如题，下面是网上看到的一个三极管控制电机调速的一个电路和解析，小弟才疏学浅看了半天还是不能理解究竟它这个电路到底是怎么一个控制调速原理，所以贴出来大家一起来讨论下。

This circuit works on the same rules as previous two transistor version, but the main improvement is increasing the gain for the reference voltage by transistor Q2. This allows us to use micropower band gap voltage reference, which is more stable than standard diodes. Another improvement from adding Q2 is the temperature compensation of  Vbe between Q1 and Q2 transistors.

Calculating this circuit starts from setting the back EMF voltage. In this circuit, the reference voltage is equal to LM385 - 2.5 V and voltage Vbe of Q2:  1.2+0.7=1.9
If we need Vbemf to be 3.8V, the voltage divider R2, R4, and R3 factor should be 2:1. Potentiometer (R3) is for fine tuning this voltage, but in this circuit changing speed using the trimmer will cause compensation change. So R3 is only for final speed tunning in small range, say 5% or less, and should only be used to compensate the tolerance of other component values.

After setting this voltage divider, choosing R6 and R7 value is easy when we know motor internal resistance. The equivalent parallel connection of R6, R7, and motor resistance should have the same ratio as R2, R3, and R4 voltage divider (with the R3 potentiometer set in middle position).