How does this circuit work?

西里古1992

How does this circuit work? [Copy link]

What is the structure of the two transistors Q8 and Q9? What is the conduction sequence of Q10, Q8 and Q9?

西里古1992

Do Q8 and Q9 form a constant current circuit???

maychang

Sirigu1992 posted on 2020-10-20 12:24 Do Q8 and Q9 form a constant current circuit? ? ?

Q8 and Q9 are not constant current circuits.

Note that the collector of Q8 is connected to the base of Q9, and the collector of Q9 is connected to the base of Q8. Both tubes are inverting amplifiers. Together, they are in phase, and the amplification factor is very large.

chunyang

Q8 and Q9 are used to implement over-current protection.

First, Q10 is turned on. When there is a load, a voltage drop will appear on R406. R403 and R405 are connected in parallel with R406, so as the voltage drop on R406 increases, the potential of the base of Q8 will decrease. When the base potential of Q8 drops to a certain level, Q8 starts to conduct. The conduction of Q8 will increase the base potential of Q9, and when the base potential of Q9 rises to a certain level, Q9 starts to conduct. Once Q9 starts to conduct, it will accelerate the decrease of the base potential of Q8, and the decrease of the base potential of Q8 will increase the conduction degree of Q8, further increase the base potential of Q9, and then Q9 will further conduct. This is a positive feedback process, and as a result, once Q9 starts to conduct, Q8 and Q9 quickly enter the saturation state, pull up the gate potential of Q10 to cut off Q10, and due to the existence of R400 and U95 photosensitive tube circuit, Q8 and Q9 will maintain the saturation state until the drive of U95 is invalid.

Obviously, Q8 and Q9 are not constant current source circuits, but are equivalent to "thyristors".