What tube is 2N3904? 2N3904 Pin diagram and parameters+2N3904 Usage+2 application examples

3/28/2025 11:11:39 AM

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Welcome all technical staff advice.

Today to share with you is: 2N3904 triode.

1--What kind of tube is 2N3904?

The 2N3904 is an NPN transistor used in general-purpose switching or low-power amplification applications.

Primarily designed for medium voltage, low power applications and operating at medium high speeds.

2--2N3904 Pin diagram and parameters

The 2N3904 transistor consists of three pins:

Pin 1 (emitter) : Current will flow through this terminal.

Pin 2 (base) : This pin controls the transistor bias.

Pin 3 (collector) : Current supply to the entire terminal.

3--2N3904 triode parameters

1. Specifications

Collector current (Ic) (Max.) : 200mA
Dc current gain or hFE (Max.) : 300
Dc current gain or hFE (min) : 100@150mA, 10V
Collector emitter voltage (Vce) : 40V
Base to emitter voltage (Vbe) : 6V
Collector-base voltage (Vcb) : 60V
Vce saturation (maximum) under Ib and Ic conditions: 500mV@50mA, 500mA
Power - Max: 800mW
Frequency to conversion: 100MHz
Package/housing: TO-39-3, TO-205AD

2. Features

High voltage gain: The 2N3904 has a high voltage gain and is suitable for use in amplification circuits.
Low noise: The transistor has a low noise figure, which is very useful in low-level signal amplification applications.
High current gain: The 2N3904 has a high current gain, which is suitable for driving loads such as relays and lights.
Fast switching speed: The transistor's fast switching speed makes it ideal for switching applications.
Low cost: 2N3904 is cheap.

4--2N3904 How transistors work

In a 2N3904 transistor, most of the charge carriers are electrons, so they are always negatively charged.

The state of the transistor can be switched on from reverse bias to forward bias based on a small voltage at the base terminal (e.g. 0.7V).

Normal working conditions:

Base voltage (Vb) > emitter voltage (Ve).
Collector voltage (Vc) > Base voltage (Vb).

If the base pin is connected to the GND terminal, both the emitter and collector terminals are reverse-biased or remain open.

Again, once the signal is fed to the base pin, it will be forward-biased.

The 2N3904 triode has a high gain value of 300, which determines its amplification capacity.

The maximum current supply at the collector terminal is 200mA, so loads consuming more than 200mA cannot be connected through this transistor.

Once a current source is supplied to the base terminal, the transistor can be biased. The IB current must be limited to 5mA.

When the 2N3904 NPN transistor is fully biased, it allows up to 200mA of current to flow through two specific terminals,

the emitter and the collector, in a specific phase called the saturation region. In addition,

the collector-emitter/collector-base terminals are capable of handling typical voltages of 40V and 60V respectively.

Once the base current is separated, the transistor will be turned off, so this stage is called the cutoff region, and the VBE may be around 600mV.

5--What triode can be used instead of 2N3904?

1. Equivalent transistor

BC636\BC547\BC549\BC639\2N2222 TO-18\2N2222 TO-92\2N2369\2N3906\2N3055\2SC5200 ...

2. 2N3904 SMD

This article will not be repeated here for the time being

6--2N3904 uses

1, 2N3904 as the switch

The 2N3904 transistor can effectively be used as a switch in electronic circuits, allowing larger currents or voltages to be controlled using smaller control signals. When used in switching mode, the transistor operates in the cut-off or saturation region, providing an "on" or "off" state for the circuit.

A step-by-step guide to using 2N3904 transistors as switches:

1) Understand pin arrangement

It is important to understand the pin configuration of the 2N3904 transistor.

The emitter is usually grounded (common reference), the base receives the control signal, and the collector is connected to the load or switched circuit;

2) Determine the load requirements

Determine the load to be controlled using the transistor switch, and determine the voltage and current specifications of the load,

which will affect the selection of suitable resistors and the power handling capability of the transistor.

3) Select the base resistance

Calculate the base current (IB) needed to saturate the transistor and fully "turn on".

The base current is determined by the load current (IL) and the transistor's current gain (hfe).

Calculate the required base current:

IB = IL / hfe

Select a base resistor (RB) that limits the base current to a safe range, usually about 1/10 of the calculated value.

