2n3904 represents an N-type transistor, and 2N3906 is a p-type transistor. This is the transistor that we use the most as the control signal in the design of the circuit. What are the characteristics of these two transistors? The main feature is that it is extremely fast! That is the switch tube we often say! When we design the control part of the circuit, we only care about signal processing. In this case, we only need low-power triodes such as 2n3904 and 2n3906. The biggest feature of this kind of tube is its relatively fast speed, which means that our biggest advantage in the control process is that it has very little delay. The second feature is that the price is extremely cheap. However, if you use an unpopular triode, this is ten times the relationship. Two types of triode achieve the same function, and we don't need to buy expensive ones. Third, we must remember that the maximum current passed by 2n3904 and 2n3906 is generally 200mA, that is, its maximum current capacity is 200mA. It is best to design it to be less than 200mA when designing.
What is the difference between SMD transistor 2N3904 and 1N3904, can you substitute it?
The prefix of 3904, 2N means a triode, and 1N means a diode, so it is impossible to substitute the two.
2N3904 is a commonly used NPN transistor. Its withstand voltage is 40V, Pcm=400mW, Icm=200mA. Many low-power NPN transistors can be directly replaced. You can directly substitute the commonly used 9014, 2N5551, 8050, C1815, and 2N3903.