**Fixed Bias Circuit Diagram – Advantages and Disadvantages:**

Fixed Bias Circuit Diagram shown in Fig. 12.5 uses two batteries V_{BB} and V_{CC}. The battery V_{BB} gives potential to the base terminal through resistance R_{B} (of the order of hundred kÎ©) and battery V_{CC} supplies power to the collector through resistance R_{C} (of the order of kÎ©).

The base current can be determined by applying Kirchhoff’s second law for the branch circuit, starting from battery V_{BB} to ground.

Since V_{BE} is very small (0.7 V in case of silicon transistor and 0.3 V in case of Ge transistor) in comparison to V_{BB}, not much error will be committed if it is neglected. The base current is then given by the simple expression

The supply voltage V_{BB} being of fixed valueâ€”once the resistance R_{B} is chosenâ€”the base current I_{B} is also fixed. Hence the name fixed bias circuit is given.

The collector-emitter section consists of the supply battery V_{CC}, the collector resistor and the transistor collector-emitter junction. The currents through the collector and emitter are about the same since I_{B} is very small in comparison to either. For linear amplifier operation, the collector current will be Î² times base current, where Î² is the transistor current gain i.e.,

The base current I_{B} is determined from the operation of the base-emitter section of the circuit. The collector current, as shown above, will be Î² times the base current and not at all dependent on the resistance in the collector circuit. Thus collector current l_{C} is controlled by base-emitter section of the circuit.

Applying Kirchhoff’s second law to the branch circuit starting from V_{CC} to ground.

The quiescent point is given by I_{C}, V_{CE}.

The coupling capacitors C_{1} and C_{2} have reactances quite small at the frequency of ac signal, so that very little voltage drop of ac signal takes place across them. These capacitors are open circuit for direct currents, so that dc biasing potentials of this amplifier stage do not affect the circuits connected at the input as well as output terminals.

The fixed bias circuit is the simplest way to bias a transistor, but from stability point of view it is the worst circuit, because with the rise in temperature of transistor, Î² changes and also there is increase of leakage current and drop in V_{BE} value.

Since base current I_{B} controls the collector current I_{C} and I_{C} is equal to Î²I_{B}, with the change in the value of Î², I_{C} changes (I_{B} remaining unchanged) and the operating point gets shifted.

The circuit shown in Fig. 12.5 may be modified by using a single battery, as shown in Fig. 12.2. From the practical point of view, it is always preferable to have an electronic circuit operated from a single battery.

The expressions for I_{B}, I_{C}Â and V_{CE}Â are given below

**Stability Factor S:**

As we know from earlier section, the stability factor

and in Fixed Bias Circuit method I_{B }is independent of I_{C}, as obvious from Eq. (12.8) or Eq. (12.11), so stability factor is given as

If Î² = 150, then S = 151 which means that collector current I_{C} increases 151 times as much as I_{CO}.Â Such a large value of S makes thermal runaway, a definite possibility with this circuit.

**Advantages and Disadvantages: **

Though this biasing circuit is very simple, as very few components (only two resistors and one battery) are required, biasing conditions can be easily set, the calculations are simple and there is no loading of the source by the biasing circuit, as no resistor is used across base-emitter junction but this method of biasing is rarely used. This is due to the following reasons.

- This method provides poor stability, as there is no means to stop a self increase in collector current I
_{C}owing to temperature rise and individual variations. For example, if Î² increases either due to temperature rise or due to replacement of transistor, then collector current, l_{C}also increases by the same factor as base current I_{B}is constant. - There are good chances of thermal runaway. This is due to high stability factor S.