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Question based on folllowing diagram (TDA audio amp):

  1. Dec 6, 2014 #1
    guys for the critical frequency for low pass is it the following: 1/2pi* 2.2*10^(-6)*100k?
  2. jcsd
  3. Dec 6, 2014 #2
    The 2.2 [itex] \mu F [/itex] capacitor and the 100k resistor connected to 1 form a high pass RC filter. This is meant to remove all of the DC that may be present on the input of an audio amplifier. The cutoff frequency is [itex] \frac{1}{2 \pi R C} [/itex] wich comes to 0.7 Hz. The output capacitor and the speaker will form a high pass filter with a much higher cutoff.
  4. Dec 7, 2014 #3
    ok got it, but how about the 22Mircofarad capacitor connected in parallel to the 100K is that for bypassing the amplifier to increase the gain? And one more thing why is there both a 22 microfarad connected to the 4.7 k and there's a 0.22 microfarad connected to the 1 ohm i can never figure that out
  5. Dec 7, 2014 #4
    Because the TDA is supply from a single power supply. We need to bias the TDA somewhere in the middle of his "linear region". And this is a job for input voltage divider (R1,R2 = biasing resistors). C5 (bypass capacitor) was added to short any signal present at voltage divider output to ground.
    For example, without C5 any noise from +Vs will appear at the "1" input. And this noise will also be amplified by the amplifier gain (1 + R4/R5).
    So we add C5 to short any AC voltage present at this node.

    We added C2 because we don't want to amplifier any DC voltage present at the input of the amplifier (for example TDA dc-offset voltage).
    Without C2 DC voltage is also amplifier (1+ R4/R5) times. So by adding C2 we "reduce" DC voltage gain to one.
    Also C2 together with R5 form a "high" pass filter Fc = 0.16/R*C = 1.55Hz.

    1 ohm resistor together with 220nF (0.22uF) capacitor form so-called Zobel network or Boucherot cell. And its job is to prevent the amplifier from oscillation at high frequency that may occur due to reactance load. Thanks to this the amp will see pure resistance load (1 Ohm) at hight frequency (F = 0.16/ (1ohm * 220nF) = 727kHz )

    Last edited: Dec 7, 2014
  6. Dec 7, 2014 #5
    cool got it thanks and one more thing is the maximum power given by the circuit Vs^(2)/RL?
    Last edited: Dec 7, 2014
  7. Dec 7, 2014 #6
    No, why R1?
    The theoretical maximum output power for a single power supply is equal to
    [tex]Pmax = \frac{(\frac{Vs}{2})^2}{2*RL} = \frac{Vs^2}{8*RL} [/tex]
    where RL - is a speaker impedance.
    But this calculations do not include voltage drop across TDA (headroom voltage/saturation voltage).
  8. Dec 7, 2014 #7
    But if Rl was supposed to be lets say 4 ohms then the Pmax would be 4.5 watts though according to the spec sheet the Power output would be 12 watts given that the Vs is 12volts
  9. Dec 7, 2014 #8
  10. Dec 7, 2014 #9
    ok thanks Jony you're awesome and by the way just to make sure the coupling capacitors are for filtering the DC from entering the amplifier right? 100 microfarad and the 100 nanofarad
  11. Dec 8, 2014 #10
  12. Dec 10, 2014 #11
    and the diodes are just biasing diodes correct?
  13. Dec 10, 2014 #12
    No, these two diodes protect the output stage from inductive kick.
  14. Dec 10, 2014 #13
    jony what do you mean by an inductive kick?
  15. Dec 10, 2014 #14
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