Fixed Bias (Base Resistor Bias)

Fixed Bias (Base Resistor Bias)

The Figure shows the fixed bias circuit. It is the simplest d.c. bias configuration. For the d.c. analysis we can replace capacitor with an open circuit because the reactance of a capacitor for d.c. is

In the base circuit,

Apply KVL, we get

V_CC = I_BR_B + V_BE

Therefore,

I_B = (V_CC - V_BE)/R_B

For a given transistor, V_BE does not vary significantly during use. As V_CC is of fixed value, on selection of R_B, the base current I_B is fixed. Therefore this type is called fixed bias type of circuit.

In the Collector circuit

Apply KVL, we get

V_CC = I_CR_C + V_CE

Therefore,

V_CE = V_CC - I_CR_C

The common-emitter current gain of a transistor is an important parameter in circuit design, and is specified on the data sheet for a particular transistor. It is denoted as β.

I_C = βI_B

In this circuit V_E =0

Stability factor S for Fixed bias circuit

Stability factor S

Merits:

·        It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (R_B).

·        A very small number of components are required.

Demerits:

·        The collector current does not remain constant with variation in temperature or power supply voltage. Therefore the operating point is unstable.

·        Changes in V_be will change I_B and thus cause R_E 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.

·        For small-signal transistors (e.g., not power transistors) with relatively high values of β (i.e., between 100 and 200), this configuration will be prone to thermal runaway. In particular, the stability factor, which is a measure of the change in collector current with changes in reverse saturation current, is approximately β+1. To ensure absolute stability of the amplifier, a stability factor of less than 25 is preferred, and so small-signal transistors have large stability factors.

Usage:

Due to the above inherent drawbacks, fixed bias is rarely used in linear circuits (i.e., 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.

Problems

1. Design the fixed bias circuit from the load line given in the figure.

2. For the circuit shown in figure. Calculate I_B,I_C,V_CE,V_B,V_C and V_BC. Assume V_BE= 0.7V and β=50.

 3. Design a fixed biased circuit using a silicon transistor having β value of 100. Vcc is 10 V and dc bias conditions are to be V_CE = 5 V and I_C = 5 mA,

Solution

Applying KVL to collector circuit,

Applying KVL to base circuit,

4. Calculate the operating point (Q-point)

Base biased CE connection

I_C = β_dc * I_B = 100 * 29µA = 2.9 mA

V_CE = V_CC - (I_C * R_C) = 15V - (2.9 mA * 3KΩ) = 6.3V

By plotting I_C (2.9 mA) and V_CE (6.3V), we get the operation point ----> Q-point (quiescent point).

Collector curve with load line and Q – point 

5. Draw the load line and Q-point.

base biased CE connection, β=50

Solution:

I_C = I_B * β = 2.15 mA

V_CE = V_CC - (R_C * I_C)= 5.7V

`                                     V_{CE (cut)} = V_CC = 3.0V