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I have a circuit with a Darlington emitter follower used as a buffer between a common emitter amplifier and a low resistance load (speaker). i wanted to know if this arrangement could alone (with the corect values ofcourse) amplify an input from a microphone and give output at the speaker. could this arrangement be used for applications such as mobile phone speakers? if i have an overall voltage gain of say, about 170? would this be enough to drive a speaker of less than 1watt power?

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Can you show us the circuit? You don't have enough reputation to post it yet, but you can upload it somewhere and give us the link. Someone will stop by and edit it in. –  Camil Staps Apr 24 '13 at 12:59
    
A single-ended drive to a loudspeaker may burn out the coil - be aware of this. The amount of voltage gain required depends on the microphone as much as the speaker (and also the power rails of the output stage.) –  Andy aka Apr 24 '13 at 13:01
    
A gain of 170 (44.6dB) will likely not be enough to boost the output of a professional, low-impedance mic to useful speaker-driving voltage, but it could work with a high impedance unbalanced microphone (which is also easy to use with your single-ended circuit). The circuit has to present a decent impedance to a high-Z microphone, however! High impedances don't come easily in BJT circuits: not without more complicated designs with differential input stages and massive global negative feedback. You might benefit from a JFET input stage. –  Kaz May 12 '13 at 6:17

2 Answers 2

A single common emitter will probably be not enough to supply sufficient (170×) gain. A Darlington emitter follower will only decrease that gain further (approx. 0.9×), so the quick answer is no.

Depending on your exact circuit, the voltage gain may be fairly easy to solve/increase. So show us the circuit.

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2SD2153 and several more on DigiKey. –  Anindo Ghosh Sep 9 '13 at 7:57

As far as I know, Darlington emitter followers only operate in one direction—no AC. Microphones and speakers need current flowing in both directions. If you only have one single-sided follower, it isn't going to work. I think that you might do better with an op amp or two.

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Both op-amps (single-supply) and Darlington circuits handle the AC signal transmission in the same way, by AC-coupling the signal at either or both ends and providing DC bias within the amplification section. Hence, this isn't relevant to the question. –  Anindo Ghosh Sep 9 '13 at 7:50

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