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I read an electronic magazine in which there is a report on one laser diode constant power drive circuit can be used. After reading, I became interested in laser diodes. So I searched some related materials and information about this topic from google. Today I come to this forum to turn to you to discuss my understandings are appropriate. Here are my personal understandings: Picture One The reference voltage of the circuit does not use the common resistor divider circuit, but uses the Vbe of the transistor Tr1 as the reference voltage, Vbe is about 0.7V, ie (Im-Ib) Vr1 = 0.7V, but Ib is small and can be ignored. Vbe has a temperature characteristic of 2 mV / ° C, the reference voltage will vary with temperature, even if the temperature characteristic is much better than the constant current drive. The entire circuit uses only two crystal transistors, Vr1 for the output adjustment and load resistance, is quite simple APC circuit. This is a circuit designed to improve reliability, sharing five crystal transistors. The main features are as follows: ● The semi-variable resistor for adjusting the output is canceled. ● If there is a short circuit between the BEs of Tr5, the current flowing through resistor R2 becomes almost the base current of Tr1 and Tr2, which will increase the output. However, the base current Ib flowing through Tr2 will make 680Ib + Vbe> 2Vbe, the result Tr4 conduction, bypass part of the current to the ground, so that the output power is subject to certain restrictions. ● If any of Tr1 and Tr2 is short-circuited between C and E, there is no overcurrent due to the presence of another transistor. ● In addition to the five transistors, the short circuit of the remaining components will not cause the output to increase. Circuit R1 is the base resistance, doubling as a current sampling resistor; R5 for the load resistance. Ps: Excuse me if I was wrong in words or expressions as I am a green hand in the field of laser diodes. I need continual learnings. What is your idea ? Do you agree with my ideas ? Any of your ideas would be highly appreciated. May someone would like to help ? thanks in advance