Calculate base resistance (RB) :

RB = (Vcontrol – VBE) / IB

Vcontrol is the control signal voltage
VBE is the base-emitter voltage drop (about 0.7V for silicon transistors)

4) Connect the transistor

Connect components according to the circuit diagram.

Connect the emitter of the transistor to the ground or a common reference point.
Connect the collector to the positive supply voltage and load.
Connect the base to the control signal via the base resistance.
Ensure correct polarity when connecting load and power;

5) Test and operation

The control signal is applied to the base of the transistor.

When the base current exceeds the threshold, the transistor enters the saturation region and conducts current between the collector and emitter, effectively "turning on" the load.

When the control signal is removed or reduced below the threshold, the transistor enters the cut-off region and "turns off" the load;

6) Protection measures

Protection elements such as flyback diodes or clamping circuits are used to suppress voltage spikes generated during switching operation,

ensuring the function of transistors and other components.

2N3904 transistor as a switch practical case

The 2N3904 NPN transistor is first used to control the simple LED on/off state as shown in the figure below,

the collector is powered, the LED is connected to the resistor (current limiting) at the emitter, and the other end is grounded.

Since no voltage is applied to the base, the 2N3904 is in a reverse-biased state,

so the LED goes out. Now, when we apply 5V voltage to the base,

2N3904 gets a forward bias and now the LED lights up, as shown below:

In the above simulation, a simple LED is controlled and a manual switch is used.

2, 2N3904 as an amplifier

The 2N3904 transistor is commonly used as a general-purpose amplifier in electronic circuits.

Its versatile nature and availability make it suitable for a variety of applications where signal amplification is required.

When used as an amplifier, the 2N3904 operates in its active region, where a small input signal is amplified to produce a larger output signal.

Step by step guide for using 2N3904 transistors as amplifiers:

1) Pin arrangement
Familiarize yourself with the pin configuration of the 2N3904 transistor, as above.

2) Bias transistor
A suitable DC bias is established for the transistor by applying a suitable voltage to the base-emitter junction.

This bias voltage determines the operating point of the transistor and ensures that it remains in the active region.

A voltage divider network of resistors is used to create the desired bias voltage. For recommended bias conditions, see Datasheet.

3) Coupling capacitance

Coupling capacitors are used to block the DC voltage, allowing only the AC components of the input and output signals to pass through.

Connect one capacitor in series with the input signal of the base terminal and the other capacitor in series with the output signal of the collector terminal.

Capacitors prevent DC bias from affecting input and output signals.

4) Load resistance

Connect a load resistor between the collector terminal and the positive supply voltage.

The load resistance determines the gain of the amplifier and provides the load for the amplified signal.

5) Input and output impedance matching

To ensure effective signal transmission between the input and output stages,

the input impedance of the amplifier matches the source impedance,

and the output impedance of the amplifier matches the load impedance.

Impedance matching optimizes power transmission and minimizes signal reflection.

6) Test and adjust

Apply the input signal to the base terminal and observe the amplified output signal at both ends of the load resistor.

Adjust the bias network, such as the voltage divider resistance, to reach the desired operating point and optimize the amplifier's performance.

Use an oscilloscope or multimeter to measure and analyze input and output signals.

7) Stability and compensation

If necessary, compensation elements such as capacitors or resistors can be added to improve stability and prevent the amplifier circuit from oscillating.

Components are usually determined based on the frequency response of the amplifier and the required bandwidth.

8) Amplifier circuit configuration

Common emitter amplifier
Common-collector amplifier
Common-base amplifier

Of the above types, the common emitter type is the most popular and most commonly used configuration.

When used as an amplifier, the transistor's DC current gain can be calculated using the following formula:

Dc current gain = Collector Current (IC)/Base Current (IB)

2N3904 as an amplifier example

In this simple amplifier circuit using transistor 2N3904, any input signal needs to pass through capacitor C1.

The input is the feed from the filter to the Q1 base after removing all signals except the sound signal.

Q1 is set to the common-emitter version of the amplifier circuit.

The voltage divider circuit of Q1 needs to connect R1 and R2. Q1 collector output signal is insufficient.

Therefore, this signal must now be increased again through Q2 and Q3.

The circuit includes the Darlington circuit of Q2 and Q3. Now, the signal has better gain, and from there is driven through the C3 to the SP1 speaker.