The constant current source composed of two transistors can drive high power

Constant current source circuits are indispensable in lighting fixtures and battery chargers. The attached figure shows a constant current source circuit composed of two PNP transistors and four resistors. By selecting high-power transistors and heat sinks, the constant current can reach more than 1A.

Transistor Tr1 and resistors R1, R4, and R2 form the main circuit. R2 and R4 are connected in series to form the upper bias resistor of the base of transistor Tr1. When the power is turned on, due to the current flowing in the base, the collector current will flow through Tr1, and the emitter current will flow through R1, resulting in a voltage drop across R1. When the voltage drop across R1 reaches 0.6V, Tr2 will be turned on, and the collector current of Tr2 will flow through R2. The base current of Tr1 will be shunted. The base current of Tr1 will decrease, and the collector current will also decrease. Therefore, after Tr2 is turned on, it will limit the base current of Tr1, so that the output current of the circuit is stable at a certain current value. The size of the constant current is determined by the resistance value of R1 and the Vbe of Tr2. Assuming that the Vbe of Tr2 is equal to 0.6V, the output current I of the circuit can be calculated using I=Vbe/R1=0.6V/1Ω=0.6A.

For a crystal triode, the temperature coefficient of Vbe is about -1mV/℃, and the value of Vbe will change greatly with the change of temperature, so this circuit cannot be used in situations where high precision of the constant current source is required.