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npn Bipolar Transistors

General Introduction

Transistors are devices that are used to drive output transducers. They deal with electrical power in response to electrical signals.

The above is a fairly concise description of what transistors do. By considering a systems approach to electronics, processes such as counters and logic circuits often need to drive larger output devices such as bulbs, motors and heaters etc. The devices used to perform the various processes are often small low power ICs that can only source or sink relatively small currents (several mA) and work at fairly low voltages. The output transducers may, on the other hand, require larger currents (several Amps) and work at higher voltages. Something needs to allow the process devices to work with the output transducers and this is where transistors are used.

Transistors are devices that require only small input currents or voltages but can handle large output currents.

There are two distinct types of transistor, the bipolar transistor and the MOSFET, and for each type of transistor there are two varieties. Bipolar transistors can be either npn type or pnp type. Bipolar transistors are devices that are operated by current. MOSFETs are operated by voltages and can be either n-channel or p-channel MOSFETs. As transducer drivers, MOSFETs tend to be better at handling high currents than bipolar transistors but bipolar transistors are better suited to low voltage circuits.


imageAn npn bipolar transistor is an active circuit component with three legs and usually made from silicon. The simple npn bipolar transistor can be used to construct all of the logic gates, and by extension all digital circuits, and also op-amps and all manner of analogue circuits. The npn bipolar transistor is therefore a fundamental building block of modern electronics.

The npn bipolar transistor can also be used as a straight forward electronic switch. Used as a switch, or transducer driver, the device can be used to control powerful devices and switch large currents. A basic electronics course only considers this simple application as a transducer driver.

npn Bipolar Transistor Basics

symbolAn npn bipolar transistor has three terminals or, in other words, three legs.

The are called the Base,Collector and Emitter. The emitter is identified by the arrow pointing outwards.

Basic action: When a small current flows into the Base, a much larger current is allowed to flow from the Collector to the Emitter. To enable a Base current to flow, the Base potential must be 0.7V higher than the Emitter potential.

The bipolar transistor is a current operated device.

Types of npn bipolar transistor

Bipolar transistor come in different shapes and sizes depending on their intended use.

bipolar transistorSignal transistors are used for signal processing, logic operations and voltage amplifiers. They come in a small metal canister style package (e.g. BC108) or a small plastic package (e.g. BC456). On the canister type, the tag on the case marks the emitter and the metal case is connected to the collector.

bipolar transistorMedium sized transistors also come as a slightly larger metal can type or as a TO-220 style plastic type with a metal mounting tag. Examples include 2N3053, BC441 and TIP41B. Again, the emitter is marked by a tag on the metal case and the metal case and metal heatsink tab are connected to the collector. These are used as transducer drivers and low power amplifiers and are capable of handling currents of 1 or 2 Amps.

bipolar transistorPower transistors can handle large currents and are used in high power amplifiers and high current regulators. Examples of power transistors include the well known 2N3055.

Used as a Transducer Driver

bipolar circuitThe circuit shows an npn bipolar transistor connected as a transducer driver.

When the button is pressed the Base voltage is greater than 0.7V and so current flows through the Base resistor and into the Base. The resistor is necessary to allow the Base-Emitter voltage to remain at 0.7V. As a small current is flowing into the Base, a larger current is allowed to flow from the power rail, through the bulb, in to the Collector and out through the Emitter to 0V. The bulb is ON.

With the button not pressed, no current flows in to the Base and no current flows through the Collector to the Emitter. The bulb is OFF. When there is no Base current, the resistance between the Collector and Emitter is very high. When current flows in to the Base, the Collector-Emitter resistance drops and allows current to flow.

bipolar circuitWhen an npn bipolar transistor is used to control a motor, solenoid, relay or any other device containing an electromagnet a protection diode is necessary to protect the transistor from the large back e.m.f. produced when the device is switched off.

Note: In all cases a low power circuit (such as a logic gate) controls the transistor and the transistor controls the high power device (such as the bulb or motor)

Base-Emitter Voltage

bipolar circuitTo turn the transistor on, the potential difference between the Base and Emitter must be 0.7V. Therefore, it makes sense to always connect the Emitter directly to 0V so that any Base voltage above 0.7V applied to the Base resistor will turn the transistor on. This is how the npn bipolar transistor is always connected when used as a transducer driver.

Transistor Current Gain

bipolar circuitThe npn bipolar transistor can be thought of a current amplifier. A small Base current is amplified to give a large Collector current. The Current Gain (hFE) is given by the ratio of the Collector current to the Base current.

hFE = Ic / Ib

The current gain of a typical signal transistor is between 100 and 200 and is less for a power transistor. Current gain can be measured directly with a suitable multimeter.

Calculating the value of the Base Resistor

example circuitThe Base resistor is important. Consider the example in the diagram.

Step 1. The power rating of the bulb tells us the required Collector current is 2A.

Step 2. The transistor current gain is x100 therefore the necessary Base current is 2 ÷ 100 = 20mA

Step 3. The input voltage is 5V and the Base-Emitter voltage is 0.7V, therefore, the potetial difference across the Base resistor is 5.0 − 0.7 = 4.3V

Step 4. The resistor equations gives RBase = 4.3 ÷ 0.02 = 215Ω

Step 5. To ensure the transistor is fully turned on, use the next lowest E24 series resistor.

Step 6. Choose RBase = 210Ω

End Note

Configured as a transducer driver, this is the most basic application of an npn bipolar transistor. Although it is acceptable to think of the transistor as a simple electronic switch in this case, there are many more complex applications where this simplistic approach is not appropriate.