2N3904 Overview and Key Specifications
The 2N3904 is a low power NPN transistor in a TO-92 through-hole package. It is designed for general purpose switching and amplification. Some of the key features and maximum ratings from the datasheet include:
| Parameter | Value |
|---|---|
| Collector-Base Voltage (VCBO) | 60 V |
| Collector-Emitter Voltage (VCEO) | 40 V |
| Emitter-Base Voltage (VEBO) | 6 V |
| Collector Current (IC) – Continuous | 200 mA |
| Total Power Dissipation (PD) @ TA = 25°C | 625 mW |
| Operating and Storage Temperature Range | -55 to 150 °C |
The maximum current and power ratings show that the 2N3904 is suited for low power applications up to a couple hundred milliamps. The 40V maximum collector-emitter voltage provides decent headroom for many circuit designs.
Electrical Characteristics
Next let’s look at some of the key electrical characteristics of the 2N3904 from the datasheet. These specs are listed at 25°C unless otherwise noted.
Current Gain
The DC current gain, hFE or β, specifies the ratio of collector current to base current. The 2N3904 has the following current gain characteristics:
| Parameter | Min | Max | Conditions |
|---|---|---|---|
| hFE | 100 | 300 | VCE = 1 V, IC = 10 mA |
| hFE | 60 | – | VCE = 1 V, IC = 50 mA |
| hFE | 30 | – | VCE = 1 V, IC = 100 mA |
The wide current gain specification between 100 and 300 at 10 mA is typical for general purpose small signal BJTs. The minimum gain remains fairly high at 30 even at the max rated current of 100 mA. This indicates the 2N3904 is useful across a wide range of collector current.
Voltages
Several voltage parameters are key for circuit design:
| Parameter | Min | Max | Conditions |
|---|---|---|---|
| Collector-Emitter Saturation Voltage (VCE(sat)) | – | 0.2 V | IC = 10 mA, IB = 1 mA |
| Collector-Emitter Saturation Voltage (VCE(sat)) | – | 0.3 V | IC = 50 mA, IB = 5 mA |
| Base-Emitter On Voltage (VBE(on)) | – | 0.85 V | VCE = 1 V, IC = 10 mA |
VCE(sat) represents the small remaining voltage across the transistor when fully turned on. This is a key parameter for switching circuits. The low sub-0.3V values enable efficient switching of the 2N3904.
VBE(on) is the forward voltage of the base-emitter diode required for the transistor to start turning on. The 0.85V maximum is a fairly typical value.
Frequency Parameters
The 2N3904 datasheet also provides several parameters related to AC and high frequency performance:
| Parameter | Min | Typ | Max | Conditions |
|---|---|---|---|---|
| Current Gain Bandwidth Product (fT) | 300 MHz | – | – | VCE = 20 V, IC = 10 mA |
| Collector-Base Capacitance (CCB) | – | 4 pF | 8 pF | VCB = 10 V, IE = 0, f = 1 MHz |
The 300 MHz minimum gain bandwidth product indicates the frequency range in which the 2N3904 provides useful current gain. The low 4 pF typical collector-base capacitance helps enable this fairly high bandwidth.
2N3904 Applications
The 2N3904’s key parameters make it suitable as a general purpose, low-power BJT useful in many applications:
Switching Circuits
The 2N3904 is commonly used to switch loads such as relays, lamps, buzzers, or logic circuits on and off. Its high DC current gain allows a low base current to control a much higher collector current. The low VCE(sat) enables efficient switching.
Some typical switching circuits with the 2N3904 include:
- Interfacing a microcontroller output to control a higher-current load
- Switching a relay on/off
- Toggling LEDs or lamps
Linear Amplification
While the 2N3904 has largely been replaced by op-amps and other ICs for many linear amplification needs, it can still be a useful building block in discrete designs such as:
- Simple common-emitter amplifier stages
- Differential amplifiers/long-tailed pairs
- Current sources and mirrors
The moderately high fT of 300 MHz also allows it to be used in some higher frequency applications such as IF amplifiers in the low VHF range.
Logic Gates
By cleverly combining transistors, diodes, and resistors, logic gates can be built out of discrete components. The 2N3904’s high gain and low VCE(sat) make it suitable for building gates such as:
- NOT/inverter
- NAND and AND
- NOR and OR
These techniques were used in vintage computing before the advent of modern logic ICs. They can still be useful for learning or in resource-constrained scenarios.
FAQ
What is the pinout of a 2N3904?
The 2N3904 pinout in a TO-92 package is:
- Emitter
- Base
- Collector
Is the 2N3904 an NPN or PNP transistor?
The 2N3904 is an NPN bipolar junction transistor. The complementary PNP part number is 2N3906.
What loads can a 2N3904 switch?
The maximum continuous collector current of 200 mA is sufficient for driving small relays, lamps, buzzers, or dozens of LEDs. However, for loads over 200 mA an external power transistor may be required.
How much current gain does a 2N3904 have?
The 2N3904 has a DC current gain (hFE) spec ranging from 100 to 300 at a collector current of 10 mA. Gain remains above 30 at the maximum rated current of 100 mA.
Can I use a 2N3904 as an amplifier?
Yes, the 2N3904 can provide voltage and current gain in common linear amplifier topologies up to its fT of 300 MHz. However, dedicated analog ICs are often more convenient for amplification.
The 2N3904 datasheet provides the key parameters needed to leverage this popular NPN transistor in many general purpose low-power circuits. Its high DC current gain, low VCE(sat), low capacitances, and moderately high fT make it suitable for switching, logic, and amplification applications. While largely replaced by ICs in modern designs, discrete transistors like the 2N3904 remain important building blocks to understand.
