Brief Introduction and Guide for 2N5551 NPN Amplifier Transistor

/ 2018-09-21 / News

The 2N5551 is factory-made in a TO-92 plastic pack. When looking at the flat aspect with the leads pointing downwards, the three leads popping out of the junction transistor are, from left to right, the leads of the emitter, the bottom and therefore the collector. 2N5551C (suffix "C") junction transistor with central collector.


A transistor is essentially a pair of diodes that share one end. The shared end referred to as the bottom and therefore the other two ends are called the emitter and collector.

The collector accepts an input current from the circuit, however it cannot send the current through the junction transistor till allowed to by the bottom.

The emitter sends a current into the circuit, but only if the base allows the collector to pass the current through the transistor to the emitter.

The base acts as a door. When a small current is applied to the base, the grid opens and a large current can flow from the collector to the transmitter.


Manufacturer: Central Semiconductor 

Product Type: Bipolar junction Transistors - BJT 

Mounting Style: Through Hole 

Package / Case: TO-92 

Transistor Polarity: NPN 

Configuration: Single 

Collector- Emitter Voltage VCEO Max: 160 V 

Collector- Base Voltage VCBO: 180 V 

Emitter- Base Voltage VEBO: 6 V 

Collector-Emitter Saturation Voltage: 0.2 V 

Maximum DC Collector Current: 0.6 A 

Gain Bandwidth Product fT: 300 MHz 

Minimum Operating Temperature: - 65 C 

Maximum Operating Temperature: + 150 C 

Continuous Collector Current: 0.6 A  

DC Collector/Base Gain hfe Min: 80  

Pd - Power Dissipation: 625 mW  

Series: 2N5551

2N5551 Pinout configuration

Pin NumberPin NameDescription
1EmitterCurrent Drains out through emitter,   normally connected to ground
2BaseControls the biasing of transistor, Used   to turn ON or OFF the transistor
3CollectorCurrent flows in through collector,   normally connected to load

2n5551 Pinout


Low power amplifiers

Current amplifiers

Small signal boosters

Audio or other signal amplifiers

Darlington pair

What areas to utilize 2N5551?

The 2N5551 is an NPN amplifier Transistor with an amplification factor (hfe) of 80 when the collector current is 10mA. It also has decent switching characteristics (Transition frequency is 100MHz) hence can amplify low-level signals.

Due to this feature, the transistor is commonly used for amplification of audio or other low power signals. So if you are looking for an NPN transistor for you amplifier circuit then this transistor might be the right choice.

The method to make use of 2N5551

As told earlier the 2N5551 NPN transistor is widely used for amplification. A very simple bare minimum circuit for a transistor to work as an amplifier is shown below. The simulation graph that shows the amplified output sine wave can also be found.


Here the input sine wave of magnitude 8mV (yellow colour) is amplified to 50mV (Pink colour) as shown in the graph.  In the above circuit the resistors R3 and R4 form a potential divider which decides the Emitter -Base voltage (VBE). The Resistor R1 is the load resistor and the resistor R2 is the emitter resistor. Changing the value of RL will affect the amplification of the output wave.

A transistor is normally a current amplifier, meaning the current flowing though the base will be amplified in the current flowing through the collector. This amplification depends on the amplification factor (hfe) which is 80 for 2N5551. This means that the collector current will be amplified by 80 times than that of the base current. Ic = βIb

Another current that we have bring into consideration is the emitter current (IE), but due to transistor action we assume that Emitter current is almost equal to the value of Collector current, however the difference between the both can be found with the value of α. Normally the value of collector current will e given by IE = IC + IB

The output is obtained across the collector that is the Collector-Emitter voltage (VCE). This output voltage depends on the Input voltage (Vcc, here 12V) while not the voltage drop across the loads electrical device (R1). so the output voltage Vout will be given as Vout = VCE = (Vcc – IcRc)

Alternative NPN Transistors:

BC549, BC636, BC639, BC547, 2N2369, 2N3055, 2N3904, 2N3906, 2SC5200

2N5551 Equivalent Transistors: 

NTE194, 2N5833, 2N5088, 2N3055, 2N5401 (PNP)

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