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.

这个三极管电路的工作原理和前两个相同，一个重要改进是使用Q2对基准源进行放大 ，这就允许我们使用低电压的带隙基准源，获得比普通二极管更好的稳定性。添加Q2获得的另外一好处是Q1和Q2构成差分对，具有更好的温度稳定性。

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.

分析这个电路要从电机的反向电动势开始。在这个电路中，参考电压等于基准源LM385-1.2V加上Q2的Vbe压降0.7V：1.2+0.7=1.9   如果需要Vbemf 达到3.8V，分压电阻R2、R4和R3的比值应该是2：1。可以通过调制电位器R3来方便的获取这个分压比。但是在这个电路中使用微调电位器改编转速，会造成电路的补偿参数发生改变。所以R3只能用来微调转速，调整5%或者更小的区间，并且只用来补偿元件的参数误差。

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).

设定分压电路后，并且当我们知道了电机的内阻后，确定R6R7参数就很容易了。R6R7并联的等效电阻 和 电机内阻的比值，应当等于R2和R4的比值（R3看做R2R4的一部分）