Basic Electronics Paper 3, Nov/Dec 2015

Question 2

 

AIM: To determine the value and stability of a transistor operating point.

(a). Connect the circuit diagram as shown in figure 2.

(b). Ask the supervisor to check the circuit connection.

(c). Copy Table 2 into your answer booklet.                                                     

               

(d). Set Vcc to 9V.

(e). Close switch S1.

(f). Measure and record in Table 2 the values of V1, V2, VBE and VCE.

(g).Open switch S1.

(h). Replace the 27kΩ resistor with the 47kΩ.

(i). Repeat steps (e) to (g).

(j). Complete Table 2.

(k).Briefly comment on the sets of values of VBE, VCE and β for the two values of   

  RB.

(l). Compare the values of in β experiment 1 and 2 and comment.

 

The candidates were expected to connect the circuit as shown, measure and record the values of the voltmeter and ammeter readings.

Observation

The expected answers were:


Table 2

 

RB (kΩ)

 

V1 (V)

 

V2 (V)

 

VBE (V)

 

VCE (V)

        V1
IB =  RB
(mA)

        VC
IC =   RC
(mA)

       IC
β =  IB

 

27

 

8.287

 

8.962

 

0.713

 

0.038

 

0.3069

 

8.962

 

29.20

 

47

 

8.292

 

8.948

 

0.708

 

0.052

 

0.1760

 

8.948

 

50.84

         
(k) (i) VBE is not the same for both values of RB, but decreased with increase in 
RB.
(ii) VCE is not the same but increased with increase in RB.
(iii) Beta (β) in experiment 1 is constant and much greater than β in   
experiment 2, which is not constant.


The Chief Examiner reported that many candidates showed good knowledge of meter readings and the required calculations. However, many candidates did not show the theoretical knowledge of the experiment.