NEED FOR BIASING OF A TRANSISTOR
Amount of bias required is important for establishing the Q- point which is determined by the mode of operation desired.
If the transistor is not biased correctly, it would
- Work inefficiently
- Produce distortion in the output signal
FACTORS AFFECTING BIAS VARIATIONS
The bias instability is the direct result of thermal instability which is produced by cumulative increase in Ic , that may lead to thermal run away.
The collector current for CE circuit is given by,
IC = β IB + ICE0
Or IC = β IB + (1+β) IC0
The three variables β, IB, IC0 are increases with temperature. Increase in IC0 produces increases in IC. This increases the power dissipation and further increase in temperature and hence IC, and that will lead to destroy transistor itself.
If we done some circuit modification, IC is made to decrease with temperature automatically, then decrease in the term β IB can be made to neutralize the increase in the term (1+β) IC0 , thereby keeping IC constant. This will achieve thermal stability resulting in bias stability.
The degree of success achieved in stabilizing IC with respect to IC0 when both β and IB is expressed in terms of current stability factor S.
S= c / C0
Larger the value of S, greater the thermal instability and vice versa.
The stability factor can be expressed as,
S= (1+ β)/ 1- β ( B/ C)
The β sensitivity of a circuit is meant the influence that the β value has on its dc operating point.
Beta sensitivity (K β) is given by,
c / C = K β . B/
DIFFERENT METHODS OF TRANSISTOR BIASING
- 1. BASE BIAS or FIXED CURRENT BIAS
This is very simple circuit (as shown in the previous post)but it is not a very satisfactory method because bias voltages and currents do not remain constant during transistor operation. For such a circuit, S = 1 and K β =1.
- 2. BASE BIAS WITH EMITTER FEEDBACK
At saturation, VCE is essentially zero, hence VCC is distributed over RL and RE.
I c (sat) = VCC/RE +RL
Ic can be found as,
I c = (VCC – VBE) / RE + (RB/β)
Ic = (VCC) / RE + (RB/β)
- 3. BASE BIAS WITH COLLECTOR FEEDBACK
This circuit is like the base bias circuit except that base resistor is returned to collector rather than to the Vcc supply. It derives its name from the fact that since voltage for RB is derived from collector, there exists a negative feedback effect which tends to stabilize IC against changes in β.
Ic(sat) = VCC/RL
Ic = (VCC– VBE )/RL+(RB/ β)
K β =1- IC/ICmax
- VOLTAGE DIVIDER BIAS
This arrangement is commonly used for transistors incorporated in integrated circuits. The term ‘voltage divider’ is derived from the fact that resistors R1 and R2 from a potential divider across VCC. The voltage drop V2 across R2 forward biases the emitter whereas Vcc supply reverse biases the collector.
Ic(sat) = VCC/RE +RL
K β =1/(1+βRE/R1//R2)