Page 43 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Transistor biasing and characteristics

Objectives: At the end of this lesson you shall be able to
• state the need and type of transistors biasing
• state the reason for shifting Q point due to temperature and b changes
dc
• state the necessity and importance of transistor characteristics
• state the importance of DC load line and meaning of Q point in transistors characteristics.

Need of biasing of transistor

Before any one rides a motor cycle or drives a car, he has to start the engine and keep the engine running. In
simple terms biasing transistors is similar to keeping the transistor started before making the real use of it. Once
the transistor is started, like the engine of a car, it can be made to amplify, like covering the distance by driving
the car.

Before an ac signal is fed to a transistor, it is necessary to set up an operating point or the quiescent(Q) point
of operation. Generally this Q point is set at the middle of the DC load line. Once the Q point is set, then the
incoming ac signals can produce fluctuations above and below this Q point as in Fig 1.

Fig 1

For the normal operation of a transistor amplifier circuit, it is essential that there should be
a a forward bias on the emitter-base junction and
b Reverse-bias on the collector-base junction
In addition, the amount of bias required is important for establishing the Q point which is dictated by the mode of
operation desired.
If the transistor is not biased correctly, it would
1 Work inefficiently and

2 Produce distortion in the output signal.
It is desirable, that once selected, the Q point should remain stable i.e. should not shift its position due to
temperature rise which cause variation in b (VBE) or leakage currents.

Further the amplitude variations in current and voltage of the input signal must not drive the transistor either into
saturation of cut off.
Stable Q point: A set Q point of a transistor amplifier may shift due to increased temperature and transistor b
value changes. Therefore, the objective of good biasing is to limit this shifting of the Q point or to achieve a stable
Q point.

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