Electronics > Beginners
why is class A audio amp saturating?
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SiliconWizard:

--- Quote from: d4n13l on August 13, 2019, 07:47:52 pm ---I made the circuirt you see in the attrachment and I'm using a small radio as input, however for some reason the output is saturated on the positive side of the wave and can't figure out why

--- End quote ---

As it is, the speaker will see a constant DC offset voltage. Never do that. Not only will it hinder the speaker's performance a lot, but it will also draw an enormous and unnecessary power, and could eventually destroy the speaker, make the coil overheat (and maybe make the insulator melt), or at least warp the speaker cone. A series capacitor would be required here (with high enough capacitance because it will form an high-pass filter with the speaker itself).

Audioguru again:
The output transistor will not play the signal if the speaker has a capacitor in series with it. When power is applied the speaker will make one POP sound as the capacitor charges then nothing after it.
Zero999:

--- Quote from: Audioguru again on August 18, 2019, 01:23:38 am ---The output transistor will not play the signal if the speaker has a capacitor in series with it. When power is applied the speaker will make one POP sound as the capacitor charges then nothing after it.

--- End quote ---
I think the idea is to add a pull-down resistor before the capacitor. The problem with that is it it either: limits the voltage swing, if it's too higher value or burns a lot of power ,if it's low enough to ensure a large voltage swing. Using a current sink, as I mentioned previously overcomes this problem, but requires another transistor which could be better used converting the amplifier to class AB.
fourfathom:

--- Quote from: Zero999 on August 18, 2019, 07:51:58 pm ---
--- Quote from: Audioguru again on August 18, 2019, 01:23:38 am ---The output transistor will not play the signal if the speaker has a capacitor in series with it. When power is applied the speaker will make one POP sound as the capacitor charges then nothing after it.

--- End quote ---
I think the idea is to add a pull-down resistor before the capacitor. The problem with that is it it either: limits the voltage swing, if it's too higher value or burns a lot of power ,if it's low enough to ensure a large voltage swing. Using a current sink, as I mentioned previously overcomes this problem, but requires another transistor which could be better used converting the amplifier to class AB.
(Attachment Link)

--- End quote ---

The current sink still has to continuously sink sufficient current to pull the speaker negative, so it's only somewhat better than the resistor when you are trying for a large output swing.
magic:
CCS is significantly more efficient.
Do the math: how much current it takes to drive 8Ω load within 1V of supply rail, what resistance is needed for that, how much current will it waste when idle.
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