What does this circuit do? What is the role of transistor Q3?

灞波儿奔

What does this circuit do? What is the role of transistor Q3? [Copy link]

Teachers, please help analyze, is the transistor Q3 PNP type functional in the circuit diagram below?

maychang

"Is the PNP transistor Q3 in the circuit diagram below functional?"

The role of Q3 is not obvious. Q3 does not have any feedback. Looking from the collector, the internal resistance is very large, and it is approximately a constant current source.

maychang

Q4, on the other hand, has a very strange connection. In the figure, Q4 is a PNP tube, but the base potential is higher than the emitter potential (which is exactly the voltage between the collector and emitter of Q1). I don't understand how Q4 works.

chunyang

The circuit is mapped, right? There should be an error, and the circuit to the right of R2 and R7 must be given.

仙猫

Q3 acts like an undervoltage switch, cutting off when the input drops below 12V.

(Although the lack of a bypass resistor between be and circuit seems unreliable)

Q2 and Q4 are too weird. No matter what the situation is, the base potential is clamped below 0.7V. I don't know what to do in the next stage.

灞波儿奔

Xianmao published on 2022-4-6 15:37 Q3 is like an undervoltage switch, which cuts off when the input is lower than 12V. (Although the lack of a bypass resistor between be and be seems unreliable) Q2 and Q4 are too weird, no matter what...

If it is undervoltage protection, can it be understood that when the input voltage is less than 12V, Q3 tube is cut off
? In this case, I think I agree with it.

I would also like to ask the teacher, if the output resistors at the back are connected using the method shown in the figure below, with a resistor connected to the midpoint, can this achieve quick shutdown?

灞波儿奔

maychang posted on 2022-4-6 10:39 Q4, on the other hand, has a rather strange connection method. In the figure, Q4 is a PNP tube, but the base potential is higher than the emitter potential (which is exactly the voltage between the collector and emitter of Q1). Not...

I mainly want to use two resistors in the totem to balance the midpoint voltage. Can I modify it as shown below?

maychang

灞波儿奔 posted on 2022-4-7 09:36 I mainly want to use two totem resistors to balance the midpoint voltage. Can I modify it as shown below?

"Can I modify it as shown below?"

"Yes" or "No" depends on what you intend to achieve. If you can achieve the goal, then it is yes; if you cannot achieve the goal, then it is no. Now I don't know what the goal is.

maychang

灞波儿奔 posted on 2022-4-7 09:36 I mainly want to use two totem resistors to balance the midpoint voltage. Can I modify it as shown below?

But if you modify it like this, it will definitely not work, that is to say, you cannot achieve any purpose.

The reason is that the collector junction of the upper transistor and the emitter junction of the lower transistor have short-circuited the power supply to ground.