Darling-ton transistor is a special arrangement of two standard NPN or PNP bipolar junction transistors (BJT) connected together. the emitter of one transistor is connected to the base of the other to produce a more sensitive transmitter with much larger current gain being useful in applications where current amplification or switching is required.

Darling ton transistor pairs can be made from two individually connected bipolar transistors or a single device commercially made in a single package with the standard : Base, emitter and collector connecting leads and are available in a wide variety of ease styles and voltage (and current) ratings in both NPN and PNP versions.

The main features of the darling ton connection is that the composite transistor acts as a single unit with a current gain that is the product of the current gain of the individual transistors. if the connections is made using two separate transistors having current gain of $\beta_1$ $\beta_2$, the darling ton connection provides a current gain of

$\beta_D$ = $\beta_1$ . $\beta_2$

If the two transistors are matched so that $\beta_1 = \beta_2 = \beta$, the darling ton connection provides a current gain of

$\beta_D = \beta^2$

A darling ton transistor connection provides a transistor having a very large current gain, typically a few thousands.

BB = $\frac{Vcc - Vbe}{RB + \beta. RE}$

$\beta_D$ > > VBE

IE = (1 + $\beta_D$) IB

IE = $\beta$D IB

The dc voltages are,

VE = IERE

Advantages : A typical darling ton transistor has a current gain of 1000 or more, so that only small base current is needed to make the pair switch on higher switching currents.

Another advantage involves providing a very high i/p impedance for the circuit which also translates into an equal decrease in output impedance.