2N5551: Pin configuration, usage and precautions

By Central Semiconductor 1114

2N5551: Pin configuration, usage and precautions

The 2N5551 transistor is a semiconductor device used to switch or amplify electronic signals and power. It has at least three terminals for connecting to external circuits. Once current or voltage is applied to one of a pair of terminals, it controls the current in the other pair of terminals.

The 2N5551 is a bipolar transistor commonly used in low to medium power amplification and switching circuits. It is an NPN polarity transistor suitable for general amplification and switching applications.

In circuits, 2N5551 can be used as a key component of an amplifier to amplify the current, voltage or power of a signal. It is often used in audio amplifiers, radio frequency amplifiers and other low-power amplifiers. In addition, due to its high switching speed and small saturation voltage drop, it can also be used as a switching tube in switching circuits to control the flow of larger current or power. It is often used in switching power supplies, relay drive circuits, etc. . At the same time, it can also be used as an exciter to amplify smaller input signals to a sufficient level to drive other devices or circuits.

Ⅰ.Specification parameters of 2N5551

•Number of pins:3
•Frequency:100 MHz
•Transistor polarity:NPN
•Installation style:Through Hole
•Pd-Power dissipation:625 mW
•Minimum operating temperature:-55℃
•Maximum operating temperature:+150℃
•Package:Ammo Pack
•Power-Maximum:625 mW
•Gain bandwidth product fT:100 MHz
•DC current gain hFE maximum:80
•Emitter-base voltage VEBO:6 V
•Collector-emitter saturation voltage:200 mV
•Collector-base voltage VCBO:180 V
•Maximum DC collector current:600 mA
•Maximum collector-emitter voltage VCEO:160 V
•Product Category:Bipolar Transistor - Bipolar Junction Transistor (BJT)
•Product type:BJTs-Bipolar Transistors
•Current-collector cutoff (max):50nA (ICBO)
•Vce saturation voltage drop (maximum value) at different Ib and Ic:200mV @ 5mA, 50mA
•DC current gain (hFE) (minimum value) at different Ic, Vce:80 @ 10mA, 5V


Ⅱ.Features of 2N5551

1.High amplification: 2N5551 has a high current amplification factor (hFE), which means that it can effectively amplify the small current of the base into a large current of the collector. This property makes it very useful in amplifier circuits.

2.High reliability: 2N5551 has undergone strict quality control and testing to ensure it has high reliability and long life. This makes it one of the transistors of choice in many applications.

3.High voltage and low current design: 2N5551 is designed to operate at higher collector-emitter voltage (VCEO) while maintaining low collector current (IC). This design makes it useful in a variety of applications requiring high voltage and low current amplification.

4.Small package: 2N5551 is usually packaged in TO-92, which is a miniaturized package that takes up less space on the circuit board and is conducive to more compact circuit design.

5.High switching speed: Due to the internal structure and material design of 2N5551, it has fast switching speed. This makes it ideal for use in switching circuits and logic circuits that require fast response.

6.Simple driving: Since the base current of 2N5551 is relatively small, it can be driven with a smaller current or voltage. This makes it useful in applications requiring low power consumption or low drive voltage.

7.Low saturation voltage drop: In the saturated state, the collector-emitter voltage (VCE) of 2N5551 is relatively low. This helps reduce power consumption and improve the efficiency of the circuit.

8.Wide temperature range: 2N5551 can work normally in a wide temperature range, with certain stability from low temperature to high temperature. This allows it to work reliably in a variety of environmental conditions.

Ⅲ.Working principle of 2N5551

•The 2N5551 transistor consists of three regions: Emitter, Base and Collector.
•These three areas are isolated from each other by PN junctions, forming two PN junctions: emitter junction (Emitter-Base) and collector junction (Collector-Base).

