Self bias

Self-Bias A better method of biasing is obtained by inserting the bias resistor directly between the base and collector, as shown in figure 2-13. By tying the collector to the base in this manner, feedback voltage can be fed from the collector to the base to develop forward bias. This arrangement is called SELF-BIAS. Now, if an increase of temperature causes an increase in collector current, the collector voltage (VC) will fall because of the increase of voltage produced across the load resistor (RL). This drop in VC will be fed back to the base and will result in a decrease in the base current. The decrease in base current will oppose the original increase in collector current and tend to stabilize it. The exact opposite effect is produced when the collector current decreases. The voltage divider is formed using external resistors R1 and R2. The voltage across R2 forward biases the emitter junction. By proper selection of resistors R1 and R2, the operating point of the transistor can be made independent of β. In this circuit, the voltage divider holds the base voltage fixed independent of base current provided the divider current is large compared to the base current. However, even with a fixed base voltage, collector current varies with temperature (for example) so an emitter resistor is added to stabilize the Q-point, similar to the above circuits with emitter resistor. In this circuit the base voltage is given by: voltage across
provided .
Also For the given circuit,
{ \frac {V_{CC}}{1+R_1/R_2} - V_{be} } {( \beta + 1)R_E + R_1 \parallel R_2 } ." class="tex" />
  Merits:
  • Unlike above circuits, only one dc supply is necessary.
  • Operating point is almost independent of β variation.
  • Operating point stabilized against shift in temperature.
Demerits:
  • In this circuit, to keep IC independent of β the following condition must be met:
{ \frac {V_{CC}}{1+R_1/R_2} - V_{be} } {( \beta + 1)R_E + R_1 \parallel R_2 } \approx \frac { \frac {V_{CC}}{1+R_1/R_2}- V_{be}} {R_E} , " class="tex" />
which is approximately the case if
where R1 || R2 denotes the equivalent resistance of R1 and R2 connected in parallel.
  • As β-value is fixed for a given transistor, this relation can be satisfied either by keeping RE fairly large, or making R1||R2 very low.
  • If RE is of large value, high VCC is necessary. This increases cost as well as precautions necessary while handling.
  • If R1 || R2 is low, either R1 is low, or R2 is low, or both are low. A low R1 raises VB closer to VC, reducing the available swing in collector voltage, and limiting how large RC can be made without driving the transistor out of active mode. A low R2 lowers Vbe, reducing the allowed collector current. Lowering both resistor values draws more current from the power supply and lowers the input resistance of the amplifier as seen from the base.
  • AC as well as DC feedback is caused by RE, which reduces the AC voltage gain of the amplifier. A method to avoid AC feedback while retaining DC feedback is discussed below.
Self-bias has two small drawbacks: (1) It is only partially effective and, therefore, is only used where moderate changes in ambient temperature are expected; (2) it reduces amplification since the signal on the collector also affects the base voltage. This is because the collector and base signals for this particular amplifier configuration are 180 degrees out of phase (opposite in polarity) and the part of the collector signal that is fed back to the base cancels some of the input signal. This process of returning a part of the output back to its input is known as DEGENERATION or NEGATIVE FEEDBACK. Sometimes degeneration is desired to prevent amplitude distortion (an output signal that fails to follow the input exactly) and self-bias may be used for this purpose. reference:http://www.tpub.com/neets/book7/25d.htm ; www.wikipedia.org  
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Wed, 05/20/2009 - 11:27

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