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).
2. **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.
3. **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:
- The base current (Ib) is determined by the supply voltage (Vcc) and the base resistor (Rb).
- Once the base current is set, the transistor enters its active region, allowing it to amplify signals.

### Advantages:
- Simple and easy to implement.
- Low component count.

### Disadvantages:
- **Thermal instability**: If the temperature changes, the transistor’s parameters can change, which can shift the operating point and cause the transistor to malfunction.
- **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:
- \( I_B \) is the base current
- \( V_{CC} \) is the supply voltage
- \( V_{BE} \) is the base-emitter voltage (typically around 0.7V for silicon BJTs)
- \( R_B \) 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.