Types of Transistor Biasing

TYPES OF TRANSISTOR BIASING

(1) Fixed bias (or) base resistor method

(2) Collector to base bias

(3) Voltage divides bias (or) self bias

Fixed Bias

The fixed bias method is shown in fig.3.54. In this method a high resistance RB is connected between positive terminal of supply VCC and base of the transistor. To find the value of resistance RB apply the KVL to the closed circuit "A B K L A".

Vcc = IB RB + VBE ……….(1)

IB = Vcc – VBE / RB

RB = Vcc -VBE / IB ………..(2)

We know that, β = IC / IB and VBE << V CC

RB = VCC / IB

R_B = βVCC / Ic

In this case, if VCC β are fixed values then IC is also fixed for a given transistor due to which RB is also a fixed value. So this method is called as fixed bias method.

Stability factor 'S':

The stability factor S can be defined as rate of change of collector current with respect to reverse saturation current assume β and VBE are constants

To obtain 'S' differentiate the equation (2) w.r.t to IC

dIB / dIc = 0 because VCC, VBE and RB are constants

S = 1+ β

Thus if β = 100 then S = 100 then S = 101. In this case 'S' is very high that is, it clearly shows that IC is more dependent upon ICO and temperature. This causes poor stabilization. These is the disadvantages of this method. Hence it is rarely used.

Example: 13

Calculate the value of the resistors on a fixed bias circuit using the following data Ico = 9 mA , VCE = 4.4 volts , β = 110, VBE = 0.7 V and VCC = 8 V.

Solution:

ICO = 9 mA , VCE = 4.4 volts , β = 110, VBE = 0.7 V and VCC = 8 V

RC = VCC - VCE / Ic

= 8 - 4.4 / 9×10-3

RC = 400Ω

IB = IC / β

IC = 0.08 mA

For base circuit

Vcc = IB RB +VBE

RB = Vcc – VBE / IB  

= 8 -  0.7 / 0.08 × 10-3

RB = 91.25Ω

Example: 14

Find the following for the fixed bias configuration (i) ICQ and VCEQ (ii) VB and VC using the following specification RB = 240 k Ω Vcc = 10 volts RC = 2kΩ β = 70.

Solution:

Vcc = IB RB+VBE

IB = VCC – VBE / RB = 10 – 0.7 / 240 = 38.75 μA

IB = 38.75 μA

ICQ = βIBQ = 70 × 38.75 × 10-6

ICQ = 2.71 mA

VCC = IC RC + VCE

VCE = VCC - ICRC

VCE = 10 - 2.71 x 10-3 × 2 × 103

VCE = 4.575 volts

Collector to Base Bias

The circuit diagram of a collector to base bias is shown in fig.3.55. This circuit is same as fixed bias circuit except that the resistor R is connected to collector rather than to Vcc. The resistor RB act as feedback resistor. In this circuit there is considerable improvement in the stability.

If the collector current IC tends to increase the DC voltage drop across RC decreases and consequently VCE decreases. As a result the base current IB also reduces. This will tend to compensate for the original increases.

Apply the KVL for the closed loop.

Vcc = (Ic + IB) Rc + IB RB + VBE………(1)

Vcc = Ic Rc + IB (RC + RB) + VBE

…………..(2)

Since VBE is almost independent of collector current, hence it is neglected from the above equation

Thus

IB = Vcc - IcRc / RC + RB …………(3)

Differentiate the above equation w.r.t IC we get

 ………….(4)

Stability factor S = ∂IC / ∂ICO |when ICO,ẞ constant

 ………….(5)

Substitue the value of dIB/dIC in the equation (5)

Example: 15

Determine the value of resistor in a collector to base bias circuit using the following specifications VCC = 8V, β = 110, VBE = 0.7V, ICQ = 9 mA and VCE = 4V.Calculate its stability factor.

Solution:

Apply KVL to the collector circuit

VCC = (IB+IC) RC + VCE

IB = IC/β = 9/110 = = 0.08 mA

RC = VCC-VCE / Ic+IB

RC = 8-44 / 9.08 × 10-3

RC = 440.5Ω

Vcc = (Ic+IB) Rc + IB RB + VBE

= (9.08 × 10-3 × 440.5) + (0.08 × 10-3 × RB) +0.7

RB = 8 - 0.7 - 9.08 × 10-3 × 440.5 / 0.08 × 10-3

RB = 41.3 kΩ

S = 51.17

Self Bias (or) Voltage Divider Bias

Most commonly used biasing is self bias (or) emitters bias. It is shown in fig.3.56. This is also known as universal bias. This method R1 and R2 acts as potential divider and are connected across the supply Vcc to provide proper biasing.

The emitter resistance RE provides stabilization. The net forward bias across the emitter junction is VBE = VB-VE

Let current I1 flow through R1 As the base current IB is very small, the current flowing through R2 can also be taken as I1. The calculation of collector current IC is as follows.

I1 = Vcc / (R1 + R2)

The voltage V2 developed across R2 is given by

V2 = Vcc R2 / R1 + R2

Apply KVL to the input circuit as shown below

Applying KVL to the input circuit

V2 = VBE + VE = VBE + IE RE

= VBE + IC RE (IE=IC)

IC = V2 - VBE / RE

Here IC is almost independent of transistor parameters β, IB, and ICO and hence good stabilisation is ensured. The collector emitter voltage VCE can be calculated as follows.

Apply KVL to output circuit

Vcc = Ic RC + VCE + IE RE

= Ic RC + VCE + IC RE

VCE = VCC - IC (RC + RB)

Stability factor S is given by

This expression shows that S = 1 if RB / RE <<1 i.e RC should be much greater than RB. But there is a limit for increasing RE because the high value of RE means increased voltage drop across emitter so that IB get reduced.