Darling-ton Pair

The Darling ton pair is cc - cc cascading having following properties.

1) It provides very high value of i/p impedance in the range of M$\Omega$

2) It provides unity voltage gain.

3) It provides large value of current gain.

4) It provides very low value of o/p Resistance.

Applications :

1) It is used in current Boasting ckt.

2) It is used in Butter Ampli

Current gain (Ai)

Let $\beta$ 1 = i c 1 / i b 1 is current gain of TX Q1.

$\beta$ 2 = ic2 / ib2 is current gain of TX Q2.

then overall current gain of Darling ton pair is given as.

Ai = $\beta_1$ x $\beta_2$

RB2 is i/p Resi locking thro' B2

$\therefore$ RB2 = r $\pi$ 2 + (1+$\beta_2$) RE

RB1 is i/p Reri looking thro' B1

RB1 = r $\pi_1$ + (1 + $\beta_1$) RB2

let Ri is overall i/p Resistance of Darling ton Pair.

$\therefore$ Ri = $R_1$ || $R_2$ || $RB_1$

Let Av = $\frac{vo}{vi}$ is voltage gain of darling ton pair.

vo = io (RE)

io = ib1 (1+ $\beta_1$ (1+ $\beta_2$)

$\therefore$ Vo = ib1 (1+ $\beta_1$) (1+ $\beta_2$) RE

Vi = ib1 . Ri

$\therefore$ Av = $\frac{(1+\beta_1)(1+\beta_2)RE}{Ri}$

Let Ro is overall o/p Resi given as