Page 44 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The Q point is nothing but a point in the output characteristic of the transistor. This point corresponds to a
particular value of IB, IC and VCE. Further , the collector current IC depends both on IB and b of the transistor.
If IB changes, Ic also changes, and hence, the Q point changes. If b changes, again IC changes, and hence, the
Q point gets shifted.
Shifting of Q point due to temperature: Remember that a transistor is a temperature sensitive device. Any
increase in the junction temperature results in leakage current. this increased leakage current in turn increases
the temperature and the effect is cumulative. This chain reaction is called thermal run away. If this thermal
run away is not stopped, it may result in the complete destruction of the transistor due to excessive heat. In
transistors, due to this increased leakage current, the base current increases, and hence, the Q point gets shifted.
This change in the set Q point affects the performance of the amplifier resulting in distortion.
Shifting of Q point due to bdc changes: Practically two transistors of the same type number may have different
value of b. this is due to the manufacturing process of transistors. Hence, when a transistor is replaced or
changed, due to different b of the replaced transistor, the Q point again gets shifted.

Therefore, a stable biasing is one which does not shift the Q-point even if temperature varies and/or the b of the
transistor changes.
Different methods for transistor biasing: There are several ways to bias a transistor for linear operation. This
means, there are several ways of setting up a Q point near the middle of the dc load line.
The methods used for providing a bias for transistors are
1 fixed bias or base bias

2 self-bias or emitter bias or emitter feed back bias
3 voltage divider bias
Fixed bias or base bias: The circuit in Fig 2 provides a fixed bias by means of the power source Vcc and the
base resistor RB

Fig 2

Self-bias arrangements are not practicable for small values of current because the dc Q point changes due to
– poor Beta sensitivity

– bias voltages and current do not remain constant during transistor operation due to temperature variation.
Hence, in a base-biased transistor, it is impossible to set up a stable Q point. Therefore, base biasing of transistors
is not generally done in linear amplifier circuits. However, base biasing is commonly used in digital circuits
(discussed in further lessons) where transistor are used as a switch and not as a linear amplifier.
2 SELF BIAS or EMITTER BIAS or emitter feedback bias: Fig 3 shows a emitter-biased transistor. This type
of biasing compensates for the variations in temperature and keeps the Q point fairly stable.
Let the temperature rise-causing rise in Ic and consequently rise in Ic. Then the current in RE increases. The
increased current in RE increases the dc voltage drop across RE, reduces the net emitter to the base bias, and
the base current, and hence reduces the collector current. Thus the presence of the self-biasing resistor RE
reduces the increase in Ic and improves the operating point stability.

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