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Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
Okay, I think that will work just as I need then! I'll prototype when i get back to school, and hope that it will work, otherwise I gotta figure out that op amp offset circuit. Thanks!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, both outputs in a BTL audio amp should sit at half the rail voltage in the absence of a signal, no matter what the volume control is set to. The signal makes them flap around this idle point.
It might help to think of your output signal as ranging from -2.5V to +2.5V instead of 0 to 5. Then using a volume control on it turns it down towards 0V.
But are we talking analog amps for audio, PWM motor drive, or what? With PWM, this setup would result in both half-bridges running at 50% duty in the absence of a signal, and their duty cycles being mirror images of each other, like you hinted at.
This is valid, and it's how BTL Class-D audio amps work, but the "both outputs low to deliver no power" method is preferred for motor drives, since it doesn't have the switches switching all the time and wasting power in switching losses.
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
This circuit is for audio, and the problem that I'm having trouble getting my head around is how the offset value changes in op amps- as soon as the input signal changes amplitude in an attempt to adjust volume, both signals are no longer centered around half of Vcc, and the speaker is seeing an average DC value. The first simulation is with an input signal of 0-5V, and the wave looks just as it should. However, as soon as the input is changed from 0-2.5V, the offset values are incorrect. I would like these waves to still be centered around 2.5V with a p-p voltage of 2.5V. Where is the best part to modify the circuit for this?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
In a real BTL audio amp, it would be done by AC coupling the input signal. Stick a capacitor in series with the output of your V2 source, and use another couple of resistors to set the DC level after the capacitor to half the rail voltage.
Or prop the bottom end of V2 up on a 2.5V DC source, if you find that concept easier to understand.
Incidentally, what's with all the resistors around U1? I'd have expected to see it configured as a follower with gain of +1.
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
Thanks Steve, that is just what I needed to hear. Simulations are working properly and i tried the capacitor, but the two resistors were absent in my design. Also, those extra resistors are gone too around U1, i guess a perfect amplifier with a gain of 1 would be a follower Thanks for knockin some sense into me
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Simple Oscillator
Thanks for knockin some sense into me 