waecE-LEARNING
Electronics Paper 1, Nov/Dec 2011  
Questions: 1 2   Main
General Comments
Weakness/Remedies
Strength

Question 2

AIM: To obtain the input characteristics and d.c. current gain for a common
                  emitter configuration.

Electronic Image

  1. Connect the circuit as shown in figure 2.
  2. Ask the supervisor to check the circuit connection.
  3. Copy Table 2 into your answer booklet.

                                         

Table 2

 

V1(V)

 

IC(mA)

 

Ib = Electronic Image

 

Electronic Image

1

 

 

 

3

 

 

 

5

 

 

 

7

 

 

 

9

 

 

 

 

 

 

 

 

  1.   Turn on the switch (S1).
  2.   Set the input voltage (V1), to 1V, measure and record in Table 2 the

   corresponding current (IC).

  1.   Repeat step(e) for the other values of V1 inTable 2.
  2.   Turn off switch S1.
  3.   Complete Table 2.
  4.   From the results obtained in Table 2, calculate the average value of the d.c.

  current gain (Ib).

  1.   Plot a graph of IC (mA) on the vertical axis against Ib (mA) on the horizontal

  axis.

  1.   Determine the slope of the graph in step (j).
  2.   Comment briefly on the value of the slope obtained in step (k) with the

  calculated average value of the d.c. current gain (Ib).

_____________________________________________________________________________________________________
OBSERVATION

The candidates were expected to connect the circuit as shown, measure and record the voltage(V) and current (mA), evaluate Ib and Electronic Image, plot a graph of IC (mA) against Ib(mA), determine and comment on the slope.

 

 

The expected answers were:
         (c)                        

 

V1(V)

 

IC(mA)

 

Ib = Electronic Image

 

Electronic Image

1

5.407

30

180.00

3

5.413

230

23.53

5

5.421

430

12.61

7

5.426

630

8.61

9

5.428

830

6.54

         

 

 

 

 

  1. Average value of  the d.c. current gain (Ib)  = 430 Electronic Image

(k)      Slope of the graph = 26.6
(l)      The average value of correct gain from the graph is about the same
          value as the calculated Electronic Image

As reported by the Chief Examiner, the readings obtained by many of the candidates that answered this question showed that they did not have adequate knowledge on how to solder and to adjust meter settings to correspond to indicated values of measured quantities.

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