2.Current control:
The current amplification effect of the 2N5551 transistor is achieved by controlling the base current to the collector current. Specifically, when the base current changes, the collector current will also change accordingly, but the magnitude of the change is much greater than the base current. This characteristic allows the transistor to control a larger current with a smaller current.

3.Working status:

•Off state: When no current flows into the base (Base), both the emitter junction and the collector junction are in a reverse biased state. At this time, no current flows through the transistor and it is in a high impedance state, which is called the cut-off state.

•Saturation state: When the base current increases to a certain level, both the emitter junction and the collector junction are in a forward biased state, and the transistor enters a saturation state. At this time, the collector current no longer increases with the increase of the base current, but tends to a stable value. In the saturated state, the internal resistance of the transistor is very low and can be regarded as a switch.

•Amplification state: When a small forward voltage (usually less than 0.7V) is applied to the base (Base), causing the emitter junction to be forward biased and the collector junction to be reverse biased, the transistor enters the amplification state. At this time, the electrons emitted from the emitter will be attracted by the electric field of the base and flow to the collector under the action of the electric field of the collector, forming a collector current. Since the base current is very small and the collector current can be very large, the transistor has the function of current amplification.

Since the 2N5551 transistor has current amplification and switching functions, it is widely used in various electronic devices, such as amplifiers, switching circuits, oscillators, etc. In audio amplifiers, the 2N5551 transistor can be used to amplify audio signals; in switching circuits, it can be used to control the on and off of the circuit; in oscillators, it can be used to generate stable oscillation signals.

Ⅳ.Maximum Ratings of 2N5551


V.2N5551 pin configuration

•Pin1(emitter): Current is discharged through the emitter, usually grounded
•Pin2(Based): Controls the bias of the transistor, used to turn the transistor on or off
•Pin3(Collector): Current flows through the collector, usually connected to the load


Ⅵ.2N5551 application fields

1.Signal processing: In signal processing circuits, 2N5551 can be used to amplify, filter, modulate or demodulate signals to meet the needs of specific applications.

•Cooperating with filter circuits: 2N5551 can be combined with resistors, capacitors and other components to construct various types of filter circuits. For example, by combining transistors with RC (resistor-capacitor) networks, low-pass, high-pass, band-pass or band-stop filters can be implemented to remove noise from a signal, separate different frequency components or adjust the spectrum of a signal.

•Signal amplification: One of the basic uses of the 2N5551 transistor is to amplify signals. By providing a small signal to the base, an amplified signal is obtained at the collector.

•Signal conversion: 2N5551 transistor can also be used to realize certain conversion functions of signals, such as voltage-current conversion, current-voltage conversion, etc. These transformations are common in signal processing and help adapt to the signal form requirements of different circuits or devices.

•Modulation and demodulation: In the modulation and demodulation process, 2N5551 may not be a direct core component, but it can participate in the implementation of the circuit as an amplification stage or switching stage. For example, in an amplitude modulation (AM) modulation circuit, transistors may be used to amplify the carrier signal and the modulating signal; during demodulation, transistors may be used to detect information in the modulated signal.

•Combination circuit: In more complex signal processing systems, 2N5551 can be used in combination with other types of transistors, operational amplifiers, digital logic circuits, etc. to achieve more complex signal processing functions, such as signal detection, signal encoding, signal decoding, etc. .

2.Amplifier: 2N5551 can be used as a key component of an amplifier to amplify the current, voltage or power of a signal. It is commonly used in audio amplifiers, radio frequency amplifiers and other low power amplifiers to enhance the amplitude and strength of signals.

•RF Amplifier: In wireless communications and radio frequency (RF) applications, the 2N5551 can also be used as part of an RF amplifier. It can amplify radio frequency signals within a specific frequency band to enhance the transmission distance of the signal or improve the sensitivity of the receiver. However, in RF applications, more factors may need to be considered, such as frequency response, noise performance, and matching networks.

