Please help analyze the function of the transistor in this circuit.

Aguilera

Please help analyze the function of the transistor in this circuit. [Copy link]

Hello, teachers. In the figure below, the function of the transistor in this circuit is shown. Why do we need to add a transistor to this circuit, and why do we need to add two transistors?
Is it to improve the stability of the output?

maychang

There should be a "dot" at the intersection of the two lines in the red circle to indicate the connection, right?

呜呼哀哉

maychang posted on 2022-6-30 15:22 There should be a "dot" (dot) at the intersection of the two lines in the red circle to indicate the connection, right?

It seems to be drawn by AD, and the cross is basically a red dot. This does not improve stability. It feels like it is just current expansion. The current change on R14 adjusts the voltage, but the current basically passes through the transistor and does not pass through TL431. The weird thing is that there is no need to expand the current in this circuit. R14=1K, and the current does not exceed 2.5mA. I can't guess why it is like this.

maychang

Alas, published on 2022-6-30 16:30 It seems to be drawn by AD. Basically, the cross-shaped ones are red dots. This does not improve stability. It feels like it is just expanding the current. The current change on R14 adjusts the voltage...

"It seems to be drawn by AD. Basically, the cross-shaped ones are red dots."

There is also a cross under the collector of 1Q2, which obviously should not be connected.

蓝的天

First, this is a voltage regulator circuit that stabilizes the 2.5V voltage. You are right.
Second, the 2.5V voltage is obtained by subtracting the voltage of 1R14 from the voltage of the 5V terminal (note, it is not necessarily 5V).

Third, when the voltage at the 2.5V end increases, the transistor can control the current flowing through 1R14 to increase, causing the voltage of 1R14 to increase, thereby reducing the voltage at the 2.5V end until it stabilizes at 2.5V; and vice versa.

Fourth, using two transistors can improve the sensitivity of stabilizing the 2.5V voltage.
Fifth, this 2.5V is not used as a power supply. You can see that the 1K resistor "blocks" between 5V and 2.5V.