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Below is my constructed audio amp and graphs that corresponds to the voltage at the collector of Q1 and the emitter of Q2. Everything works great until I add the capacitor to protect the speaker (8Ω). Then for some reason the emitter voltage becomes cut off. This is my first design so I am a newbie. I expected the capacitor to just act like an open circuit at the high frequency..? enter image description here

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  • \$\begingroup\$ I expected C2 to act like a (short circuit)* high frequencies \$\endgroup\$ – thecaptain225 Apr 19 at 16:09
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Then for some reason the emitter voltage becomes cut off.

When the signal at the collector of Q1 is high, Q2 (an emitter follower) is a good source of current into the 8 ohm load (via the 330 uF capacitor, C2) but, when the signal at Q1's collector goes low, because the load (8 ohms) is such a low value AND C2 is charged-up, all you have in the circuit is the 500 ohm resistor (R3) trying to discharge the 330 uF capacitor and it just isn't man enough for the job. This is one reason why we use push-pull stages to drive loudspeakers.

You could try lowering R3 in value and see the situation somewhat improve but then you'll find that the circuit's quiescent current rises quite high.

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  • \$\begingroup\$ Thank you for the help. This is actually a project for school. They gave me a design and I have to set the values to ensure the speaker receives a good signal, with a constraint. No resistor can dissipate more than 1/4 W. Any help or idea to achieve this will be much appreciated. When i lowered R3 it worked great. But the power it dissipated went well over the constraint. \$\endgroup\$ – thecaptain225 Apr 19 at 16:55
  • \$\begingroup\$ Well, I don't know if this would be regarded as cheating but, if you used several resistors in parallel to make "R3" then each might only consume less than 0.25 watts but together, the net resistance would be low enough to make it work. You can also try lowering the 330 uF to maybe 47 uF - at this value you might not get the low frequency performance required? \$\endgroup\$ – Andy aka Apr 19 at 17:39

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