As transistor amplifier Flow

Darlington pair
The advantage of using an amplifier with darlington pair is:
a. Strengthening great.
b. High input impedance but low output impedance.

Figure (a) Basic circuit Darlington Couples

Input resistance:

Description:
hoe = output resistance (Ro)
HFE = reinforcement transistor
r'E = resistance in RE
RE >> r'e r'e to negate the effect, but the RE too large causing the voltage gain will be reduced once.

where:

From Figure (b) may be made following equations:

Resistance input:

Transistor