Darlington configuration is nothing but CC-CC cascade configuration as shown below,

Figure 1: Darlington Configuration

Feature:

1. It provides very large current gain of order of 8,000 to 10,000. Since current gain is very large, it is also known as 'Super beta' transistor. Actually overall current gain is a product of two gain of transistor, as explain below,

$A_i = \frac{ie2}{ib1} = \frac{ie2}{ib2} * \frac{ib2}{ib1}$

$A_i = (1+ \beta_2) \frac{ib2}{ib1}$ ... since $ib2 = ie1$

$A_i = (1+ \beta_2) \frac{ie1}{ib1}$

$A_i = (1+ \beta_2) * (1+ \beta_1)$

$\beta \gt\gt 1 $

$A_i = \beta_1 \beta_2$

1. Input resistance of darlington pair is very very large as explained below,

$Rin = hie2 (1+hfe1) + hie1$

1. Drawback of darlington pair is they start conducting after 1.4V as shown in below figure,

Application:

• It can be used to control speed of high power DC motor.
• It is also used in linear regulator power supply.