Fixed bias circuit

Base-Current Bias (Fixed Bias) The first biasing method, called BASE CURRENT BIAS or sometimes FIXED BIAS, was used in figure 2-12. As you recall, it consisted basically of a resistor (RB) connected between the collector supply voltage and the base. Unfortunately, this simple arrangement is quite thermally unstable. If the temperature of the transistor rises for any reason (due to a rise in ambient temperature or due to current flow through it), collector current will increase. This increase in current also causes the dc operating point, sometimes called the quiescent or static point, to move away from its desired position (level). This reaction to temperature is undesirable because it affects amplifier gain (the number of times of amplification) and could result in distortion, as you will see later in this discussion. Fixed bias (base bias)   This form of biasing is also called base bias. In the example image on the right, the single power source (for example, a battery) is used for both collector and base of transistor, although separate batteries can also be used. In the given circuit, VCC = IBRB + Vbe Therefore, IB = (VCC - Vbe)/RB For a given transistor, Vbe does not vary significantly during use. As VCC is of fixed value, on selection of RB, the base current IB is fixed. Therefore this type is called fixed bias type of circuit. Also for given circuit, VCC = ICRC + Vce Therefore, Vce = VCC - ICRC From this equation we can obtain Vce. Since IC = βIB, we can obtain IC as well. In this manner, operating point given as (VCE,IC) can be set for given transistor.      Merits:
  • It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (RB).
  • Very few number of components are required.
  • The collector current does not remain constant with variation in temperature or power supply voltage. Therefore the operating point is unstable.
  • Changes in Vbe will change IB and thus cause RE to change. This in turn will alter the gain of the stage.
  • When the transistor is replaced with another one, considerable change in the value of β can be expected. Due to this change the operating point will shift.
  Usage: Due to the above inherent drawbacks, fixed bias is rarely used in linear circuits, ie. those circuits which use the transistor as a current source. Instead it is often used in circuits where transistor is used as a switch. However, one application of 'fixed' bias is to achieve crude automatic gain control in the transistor by feeding the base resistor from a dc signal derived from the ac output of a later stage.       Reference: ;

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Wed, 05/20/2009 - 10:42