I am working on an amplifier based on 555 triangle generator, a comparator and the IRAUDAMP1 as a guideline.

My problem is that i have high DC offset voltage at the output (-1.6V). What is the cause of this?

enter image description here

Is the problem right at the beggining, on the comparator circuit? Is the only way to fix this to use separate inductors for filtering for each mosfet?


The problem was the 555 generator. The exponential triangle causes DC offset problems.

  • \$\begingroup\$ So it would be better to use a single comparator, say like LM311 (which has offset nullification capabilities) and use that to drive both high and low side by using an inverter on one side? \$\endgroup\$ – C Cezar Jul 25 '16 at 8:36
  • \$\begingroup\$ It would eliminate one source of imbalance, result in a component saving, and be less likely to inadvertently drive the driver with an overlapping 'on' waveform that would cause shoot-through in the FETs, all reasons for going to one comparator. Given the other imbalances, it might not be a total fix. The offset null might be a convenient hook to feed the DC back to, but it's not necessary. \$\endgroup\$ – Neil_UK Jul 25 '16 at 8:45

You are trying to get zero out for zero in by dead-reckoning.

Any imbalance anywhere will give rise to a DC offset. Although all the components in the two sides are nominally the same, you cannot assume exact matching.

You use seperate comparators for the H and L FETs, which will have different offsets and speeds, level translators with different speeds, the high and low channel in the 2110 driver will be different, two physically different FETs, and a power supply that may or may not be balanced accurately. The elephant in the matching of course is the high/lowside driver, that uses a bootstrap high voltage supply, and two FETs of the same polarity to do a job that should (for balance) use opposite polarity FETS.

It is a wonder that you get as close to zero offset as you do.

Do not change the output topology to filter the FETs separately, keep the single inductor from the FET junction as you have it.

If you need a zero output offset, then it may be worth closing a low frequency filtered DC correction loop around the whole amplifier.

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  • \$\begingroup\$ Before switching to one comparator, i just added a quick op-amp in front and a negative feedback. The output looks good until the peaks. What could this be? Also, sidenote, using a discrete inverter (two complementary inverter) instead of an IC will cause me great trouble? I know that in ICs the transistors are very precisely built and using discrete there will be more imbalances. The question is, is it still better than using two comparators? \$\endgroup\$ – C Cezar Jul 25 '16 at 10:50
  • \$\begingroup\$ The image, it didn't let me edit after 5 minutes : image \$\endgroup\$ – C Cezar Jul 25 '16 at 10:57
  • \$\begingroup\$ @CCezar use one channel of the 393 and one level translator, drive both H and L inputs in parallel. What do you mean ...until the peaks - ah - I was suggesting a very slow DC correction loop, not signal frequency negative feedback. \$\endgroup\$ – Neil_UK Jul 25 '16 at 12:46

I would say the a better solution would be to use a single comparator and a mosfet driver that takes a single input. Like the Microchip MCP14628 or something similar that can handle +/- 30v (which is huge). Then you could use a simple zero balance pot to calibrate the circuit by setting the average DC level for the 555 (that might work with the existing circuit, too). Also, with that chip, you would not need Q1 or Q3 because the MCP14628 has TTL level inputs. Another advantage is the built in non-shoot thru control which can be very important at high power levels.

A simple back of the napkin calculation shows that for a 60v differential across 4 ohms yields 900 watts. What power output are you aiming for? If you're looking for that sort of output, I don't think a few volts at idle would matter too much to the speakers that could handle that.

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  • \$\begingroup\$ Unfortunately i already have the components and it's hard for me to source. I build this in order to learn, i only care to get as much performance as I can with what I have available. Having that said the 555 DC level is a good ideea. Also, the output load is connected to ground, so peak voltage will be 30volts (not even this because of the low pass passive filter). I'm aiming for around 100-150W at the output. (RMS, not PEAK) Right now I'm just trying to find out where is this offset coming from more specifically. \$\endgroup\$ – C Cezar Jul 25 '16 at 12:31

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