Understanding How to Use Bipolar Junction Transistors (BJTs) as Switches

Employing a Bipolar Junction Transistor (BJT) as a switch is a common practice in electronic circuits. A BJT operates in two primary regions: the cutoff region (off state) and the saturation region (on state). This article provides a detailed guide on how to use a BJT as a switch, covering the basics, circuit configurations, and key points to ensure optimal performance.

Understanding BJT Basics

Types of BJTs

The two main types of BJTs are NPN and PNP. This guide will focus on the NPN type, but the principles apply similarly to PNP.

Regions of Operation

BJTs operate in two primary regions:

Cutoff Region (Off State)

In the cutoff region, the transistor is off, and no current flows from the collector to the emitter.

saturation Region (On State)

In the saturation region, the transistor is on, and maximum current flows from the collector to the emitter.

Circuit Configuration

Common Emitter Configuration

This is the most commonly used configuration for a BJT as a switch. The basic components include:

Transistor: NPN type Load Resistor R_load: The device you want to control (e.g., a lamp) Base Resistor R_B: Limits current to the base of the transistor Power Supply V_CC: Provides power to the load

Turning the BJT On: Saturation

Apply Base Current

To turn the BJT on, a small current must flow into the base. This base current, denoted as I_B, controls a larger current flowing from the collector to the emitter, denoted as I_C.

Condition for Saturation

The BJT is in saturation when:

I_C ≈ β · I_B

Where β is the current gain of the transistor. To ensure saturation, the base current should be designed to be sufficient:

I_B I_C / β

Base Resistor Calculation

The base resistor can be calculated using Ohm’s Law:

R_B (V_{BB} - V_{BE}) / I_B

Where V_{BB} is the base voltage and V_{BE} is typically around 0.7V for silicon BJTs.

Turning the BJT Off: Cutoff

To turn the BJT off, ensure that no current flows into the base (I_B 0). This can be achieved by:

Disconnecting the base from the voltage source. Pulling the base low.

Example Circuit

Here is a simple example circuit using an NPN transistor as a switch:

Where:

V_CC is the positive terminal of the power supply. R_load is the load, such as an LED. R_B is the base resistor. C is the collector of the transistor. B is the base of the transistor. RB is the base resistor. E is the emitter of the transistor. GND is the ground.

Key Points

Saturation Voltage

When the BJT is in saturation, the voltage drop across the collector-emitter (VCE) is very low, typically around 0.2V.

Switching Speed

BJTs can switch on and off quickly. However, for high-speed applications, consider using a MOSFET instead.

Heat Dissipation

Ensure the transistor is rated for the load current and includes proper heat sinking if necessary.

Conclusion

Using a BJT as a switch is straightforward once you understand its operating regions and how to control the base current. This method is widely used in electronic circuits to control devices like motors, lights, and other loads.