Bipolar Junction Transistor (BJT)

What is a BJT?

A BJT (Bipolar Junction Transistor) is a three-terminal semiconductor device used for amplification and switching.

The three terminals are:

• Base (B)
• Collector (C)
• Emitter (E)

The two main types of BJTs are NPN and PNP.

NPN BJT

In an NPN transistor, a small base current controls a much larger collector current.

Basic BJT Current Relationship

I_E = I_C + I_B

The collector current is approximately related to the base current by:

I_C = β I_B

Here, β (beta) is the DC current gain of the transistor.

Example:

If β = 100 and I_B = 20 μA:

I_C = 100 × 20 μA = 2 mA

How Amplification Works

The input signal is applied to the base. A small change in base current produces a larger change in collector current.

I_C = β I_B

The collector resistor converts this current change into a voltage change:

V_OUT = V_CC − I_CR_C

Therefore:

Input increases → Base current increases → Collector current increases → Output voltage decreases.

Thus, the common-emitter amplifier produces an output voltage that is approximately 180° out of phase with the input.

Remember that a small change in the base current can produce a much larger change in the output voltage. This is the fundamental principle behind amplification.

Summary

• A BJT has three terminals: base, collector, and emitter.
• BJTs are available as NPN and PNP types.
• A BJT can be used for amplification and switching.
• In a BJT, collector current is controlled by base current.
• The common-emitter configuration provides current, voltage, and power gain.
• In a common-emitter amplifier, the output voltage is inverted relative to the input.