Design Darlington Pair as a buffer.

Discussion in 'General Electronics Chat' started by gusmas, Aug 23, 2009.

  1. gusmas

    Thread Starter Active Member

    Sep 27, 2008
    OK now i have a problem. I designed a Common Emitter Amplifier with a gain of 50(BC 107 Transistor). After calculating my Attenuation and multiplying it with my desired gain i got a answer of 30.something.

    Now my source is a electret mic with a R(intenal) of 1.8K. I need to prevent my transistor from loading my microphone so i need a Darlington Pair configured as a buffer to make the Rin(total) of the transistor bigger so it does not drain to much current from my electret microphone and it will decrease my Attenuation and increase my gain.

    My understanding of what is happening and what must happen might be wrong but if its not i need guidelines to design a Darlington pair as a Buffer.
    All that i know so far from a darlington pair is that the gain of it is approxamtly 1 and that it will increase the RL on my microphone.

    So how and where do i start to design a Darlinton Pair configured as a buffer
  2. bertus


    Apr 5, 2008
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1.8K is relatively low impedance.

    A common emmitter amp is fine!
    Use a higher gain, lower noise transistor such as a BC109C to start with.

    Try a dead standard circuit: low value emitter resistor (470R?) bypassed by an electrolytic cap, medium value (4k7?)collector resistor to power and a bias divider with two higher value resistors from base to 0V and V+ supply. Adjust the bias divider ratio to give somewhere around 60% of the supply voltage on the collector.

    Bias the electret mike with something like 4k7 or 10k to V+ and use a 10uF coupling cap from the mike in to the base of the transistor, plus another 10uF across the supply voltage.

    You should find it all works OK.