What is Transistor Biasing? Circuit Diagram & Types (Fixed Bias, Collector to Base Bias, Voltage Divider Bias)

For the transistor to act as an amplifier is must be operated in its active region. The method of applying external voltages to operate the transistor in the active region is known as Transistor Biasing. For achieving a perfect amplification in amplifier circuit proper biasing is needed.

Types (or Methods) of Transistor Biasing

To accomplish this various biasing circuits are designed and the list of which is as follows,

1. Fixed bias circuit
2. Collector to base bias
3. Potential divider bias.

Fixed Bias Circuit

Fixed bias is also known as base resistor bias. In this, a common emitter amplifier is biased by connecting a resistor ‘R_B‘ across the base and power supply terminals as shown in the figure below,

Applying KVL at the input side of the circuit in figure,

\[{{V}_{CC}}={{I}_{B}}{{R}_{B}}+{{V}_{BE}}…(1)\]

Rearranging equation (1),

\[{{I}_{B}}=\frac{{{V}_{CC}}-{{V}_{BE}}}{{{R}_{B}}}…(2)\]

The above equation is independent of I_C.

Differentiating equation (2) with respect to I_C,

\[\frac{d{{l}_{B}}}{d{{l}_{C}}}=0…(3)\]

The expression for stability factor ‘S’ for a CE configuration is,

\[S=\frac{1+\beta }{1-\beta \left( \frac{d{{l}_{B}}}{d{{l}_{C}}} \right)}…(4)\]

Using equation (3), equation (4) can be written as,

\[S=\frac{1+\beta }{1-\beta (0)}\] \[S=1+\beta \]

Since β is a large quantity, S is also very high. As a result, the circuit is very poor in stability. In practice, this circuit is not used for biasing the base.

Advantages of Fixed Bias Circuit

1. The circuit is very simple.
2. Number of components required are less.

Disadvantage Fixed Bias Circuit

1. The circuit has very high value of stability factor S.
2. Therefore it is highly unstable.

Collector to Base Bias

In this CE amplifier is biased by connecting a resistance ‘R_B‘ across the collector and base terminals as shown in figure below.  From the figure, it is clear that collector to base voltage forward biases the BE junction. Therefore sufficient base current flows through the base resistance ‘R_B‘ and it causes zero signal collector current flowing in the circuit.

Applying KVL at the input side of circuit is shown in figure.

\[{{V}_{CC}}={{I}_{C}}{{R}_{C}}+{{I}_{B}}{{R}_{B}}+{{V}_{BE}}\]

And

\[{{I}_{C}}=\beta {{I}_{B}}\]

Then,

\[{{V}_{CC}}=\beta {{I}_{B}}{{R}_{C}}+{{I}_{B}}{{R}_{B}}+{{V}_{BE}}\]

\[{{I}_{B}}{{R}_{B}}={{V}_{CC}}-\beta {{I}_{B}}{{R}_{C}}-{{V}_{BE}}\]

\[{{R}_{B}}=\frac{{{V}_{CC}}-\beta {{I}_{B}}{{R}_{C}}-{{V}_{BE}}}{{{I}_{B}}}\]

Base resistance,

\[{{R}_{B}}=\frac{{{V}_{CC}}-\beta {{I}_{B}}{{R}_{C}}-{{V}_{BE}}}{{{I}_{B}}}\]

Alternatively,

\[{{V}_{CE}}={{V}_{BE}}+{{V}_{CB}}\]

\[{{R}_{B}}=\frac{{{V}_{CB}}}{{{I}_{B}}}=\frac{{{V}_{CE}}-{{V}_{BE}}}{{{I}_{B}}}\]

Thus,

\[{{R}_{B}} =\frac{{{V}_{CE}}-{{V}_{BE}}}{{{I}_{B}}}\]

Advantages of Collector to Base Bias Circuit

1. It is easy and simple to design.
2. It requires only ‘R_B‘ for biasing.

Disadvantage Collector to Base Bias Circuit

1. The circuit provides negative feedback that reduces the gain of an amplifier.
2. Stability factor is fairly high.

Voltage Divider Bias (or) Self Bias

The most extensively used biasing circuit for maintaining stabilization is voltage divider bias circuit. It is as shown in figure.  The circuit contains two resistors R₁ and R₂ connected to V_CC in the base section of the transistor. The voltage drop across the resistor R₂ forward biases the emitter base junction. The resistor R_E connected to the emitter grants stabilization.

