Question

What is fixed bias in BJT?

Answer 1

Fixed bias in a BJT (Bipolar Junction Transistor) refers to a method of biasing the transistor where a fixed external voltage is applied to the base of the transistor through a resistor. This method sets the operating point (also called the Q-point) of the transistor.

In a fixed bias configuration:

1. Base Resistor (Rb): A resistor is placed between the base of the transistor and the supply voltage (Vcc). This resistor limits the base current (Ib).
   

1. Emitter: The emitter is typically connected to ground (0V) in this configuration, although it could also be connected to a small voltage for certain designs.
   

1. Collector Resistor (Rc): There is often a resistor in the collector circuit, but it's not directly involved in the biasing itself—it's for controlling the output current.
   

Working of Fixed Bias:

1. The base current (Ib) is determined by the supply voltage (Vcc) and the base resistor (Rb).
   

1. Once the base current is set, the transistor enters its active region, allowing it to amplify signals.
   

Advantages:

1. Simple and easy to implement.
   

1. Low component count.
   

Disadvantages:

1. Thermal instability: If the temperature changes, the transistor’s parameters can change, which can shift the operating point and cause the transistor to malfunction.
   

1. Poor performance: The fixed bias doesn't provide much control over the Q-point, leading to less stability in the transistor’s operation.
   

Formula:

The base current in the fixed bias circuit can be calculated by:
\[
I_B = \frac{V_{CC} - V_{BE}}{R_B}
\]
Where:

1. \( I_B \) is the base current
   

1. \( V_{CC} \) is the supply voltage
   

1. \( V_{BE} \) is the base-emitter voltage (typically around 0.7V for silicon BJTs)
   

1. \( R_B \) is the base resistor
   

In short, fixed bias is a simple way to bias a transistor, but it’s not the most stable method for many applications.