Input & Output Characteristics of an NPN Transistor

Written by david sandoval
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Input & Output Characteristics of an NPN Transistor
Transistor (lower right) in a circuit (Image by, courtesy of Windell Oskay)

A transistor is a semiconductor device that is used in many electronic applications. Transistors are primarily used in two ways: as a switch or as an amplifier. A particular type of transistor, the bipolar junction transistor, or BJT, is commonly used in analogue electronics due to its reliability and relative simplicity. One type of bipolar junction transistor, the NPN transistor, is commonly used by hobbyists either as a part of a switching circuit or in an amplifier circuit.

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NPN Transistor

An NPN transistor is a type of bipolar junction transistor. A bipolar junction transistor is created to have two P-N junctions, which are electrically active areas unique to transistors, diodes and other active components in the field of electronics. Electrical current can flow only one way (from the P-side of the P-N junction to the N-side) in an electrical circuit where a P-N junction exists, so a transistor will only allow current flow when there is a difference in electrical voltage that is higher at the P-end of the transistor. For example, if 1 volt is applied to the P-end of a P-N junction, and the N-end is connected to electrical ground (zero volts), electrical current will flow across the P-N junction.

An NPN transistor has three electrical leads: base (which is a P-end), emitter (which is an N-end) and collector (which is an N-end).

Input and Output Characteristics of an NPN Transistor in an Amplifier Circuit

Connecting an NPN transistor in an amplifier circuit will allow for an AC (alternating current) signal to be amplified. The amount of amplification will vary upon the resistance values of electrical resistors used in the circuit.

In a Class A amplification circuit, amplification is achieved by applying an AC signal to the base connection of a transistor. The amplified signal will be obtained by making an electrical connection at the collector connection.

Output Distortion When Used in an Amplifier Circuit

An NPN transistor can be used to amplify an AC signal; however, if a signal is amplified beyond the limits of the transistor's power rating, the transistor will "clip" the output at the limit of the power rating, thus causing distortion of signal. Continued overamplification of the input signal will cause premature failure of the NPN transistor.

Input and Output Characteristics of an NPN Transistor in a Switch Circuit

Active components such as transistors require a minimum voltage to allow electrical current to flow across the P-N junction. The typical minimum voltage to start the flow of current is 0.7 V applied to a P-N junction on a silicon-based transistor. Therefore, a transistor can be used as a switching mechanism.

In essence, when the voltage applied to the base connector on a transistor is 0.7 V or greater, the electrical circuit will operate in an "On" condition. When the voltage applied to the base connector falls below 0.7 V, the electrical circuit will turn "Off."

Power Input Characteristics of Different NPN Transistors

While silicon-based transistors are most commonly used, these are not the only type of transistor available. Two other types of NPN transistors are also available: gallium-arsenide transistors and germanium transistors.

Gallium-arsenide transistors require at least 1 V to start the flow of current through the transistor. Gallium-arsenide transistors have a very quick response time, so these types of transistors work well in switching circuits.

Germanium transistors are a common type of transistor and require approximately 0.3 V to start the flow of current through the transistor. While germanium transistors have a quicker response time than silicon transistors, germanium transistors must be operated at lower voltage and temperature settings than silicon transistors.

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