amplifieraudiopower
How can I implement a Power Amplifier stage for powering an 8 ohm load? I tried making an AB amplifier, but either it doesn't work or I'm just not able to make one. The output of the third op-amp is a 20V sine wave. This, however, can be changed. I just can't seem to be able to make a power amplifier stage for such a low output impedance…
The OPA541 data sheet says it can operate on power supplies as high as 80 volts and produce a peak current of 10 amps.
Now, if these are translated to a sinewave, the RMS voltage would be 80/2.828 = 28.3 volts and the current would be 10A/2.828 = 3.53 amps.
This would lead to a power output of 100 watts.
As you require 120V AC at 3A (power = 360 watts) you are well off-the-mark with this device and I have given it the most favourable chances in my analysis such as assuming it is a rail-to-rail output device.
For instance, if you dug deeper into the data sheet you would read that the output voltage swing on a 5A load and +/-35 volt power supplies is about +/-30V and this ratio won't be any better at +/-40V rails so, I'd expect the maximum output voltage to be only 70Vp-p or about 25 volts, leading to a power output of only 87 watts.
You would do better buying one of these: -
They cost about £20 from Maplin (UK source) and are rated at 200 watts into 4 ohms with a frequency response from 3Hz to 200kHz.
A low-count buffer amp might consist of a class AB emitter follower:
simulate this circuit – Schematic created using CircuitLab
Q2, a NPN general-purpose transistor will run a bit warmer than the lower PNP transistor. Something that can dissipate a bit more heat like TIP29 or MJE180 might be more robust.
C1 will be an electrolytic capacitor of greater than 6v rating, with positive end facing the two transistor emitters.
Note that this buffer depends on your instrumentation amplifier to provide +3v bias - "input" must be connected directly to op-amp output.
Best Answer
This is a complicated topic, and entire books have been written on it. It would be wise to start with those.
A resource I recommend is Bob Cordell's Designing Audio Power Amplifiers. Cordell does an excellent job explaining how to design high-quality Class-AB audio amplifiers.