Fixed Bias

Types of bias circuit for class-A amplifiersThe following discussion treats five common biasing circuits used with class-A bipolar transistor amplifiers:

1. Fixed bias

1. Collector-to-base bias

1. Fixed bias with emitter resistor

1. Voltage divider bias or potential divider

1. Emitter bias

Fixed bias (base bias)Fixed bias (Base bias)
This form of biasing is also called base bias. In the example image on the right, the single power source (for example, a battery) is used for both collector and base of a transistor, although separate batteries can also be used.
In the given circuit,
V_cc = I_BR_B + V_beTherefore,
I_B = (V_cc − V_be)/R_BFor 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.
Also for given circuit,
V_cc = I_CR_C + V_ceTherefore,
V_ce = V_cc − I_CR_CThe 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 on this page.
Because
I_C = βI_Bwe can obtain I_C as well. In this manner, operating point given as (V_ce,I_C) can be set for given transistor.
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 I_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.