understanding amp circuit

Discussion in 'Homework Help' started by fran1942, Aug 6, 2011.

  1. fran1942

    Thread Starter Member

    Jul 26, 2010
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    Hello, I have attached a circuit diagram to amplify an electret mic (suitable for feeding into a Picaxe chip.
    I am trying to understand the function of each of the components.

    Can someone please help me with the following:

    - What is the purpose and method of calculating the 10K and 1M ohm resistors ?

    Also, I understand that the .1 microfarad capacitor is there to block unwanted DC noise. Is .1 a standard value to use or was it calculated somehow ?

    Thanks kindly for any help.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The 1 meg resistor could probably be left out and not affect the circuit much. I believe it gives a small amount of negative feedback. Very small really. A small resistor could be added in series with the base to reduce gain, or a pot to control gain level

    The 10K resistor sets current for the amp and develops the voltage at output.
     
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    I take that all back. The one meg resistor is giving a DC bias to the base. It shouldn't be that large though. As a matter of fact the circuit doesn't seem to be likely to work very well at all. The transistor needs to be biased correctly for starters.
     
  4. Kermit2

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    edit: content removed as not relevant to the subject
     
    Last edited: Aug 7, 2011
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    With the low base bias resistors shown, plus the effect of negative feedback from the collector, the input impedance will be low, severely loading down the output from the microphone. The original circuit would have had pretty poor bias stability, but the input impedance would have been considerably higher.

    The loading effect won't be obvious if you use a sine voltage source directly, but if you add a few kΩ in series to simulate a real microphone the effect should become clear.
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    The original picture shows the output going to a 555, I guess to the trigger input? The purpose seems to have a high impedance transistor input, that will imprint an AC on it's output when noise/sound of a certain intensity is detected. Someone correct me if I'm wrong.
     
    Last edited: Aug 6, 2011
  7. Kermit2

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    Feb 5, 2010
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    edit: removed as not applicable to OP's question
     
    Last edited: Aug 7, 2011
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