2N5551 Transistor : Pin Configuration & Its Applications

2N5551 transistor is one kind of semiconductor device, used to switch or amplify electronic signals as well as electrical power. It is available with a minimum of three terminals which are used to give connection to external circuits. Once current or voltage is applied to one of the pair of terminals then it controls the flow of current in another terminals pair.


Generally, a transistor amplifies a signal when the output power is higher as compared to input power This article discusses an overview of a 2N5551 transistor, pinout, specifications, circuit working & its applications

What is a 2N5551 Transistor?

2N5551 NPN amplifier transistor is mainly designed to use in high voltage circuits for general purposes like switching and amplification. The voltage from collector terminal to emitter is 160V & the voltage from collector terminal to the base is 180V because of which it can be simply used in the circuit below 160 volts. The output load max handled for this transistor is 600mA & the maximum dissipation of the collector terminal is 625mW.

2N5551 Transistor
2N5551 Transistor

Pin Configuration

The pin configuration of the 2N5551 Transistor is shown below. This transistor includes three terminals which are discussed below.

2N5551 Transistor Pin Configuration
2N5551 Transistor Pin Configuration
  • Pin1 (Emitter): This terminal is generally connected to the GND terminal where the flow of current drains out throughout this terminal.
  • Pin2 (Base): This pin controls the transistor biasing, so this pin controls the transistor
  • Pin3 (Collector): This terminal is connected to load and current supplies throughout this terminal.

As compared to JFET, it is very different, so it is known as a voltage-controlled device. Once the voltage is applied at the base terminal of this transistor then the transistor will be activated by allowing the current to flow through it. Here, the flow of current works like a bridge in between collector & emitter terminals.

Features & Specifications

The features & specifications of the 2N5551 transistor include the following.

  • The process technology used in this is advanced
  • Less error voltage
  • Switching speed is very fast
  • Complete operation of the voltage
  • Handing capacity of power & current is high
  • It is an NPN amplifier transistor
  • DC gain is high like 80 whenever IC=10mA
  • Nonstop collector current or IC is 600mA
  • Voltage from collector to emitter is VCE = 160 V
  • Voltage from collector to the base is VCB = 180V
  • Voltage from the emitter to the base is VBE = 6V
  • Available in To-92 Package
  • 100MHz transition frequency
  • Collector current (Max) is 6A/600mA
  • Dissipation of collector terminal (Max) is 625 mW
  • DC gain min & max is 80 to 250
  • Operating & storage temperature must be (Max) -55 to +150 C

Complementary 2N5551 transistor is 2N5401 and equivalent 2N5551 transistors are 2N5551K, BC637, NTE194, BC639, 2N5833, BC487 & 2N5551G. The similar 2N5551 transistor is 2N5550.

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How to Run a 2N5551 Transistor Safely?

To obtain better performance from this 2N5551 transistor & also to run for a long time in an electronic circuit, it is advised not to use the voltage above 160V. Need to maintain 5V – 10V under from maximum ratings to be secure. Always utilize an appropriate base resistor for providing the necessary base current, do not control load over 600mA. The temperature range must be > -55 centigrade & < +150 centigrade.

How to use 2N5551 Transistor?

When the 2N5551 transistor is biased, then the current in the emitter terminal is equivalent to the sum of the remaining two terminals like base & collector. Here, the voltage supply at the base terminal should be positive as compared to the flow of current from the emitter (E) to collector (C).

The transistor characteristics are mainly determined by the gain of forwarding current which is known as Beta (β). Here ‘β’ is a ratio of the collector current & base current. This value is known as the amplification factor which will decide the value of amplified current.


Here, the ‘β’ value ranges from 20-1000 however it is available with a typical value like 200. The transistor’s current gain is one more essential factor known as alpha, denoted with ‘α’. The alpha factor is a ratio between the current at collector & emitter terminals. This value mainly ranges from 0.95 – 0.99 however in most cases, it is used like unity.

Circuit Diagram with 2N5551 NPN Transistor

Generally, the 2N5551 transistor is used widely for amplification. So here is the simple example circuit which shows an amplified sine wave o/p.

Here the magnitude of the input sine wave is amplified from 8mV to 50mV. In the following application circuit, both the resistors like R3 & R4 are used to form a potential divider to decide the VBE (Emitter -Base voltage). Here, the ‘R1’ resistor is the load resistor whereas the ‘R2’ resistor is the emitter resistor. The output signal’s amplification can be affected by changing the ‘RL’ value.

2N5551 Transistor Circuit
2N5551 Transistor Circuit

Generally in a transistor, when the current supplies throughout the base terminal can be amplified within the current supplying throughout the collector terminal. So, this amplification mainly depends on the hfe (amplification factor). This factor value is 80 that means the collector current (CC) will be changed by eighty times as compared to the base current.

Ic = β*Ib

The emitter current of this transistor-like ‘IE’ is almost equivalent to the collector current value because of the transistor action. The main difference between these two can be found through the ‘α ‘value. Generally, the collector current value can be given as

IE = IC + IB

Here, the output can be obtained beyond the collector terminal which is the VCE (Collector-Emitter). The o/p voltage mainly depends on the Vin = 12V without any voltage drop across the R1. Thus the o/p voltage like Vout can be expressed as

Vout = VCE = (Vcc – IcRc)

Where to use 2N5551 Transistor?/Applications

The applications of the 2N5551 transistor include the following.

  • This transistor is used in the circuits which operate with high voltage to achieve general-purpose applications.
  • It is mainly used for audio amplification so that you can utilize it to increase audio otherwise other electronic signals. In addition, this kind of transistor is used where the general-purpose transistors are used such as 2N3904/ BC547 to drive LEDs, ICs, controlling electronic circuits, etc.
  • Darlington pair
  • To amplify low power signals
  • Audio frequency-based driver stages
  • Audio amplifiers with low power
  • For small loads below 600mA
  • Amplifiers like low power, current amplifiers, audio or another signal, small signal based boosters
  • This transistor includes high breakdown voltage, so used within display drivers with gas discharge.
  • The feature of this transistor-like amplification will make it to use by different professionals.

Thus, this is all about an overview of the 2N5551 transistor datasheet which includes its pin configuration, specifications, features, circuit, and applications. This NPN transistor is used for switching and amplifying applications. It is mainly designed for low power, current, medium voltage & can work fairly at high speeds. Here is a question for you, what are the different types of amplifier transistors available in the market?

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