What is a constant current source

What is a constant current source ?

As the name suggests, a constant current source is a power supply that can output a constant current. r in Figure 5 is the internal resistance of the power supply E, and RL is the load resistance . According to Ohm's law: the current flowing through RL is I=E/r+R. If r is large, such as 500K, then RL is at 1K at this time--- When 10K changes, I will basically remain unchanged (only a small change) because RL is too insignificant relative to r. At this time, we can consider E to be a constant current source.

For this reason, we deduce that the constant current source is a power supply with a very large internal resistance.

In electronic circuits (such as transistor amplifier circuits) we often need amplifiers with larger voltage gains. For this reason, the load resistance of the transistor collector is often designed to be as large as possible. However, if this resistance is too large, the transistor will easily enter a saturated state. At this time, we can use a transistor to replace this large resistor. In this way, a large resistance can be obtained, and the DC voltage drop is not large at the same time, as shown in Figure 6.

The voltage stabilizing circuit composed of voltage stabilizing tube D and resistor R2 in the figure is used to bias the working point of BG1 and ensure the stability of the working point (BG2 is an amplifier tube). It can be seen from the output characteristics of the transistor that when the collector -emitter voltage VEC is greater than 1-2V, the characteristic curve is almost flat, that is, when VEC changes, the IC basically remains unchanged. In other words, the output resistance of the transistor BG1 Very large (above hundreds of kiloohms). In the figure, since the current of BG1 is basically constant, BG1 is called a constant current load of BG2. Since the amplifier with a constant current source load has a large load resistance , this amplifier circuit has a huge voltage gain. In fact, this circuit is used inside many integrated circuits.