The current flowing through the resistance ‘R₁‘ is I₁. Since, the current flowing through the base is very small it can be assumed that the same current I₁ flows through resistance by R₂. Therefore I₁ is given as,

\[{{I}_{1}}=\frac{{{V}_{CC}}}{{{R}_{1}}+{{R}_{2}}}\]

Further, since same current I₁ is flowing through R₁ and R₂ it can be assumed that R₁ and R₂ are in series. Therefore, by using voltage division rule, the voltage drop across R2 is given as,

\[{{V}_{2}}=\left( \frac{{{R}_{2}}}{{{R}_{1}}+{{R}_{2}}} \right).{{V}_{CC}}…(4)\]

Apply KVL to the loop 2 shown in figure.

\[{{V}_{2}}={{V}_{BE}}+{{V}_{E}}\]

\[{{V}_{2}}={{V}_{BE}}+{{I}_{E}}{{R}_{E}}\]

\[{{I}_{E}}=\frac{{{V}_{2}}-{{V}_{BE}}}{{{R}_{E}}}\]

Thus,

\[{{I}_{E}}=\frac{{{V}_{2}}-{{V}_{BE}}}{{{R}_{E}}}\]

Generally for a CE configuration I_E ≈ I_C. Hence equation (4) can be written as,

\[{{I}_{C}}=\frac{{{V}_{2}}+{{V}_{BE}}}{{{R}_{E}}}…(5)\]

Where,

V₂ – Voltage drop across ‘R₂‘

V_BE – Base emitter voltage.

Applying KVL to the output side of the circuit in figure,

\[{{V}_{CC}}={{I}_{C}}{{R}_{C}}+{{V}_{CE}}+{{V}_{E}}\]

\[{{V}_{CC}}={{I}_{C}}{{R}_{C}}+{{V}_{CE}}+{{I}_{E}}{{R}_{E}}\]

\[={{I}_{C}}({{R}_{C}}+{{R}_{E}})+{{V}_{CE}}\text{    (}{{I}_{C}}\approx {{I}_{E}}\text{)}\]

\[={{V}_{CE}}+{{I}_{C}}({{R}_{C}}+{{R}_{E}})\]

\[{{V}_{CE}}={{V}_{CC}}-{{I}_{C}}({{R}_{C}}+{{R}_{E}})\]

The circuit provides good stabilization through resistance ‘R_E‘. It is explained using equation (5), that is,

\[{{V}_{2}}={{V}_{BE}}+{{I}_{C}}{{R}_{E}}\]

In the above expression, voltage V₂ (across R₂) is independent of I_C and is given as,

\[{{V}_{2}}=\left( \frac{{{V}_{CC}}}{{{R}_{1}}+{{R}_{2}}} \right){{R}_{2}}\]

Since R₁, R₂ and V_CC are constant, V₂ is also constant. An increase in I_C causes an increase in potential across R_E (that is, I_CR_E). Since, V₂ is constant an increase in I_CR_E must be compensated by a decrease in V_BE Therefore, an increase in I_CR_E causes V_BE to decrease and vice versa. In this manner the circuit provides good stabilization.

Advantages of Voltage Divider Bias Circuit

1. It is most popular biasing circuit.
2. Simple ‘R_E‘ resistance provides good stability.
3. Stability factor (S) in controlled by R_E and R_th.
4. Stability factor is small for this circuit compared to other techniques.