What is the use of Resistors in between stages

Discussion in 'General Electronics Chat' started by setsunaseiei, Mar 25, 2009.

  1. setsunaseiei

    Thread Starter Member

    Mar 25, 2009
    12
    0
    Based on the circuit below:

    [​IMG]
    [​IMG]
    What is the use of the resistors connected from the voltage supply put in between bypass caps, e.g. R3 (330) and R7 (27k)?
    I think it affect the DC bias of the circuit but how and why?
     
    Last edited: Mar 25, 2009
  2. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    R3, together with C5 decouple this part of the circuit from the rest of the supply so that signals in the supply line are not amplified by the first stage's gain.

    The same is true for R7 and C2 for your second stage.

    The 3dB points are at f=1/(2*pi*R*C). Stray signals get attenuated.
     
  3. setsunaseiei

    Thread Starter Member

    Mar 25, 2009
    12
    0
    Pls. tell me how it decouples the first from the second and the second from the last. How do I find the practical values for resistance and capacitance for each?
     
  4. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    R3-C5 de-couple the base of Q1 so that it will not amplify very low frequencies. Since you're using a battery there is no ripple voltage present and so I can't see why this decoupling is even there.

    R7-C2 do the same. I can only think that both decoupling circuits are meant to attenuate higher frequency signals to prevent the whole circuit from oscillating.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Decoupling is especially needed with a battery (a little 9V battery) because the battery has a fairly high internal resistance that keeps rising as the battery runs down.
    The internal resistance causes the battery voltage to jump up and down with the load current.
    Without decoupling then the jumping supply voltage feeds into the input stages then the amplifier amplifies it, causing putt, putt, putt motorboating oscillation.
     
  6. setsunaseiei

    Thread Starter Member

    Mar 25, 2009
    12
    0
    what are the best values for the R and C to use when designing, like any rule of thumb for this? thanks to all who answered.
     
  7. Darren Holdstock

    Active Member

    Feb 10, 2009
    262
    11
    RC values: You want the -3dB rolloff frequency way below the lowest signal frequency. Here R3 and C5 give a rolloff at ≈ 13 mHz, which is plenty low enough for audio.
     
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