•Audio amplifier: 2N5551 transistor can be used in the design of audio amplifiers, especially low-power audio amplifier circuits. By configuring it as a common-emitter amplifier, it can receive weak audio signals from sources (such as microphones, CD players, etc.) and amplify them to a level high enough to drive speakers or headphones.

3.Power management: In the power management system, 2N5551 can be used to control the power switch, adjust the output voltage or current, etc. Specifically, 2N5551 can be used as a switching tube to control the power on and off. When the appropriate voltage is applied to the base, the transistor enters a conducting state, allowing current to flow from the collector to the emitter, turning the power on. Conversely, when the base voltage is reduced or removed, the transistor enters a cut-off state, cutting off the flow of current and turning off the power supply.

4.Control circuit: 2N5551 can be used in various control circuits, such as temperature control, voltage regulation, etc.

•In temperature control circuits, 2N5551 can be used with temperature sensors. When the temperature changes, the temperature sensor will produce corresponding voltage or current changes. These changes are amplified and processed by the 2N5551, thereby driving the control circuit to make corresponding adjustments, such as controlling the switch of heating or refrigeration equipment.

•In the voltage regulation circuit, 2N5551 can be used as a regulating tube. By detecting the feedback signal of the output voltage and comparing it with the reference voltage, an error signal can be obtained. After this error signal is amplified and processed by 2N5551, it can drive the control circuit to adjust the output voltage so that it remains stable at the set value.

5.Driver: In the drive circuit, 2N5551 can be used to drive various loads, such as relays, motors, LEDs, etc.

•Relay driver: When using the 2N5551 to drive a relay, it is usually configured as a switching circuit. By controlling the base current of 2N5551, the current between the collector and emitter can be controlled, thereby controlling the coil current of the relay. When the base current is large enough, the collector and emitter are connected, the relay coil is energized, and the relay contacts are closed or opened to control the circuit.

•Motor drive: For small motors, such as DC motors or stepper motors, the 2N5551 can be used as a drive switch. Through PWM (Pulse Width Modulation) or other control signals, the base current of the 2N5551 can be adjusted to control the speed and direction of the motor. It should be noted that for larger power motors, additional drive circuits and power amplifiers may be required to provide sufficient current and voltage.

•LED driver: In the LED driver circuit, 2N5551 can be directly used to drive low-current LEDs. By adjusting the base current, the brightness of the LED can be controlled. However, for LEDs with high brightness or that require precise brightness control, it may be necessary to use a specialized LED drive circuit or constant current source to ensure the stable operation of the LED.

6.Forward bias component: In some circuits, 2N5551 can be used as a forward bias component, connecting the base to the positive power supply to ensure stable operation. Forward biasing sets the voltage between the base and emitter of a transistor to a positive value, causing the transistor to conduct. In the case of the 2N5551, this is usually accomplished by connecting the base to the positive supply through a resistor. The resistance of the resistor depends on the specific application and the required bias current.

7.Switching circuit: Since 2N5551 has a high switching speed and a small saturation voltage drop, it can be used as a switching tube and used in switching circuits. It can control the flow of larger current or power and is often used in switching power supplies, relay drive circuits, etc. In switching power supplies, 2N5551 is used as a key switching element, responsible for turning on or off the power supply at a specific point in time, thereby regulating the output voltage and current. In the relay drive circuit, 2N5551 can receive smaller input signals and drive the relay through its amplification and switching functions to control larger power loads.

8.Exciter: The 2N5551 can be used as an exciter to amplify smaller input signals to a sufficient level to drive other devices or circuits. In the circuit, 2N5551, as an NPN silicon transistor, has current amplification characteristics. When a small input signal is applied to the base of the 2N5551, it produces an amplified output signal between the collector and emitter. This amplified signal can be used to drive other electronic devices or circuits that require larger signals to work.


