Master the Basics: How To Use a BJT As An Amplifier

While BJTs (Bipolar Junction Transistors) are commonly used as switches, they are also fundamental to building amplifier circuits. Amplifiers increase the power of a signal by drawing energy from a power supply and using it to boost the input signal's voltage or current.

What Is a Transistor Amplifier?

A BJT amplifier draws power from a separate supply and uses it to make an input signal stronger. The transistor operates in its active region, where small changes at the base terminal cause proportional, larger changes at the collector-emitter current path.

There are three basic amplifier configurations:

• Common Collector (Emitter Follower)

• Common Base

• Common Emitter

Each configuration has unique voltage and current gain, input/output impedance, and signal inversion behavior.

Common Amplifier Configurations

Common Collector (Emitter Follower)

• Input at base, output from emitter, collector tied to power
• Voltage gain ≈ 1, high current gain, high input impedance, low output impedance
• Used for voltage buffering

Common Base

• Input at emitter, output from collector, base at a fixed voltage
• High voltage gain, low input impedance, high output impedance
• Useful for current buffering

Common Emitter

• Input at base, output from collector, emitter tied to ground
• Provides voltage and current gain, but inverts the signal
• Most commonly used configuration for voltage amplification

Building a Common Emitter Amplifier

To build a basic common emitter amplifier, you’ll need:

• A BJT (e.g., 2N3904)
• Resistors for biasing (e.g., 100kΩ and 10kΩ)
• A 12V DC power supply
• Coupling capacitors to isolate AC signals

Step-by-step setup:

1. Use a voltage divider (100kΩ + 10kΩ) to bias the base to ~1.1V
2. Connect a 1kΩ resistor to the emitter, dropping ~0.4V → emitter current = 0.4 mA
3. Connect a 10kΩ resistor to the collector → voltage drop = 4V
4. With 12V supply, the collector sits at 8V (quiescent or idle state)

Add AC input via a coupling capacitor to block DC bias. Add another capacitor on the output to block the DC offset.

Amplifier Behavior with Input Signal

• When input increases by +100mV, collector current increases, voltage drop across collector resistor increases, output voltage drops
• When input drops by –100mV, collector current decreases, voltage drop decreases, output voltage rises

This inversion creates a voltage gain of -10 in the example.

Key Concepts and Components

• Biasing keeps the transistor in the active region
• Coupling capacitors allow AC to pass while blocking DC
• Gain = Output Voltage / Input Voltage

Key Takeways

BJTs are the building blocks of analog amplification. While op-amps may be easier to use in many cases, understanding how common emitter, collector, and base configurations work will help you build a solid foundation in electronics. With careful biasing and component selection, you can create powerful analog amplifiers for a wide range of signals.