Sometimes, this gain and input impedance of an emitter follower are insufficient to satisfy
the requirement. so asto extend the general values of circuit current gain (Ai
) and input impedance, two transistors are connect asynchronous in emitter follower configuration as shown in Fig-1.
Such a circuit is named Darlington amplifier.
Note that emitter of first transistor is hook up with the bottom of the second transistor and therefore the collector terminals of the 2 transistors are connected together.
The result's that emitter current of the primary transistor is that the base current of the second transistor.
Therefore, the present gain of the pair is capable product of individual current gains i.e.
*β = β1 β2
Note that prime current gain is achieve with a minimum use of components.
The biasing analysis is analogous there to for one transistor except that two VBE drops are to be considere.
Thus concerning Fig. 1,
Voltage across R2, V2 =Vcc/(R1+R2)*R2
Voltage across RE, VE =V2 − 2VBE
Since the transistors are directly couple, IE1 = IB2. Now IB2 = IE2/β2.
IE1 = (IE2/β2 )
Input impedance of the darlington amplifier is
Zin = β1β2 RE … neglecting r′
In practice, the 2 transistors are put inside one transistor housing and three terminals E, B
and C are bring out as show in Fig- 2. This three terminal device is know as a Darlington
transistor. The Darlington transistor acts sort of a single transistor that has high current gain and high
IE1 = β1 IB1 (∵ IE1 = IC1)
Now IE1 is that the base current of Q2 i.e. IE1 = IB2.
Characteristics. the subsequent are the important characteristics of Darlington amplifier :
(i) Extremely high input impedance (MΩ).
(ii) Extremely high current gain (several thousands).
(iii) Extremely low output impedance (a few Ω).
Since the characteristics of the Darlington amplifier are basically the identical as those of the emitter follower, the 2 circuits are use for similar applications.
When you need higher input impedance and current gain
and/or lower output impedance than the quality emitter follower can provide, you employ a Darlington amplifier.
Darlington transistors are commonly available.
Like standard transistors, they need only three terminals but they have much higher values of current gain and input impedance.
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