Transistor Biasing

Biasing is the process of providing DC voltage which helps in the functioning of the circuit. A transistor is based in order to make the emitter base junction forward biased and collector base junction reverse biased, so that it maintains in active region, to work as an amplifier.

In the previous chapter, we explained how a transistor acts as a good amplifier, if both the input and output sections are biased.

Transistor Biasing

The proper flow of zero signal collector current and the maintenance of proper collectoremitter voltage during the passage of signal is known as Transistor Biasing. The circuit which provides transistor biasing is called as Biasing Circuit.

Need for DC biasing

If a signal of very small voltage is given to the input of BJT, it cannot be amplified. Because, for a BJT, to amplify a signal, two conditions have to be met.

• The input voltage should exceed cut-in voltage for the transistor to be ON.

• The BJT should be in the active region, to be operated as an amplifier.

If appropriate DC voltages and currents are given through BJT by external sources, so that BJT operates in active region and superimpose the AC signals to be amplified, then this problem can be avoided. The given DC voltage and currents are so chosen that the transistor remains in active region for entire input AC cycle. Hence DC biasing is needed.

The below figure shows a transistor amplifier that is provided with DC biasing on both input and output circuits.

For a transistor to be operated as a faithful amplifier, the operating point should be stabilized. Let us have a look at the factors that affect the stabilization of operating point.

Factors affecting the operating point

The main factor that affect the operating point is the temperature. The operating point shifts due to change in temperature.

As temperature increases, the values of I_CE, β, V_BE gets affected.

• I_CBO gets doubled (for every 10^o rise)
• V_BE decreases by 2.5mv (for every 1^o rise)

So the main problem which affects the operating point is temperature. Hence operating point should be made independent of the temperature so as to achieve stability. To achieve this, biasing circuits are introduced.

Stabilization

The process of making the operating point independent of temperature changes or variations in transistor parameters is known as Stabilization.

Once the stabilization is achieved, the values of I_C and V_CE become independent of temperature variations or replacement of transistor. A good biasing circuit helps in the stabilization of operating point.

Need for Stabilization

Stabilization of the operating point has to be achieved due to the following reasons.

• Temperature dependence of I_C
• Individual variations
• Thermal runaway

Let us understand these concepts in detail.

Temperature Dependence of I_C

As the expression for collector current I_C is

$$I_C = \beta I_B + I_{CEO}$$

$$= \beta I_B + (\beta + 1) I_{CBO}$$

The collector leakage current I_CBO is greatly influenced by temperature variations. To come out of this, the biasing conditions are set so that zero signal collector current I_C = 1 mA. Therefore, the operating point needs to be stabilized i.e. it is necessary to keep I_C constant.

Individual Variations

As the value of β and the value of V_BE are not same for every transistor, whenever a transistor is replaced, the operating point tends to change. Hence it is necessary to stabilize the operating point.

Thermal Runaway

As the expression for collector current I_C is

$$I_C = \beta I_B + I_{CEO}$$

$$= \beta I_B + (\beta + 1)I_{CBO}$$

The flow of collector current and also the collector leakage current causes heat dissipation. If the operating point is not stabilized, there occurs a cumulative effect which increases this heat dissipation.

The self-destruction of such an unstabilized transistor is known as Thermal run away.

In order to avoid thermal runaway and the destruction of transistor, it is necessary to stabilize the operating point, i.e., to keep I_C constant.

Stability Factor

It is understood that I_C should be kept constant in spite of variations of I_CBO or I_CO. The extent to which a biasing circuit is successful in maintaining this is measured by Stability factor. It denoted by S.

By definition, the rate of change of collector current I_C with respect to the collector leakage current I_CO at constant β and I_B is called Stability factor.

$S = \frac{d I_C}{d I_{CO}}$ at constant I_B and β

Hence we can understand that any change in collector leakage current changes the collector current to a great extent. The stability factor should be as low as possible so that the collector current doesn’t get affected. S=1 is the ideal value.

The general expression of stability factor for a CE configuration can be obtained as under.

$$I_C = \beta I_B + (\beta + 1)I_{CO}$$

Differentiating above expression with respect to I_C, we get

$$1 = \beta \frac{d I_B}{d I_C} + (\beta + 1)\frac{d I_{CO}}{dI_C}$$

Or

$$1 = \beta \frac{d I_B}{d I_C} + \frac{(\beta + 1)}{S}$$

Since $\frac{d I_{CO}}{d I_C} = \frac{1}{S}$

Or

$$S = \frac{\beta + 1}{1 - \beta \left (\frac{d I_B}{d I_C} \right )}$$

Hence the stability factor S depends on β, I_B and I_C.