Page 47 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The reason for deviation of the characteristic curve for V , 5V and 10V from V 0 volt is, at higher values of VCE
CE
CE
the collector gathers a few more electrons flowing through the emitter. This reduces the base current. Hence the
curve with higher V has slightly less base current for a given V . This phenomenon is known as early effect.
CE
BE
However for the practical purposes the difference in gap is so small it can be regarded as negligible.
The CE input characteristic curves resemble the forward characteristic of a PN diode. The input resistance can
be calculated by using the formula.

The voltage gain can be calculated by using the formula:

Output CE characteristics: To find the output characteristics, keep I =0 micro-amp constant, increase V in
B
CE
regular steps of 1 and note the value of I at each step. Repeat the above procedure for IB = 20 micro-amp, 40
V
B
micro-amp and 60 micro–amp.
Output characteristics curves can be obtained by plotting Ic on the Y axis against V on the X axis. A typical
CE
output characteristics curve is shown in Fig 9.
It is seen that as V increases from zero, Ic rapidly increases to a near saturation level for a fixed value of I . As
CE
B
shown, a small amount of collector current flows even when I = 0. It is called leakage current I CEO . Since the main
B
collector current is zero, the transistor is said to be cut-off.
For simplicity in understanding consider on the output characteristic curve where I = 40 µA.
B
The output resistance can be calculated by the formula

Current gain can be calculated by the formula

In the common base configuration, the current gain can be calculated by the formula:

Transistor as a switch, series voltage regulator and

amplifiers

Objectives: At the end of this lesson you shall be able to
• explain the function of the transistor at cut-off and saturation condition
• explain the operation of a transistor as a switch and its application
• state the working of series voltage regulator using transistor
• state the classification of amplifiers.

The operation of transistor as switch: The switching action for Q1 in Fig 1 illustrates how the output current can
be controlled at the input. Note the following important operating characteristics.

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CITS : Power - Electrician & Wireman - Lesson 60-69