When we make the emitter terminal sufficiently positive with respect to the base, the
majority ca
iers in the emitter i.e. holes start drifting from the emitter to the base due to forward
iasing of base-emitter junction.

Fig. 2a(i)
Base of the transistor is relatively lightly doped with electrons and also the base width is smaller
than the diffusion length of the electrons. Hence only a few electron-hole recombinations take
place, around 0.5%, and hence most of the holes entering the base, do not combine with electrons.

The base-collector junction is RB to e- in the base, but FB to holes in the base region. As the base
width is very small and is packed with holes presently, these holes pass through the base-emitter
junction and then move towards the negative potential of the collector terminal. Hence, the control
of cu
ent is independent of the collector-base voltage and almost governed by the emitter-base
voltage. The essence of transistor action is this cu
ent control by means of the base-emitter voltage.

Fig. 2(ii)
The reason for its name as Bipolar Junction Transistor is because herein a transistor both types of
charge ca
iers (holes and electrons) take part in the cu
ent flow and hence the name has the term
ipolar. The transistor also comprises two p-n junctions and hence it is a junction transistor; hence
the name – bipolar junction transistor.

Solution: 2(b) The circuit diagram for an n-p-n transistor connected in common-base configuration
to obtain the input and output characteristics is as...