1. Basic concepts and principles of triodes
Transistor, as a key member of the transistor family, plays an indispensable role in electronic circuits. It contains three basic parts: Base, Emitter and Collector. Here, we mainly focus on NPN transistors. The core characteristics of an NPN transistor can be described by an equivalent circuit, in which the connection between the base and the emitter is equivalent to a diode, and the connection between the collector and the emitter can be regarded as an adjustable resistor. The resistance of this resistor varies widely, from a few ohms to infinity (open circuit state).
Before discussing in depth, we must clarify the characteristic equation of the NPN transistor: Ic=βIb. In this equation, Ib represents the current from the base to the emitter, Ic is the current from the collector to the emitter, and β is the amplification factor of the triode. This multiple is a constant determined based on the production process, and its value is usually between tens and hundreds. However, it should be noted that the triode achieves this amplification effect by adjusting the equivalent resistance (Rce) between the collector and emitter. When Rce is adjusted to an extremely low value but still cannot achieve Ic=βIb, we call it a "saturation" state; conversely, when Rce is adjusted to an extremely high value but still cannot achieve Ic=βIb, it is called a "cut-off" state. Ideally, the transistor should work in the amplification region, that is, the state of Ic=βIb.
2. Construction and analysis of NPN transistor constant current source discharge circuit
In electronic circuit design, the application of constant current sources is crucial. Taking a conventional capacitor discharge circuit as an example, the discharge current Ic=Uc/R, where Uc represents the voltage of the capacitor. Since the capacitor voltage decreases over time, the traditional discharge current is not constant. However, by using NPN transistors, we can build a constant current discharge circuit.
In such a circuit design, the discharge current of the capacitor is independent of its voltage. For example, assuming that the Ve value of the circuit is 4.3V (calculated as 5V minus 0.7V), then we can find that Ic (the collector current) is approximately equal to Ie (the emitter current), calculated as Ve divided by Re ( emitter resistor). This calculation process is based on an important premise: the triode must work in the amplification area, that is, Ic=βIb must be satisfied. Considering that the general value of β is on the order of 100 times, Ie can be considered to be approximately equal to Ic.
3. Solution process of triode circuit
When designing and analyzing transistor circuits, we usually follow the following steps: first assume that the transistor works in the amplification region and meet the conditions of Ic=βIb and Ic≈Ie; then inversely deduce Uce (the voltage between the collector and the emitter) based on the calculation results ) is reasonable to determine whether the previous assumptions are true. For example, assuming the voltage across the capacitor is 10V, we can calculate Uce to be 5.7V, which in turn gives Rce a value of 5.7K ohms. This means that by adjusting Rce to 5.7K ohms, the transistor can maintain the discharge current of the capacitor at 1mA. Similarly, when the capacitor voltage is 8V, Uce is 3.7V and Rce is 3.7K ohms, so that the discharge current is still maintained at 1mA.
However, when the capacitor voltage drops below a certain threshold, such as 3V, we will find that the calculated result of Uce becomes a negative value (-1.3V), which is obviously unreasonable. This shows that even if Rce drops to 0 ohms, the condition of Ic=βIb cannot be satisfied. Therefore, when the capacitor voltage drops below 4.3V, the transistor will no longer operate in the amplification region but enter the saturation region. It is worth noting that in practical applications, the resistance between the collector and the emitter cannot be reduced to 0Ω, so the lowest value of Uce can generally only be reduced to about 0.2V. This value is called the saturated tube voltage drop Uces.
4. Application of PNP transistor in constant current source charging circuit
Different from NPN transistors, to implement a constant current source charging circuit, we must use PNP transistors. The working principle and structure of the PNP transistor are different from NPN, but it plays a vital role in realizing the constant current source charging circuit. In a PNP transistor, the direction of current flow is opposite to that of an NPN transistor, which provides greater flexibility in designing different types of electronic circuits.