A small signal transistor is designed for applications dealing with low voltages and currents, definitely not for power amplifiers. The 2N3904 is a small signal transistor, registered by Motorola in the mid-1960s, used for general purpose switching (100 mA) and amplification (100 MHz) applications. It is an NPN BJT transistor yet comes in PNP configuration (2N3906) as well. Knowing how to read the 2N3904 datasheet helps engineers better design it for reliable amplification and switching applications.
2N3904 Datasheet Analysis
The 2N3904 transistor has maximum ratings, as shown in the table below.
As you can see, the transistor can conduct a maximum 200mAdc with a maximum input voltage of 6 Vdc applied between base and emitter. The power dissipation is in milliwatts, clearly showing that the device is not designed for high-power applications. Now let’s take a look at the gain table of the device.
The table below shows the ON characteristics of 2N3904.
The 2N3904 datasheet also lists the DC current gain (hFE) of its alternative 2N3903 (PNP) in separate rows. You can see the PNP type has 50% less gain than the NPN type- the biggest reason why the NPN type is so popular. The minimum gain for 2N3904 is 40 for IC = 0.1mA which goes up to max 300 for IC = 10 mAdc at VCE = 1.0 Vdc. The gain at this point is ten times of the minimum gain that the transistor gradually achieves at the end for IC = 100 mAdc at VCE = 1.0 Vdc.
The point of this analysis is to acknowledge the importance of designing the circuit of the transistor for a hassle-free operation despite a high gain bandwidth.
The maximum voltage drop across the device (Vce) during the ON condition can have two different values as follows:
- 0.2Vdc when the IC = 10 mA and the base is driven with 1 mA (hFE = 10)
- 0.3Vdc when the IC = 50 mA and the base is driven with 5 mA (hFE = 10)
Small Signal Characteristics
Here the term fT represents the frequency at which the output signal current will be equivalent to or less than the input signal current, irrespective of the set amplification. That means, at this frequency, the transistor can no longer amplify the signal.
Here Cibo and Cobo represent input and output capacitance of the transistor, respectively, in a common base configuration with output open as shown in the image below.
Knowing these maximum input and output capacitance values helps designers to acknowledge the maximum frequency input signal to drive through the transistor to avoid leakage current.
The term NF represents the internally generated noise by the transistor due to temperature fluctuation, which should be no worse than 5 dB in this case, as per the 2N3904 datasheet. If your application is sensitive to this much noise, e.g., RF amplifiers, and you require an extremely quiet device, transistors like 2SC3324 or 2N5088 would be your ideal bet.
Due to inherent capacitance of the transistor (Cce, Ccb, Ceb), it takes some time for the current to rise and fall. It’s not instantaneous as the capacitor takes some time to discharge.
Alt Text: 2N3904 Switching Characteristics from the 2N3904 datasheet
As per the 2N3904 datasheet, the transistor takes 35ns to rise from its initial value to 10 % of its final value (delay time) and also takes the same time from there to reach 90% of its final value (rise time). The storage time indicates the amount of time required to get the carriers out of the base of the transistors.
Applications of the 2N3904
The 2N3904 can be used in these following ways:
- A minimum load switch (up to 200mA) for having less saturation voltage and maximum gain
- An amplifier modules such as audio amplifiers and signal amplifiers, because of their high current gain
- For automatic Switching applications
- For fast switching applications such as pulse width modulation
- Within home appliances such as TV and stereo systems
Packaging and Dimensions
The transistor is available in through-hole (TO-92) and SMD (SOT-23 and SOT-223) packages. The package dimension of the TO-92 package is shown below.
It is imperative to go through the 2N3904 datasheet in order to best understand the pin layout and design configuration of each package type. However, collecting all that information from different sources could take time.
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