Ⅶ.2N5551 usage and precautions


1.Pin connection: 2N5551 has three pins, namely emitter (E), base (B) and collector (C). When making connections, you need to ensure that these pins are connected correctly to the corresponding parts of the circuit.
Emitter (E): Usually connected to the negative supply (GND) or ground of the circuit.
Collector (C): A load or other part connected to a circuit for outputting an amplified signal.
Base (B): used to receive input signals and control the on and off of the transistor by controlling the base current.

2.Signal amplification: One of the main functions of 2N5551 is to amplify the input signal. When the base receives an input signal, the transistor amplifies it and outputs it to the collector. The amplification factor depends on the characteristics of the transistor and the design of the circuit.

3.Bias circuit: In order for the 2N5551 to work properly, it needs to be provided with a suitable bias circuit. The function of the bias circuit is to provide a stable DC voltage or current to the base of the transistor to ensure that the transistor operates at the appropriate operating point.

4.Switching application: 2N5551 can also be used as a switching tube. By controlling the voltage or current at the base, the transistor can be switched between the on and off states, thereby controlling the circuit on and off.


1.To avoid damaging the transistor, avoid working at too high or too low temperatures.

2.When designing the circuit, make sure to provide appropriate heat dissipation measures for the transistors to prevent overheating.

3.When using 2N5551, you need to pay attention to its maximum operating voltage, maximum collector current, maximum power consumption and other parameters to ensure that it does not exceed its operating range.

Ⅷ.How to properly bias the 2N5551 transistor to ensure its optimal performance

1.Understand the transistor characteristics: First, you need to be familiar with the technical parameters of 2N5551, such as maximum drain voltage (BVCEO), maximum collector current (IC), maximum power dissipation (PD), etc. These parameters will determine the voltage and current range you can safely apply.

2.Select the appropriate bias circuit:

•Current Source Biasing: A current source is used to drive the base of a transistor. This approach can provide very stable biasing, but can be more complex to implement.

•Fixed Bias: Connect the base of the transistor to the positive supply using a resistor to provide a stable DC voltage to the base. This method is simple and easy to implement, but may not be stable enough when temperature changes.

•Voltage-divider bias: Two resistor voltage dividers are used to provide the base voltage. This approach provides more stable biasing because it partially compensates for the effects of temperature changes on transistor parameters.

3.Calculate the bias resistor value: Calculate the required bias resistor value based on the parameters of the selected bias circuit and transistor. Make sure these resistor values ​​are within acceptable limits, taking into account temperature coefficients and tolerances.

4.Connect the circuit: Connect the transistor and resistor to the circuit according to the calculated resistance value and bias circuit diagram. Make sure the connections are correct and there are no shorts or breaks.

5.Test and adjust: Use appropriate test equipment (such as multimeter, oscilloscope, etc.) to check the performance of the circuit. Observe whether the collector current, voltage gain and other parameters of the transistor are as expected. If necessary, the bias resistor values ​​can be fine-tuned to optimize performance.

6.Temperature stability: Pay attention to the impact of temperature changes on transistor performance. If the circuit needs to operate over a wide temperature range, consider using a temperature compensation circuit to stabilize the bias.

7.Protection circuit: Add appropriate protection components (such as current-limiting resistors, fuses, etc.) to the circuit to prevent the transistor from being damaged due to overcurrent, overvoltage and other abnormal conditions.


Ⅸ.Replacement model of 2N5551


Frequently Asked Questions

1.Can 2N5551 be used as a switch?
Yes, 2N5551 can be used as a switch in electronic circuits to control the flow of current based on the input signal applied to its base.

2.How does 2N5551 work as an amplifier?
In amplification applications, the input signal applied to the base controls the collector current, thus amplifying the input signal at the collector.

3.What are the typical voltage and current ratings of 2N5551?
The typical voltage ratings of 2N5551 include a collector-emitter voltage (VCEO) of around 160V and a base-emitter voltage (VBE) of around 6V. The collector current (IC) is typically around 600mA.