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LM4755: LM4755 Clipping (1 kHz test,sine)

Gerardo T.
Prodigy 10 points
Part Number: LM4755

Hi,

I constructed a prototype using the schematic as shown on page 2 of datasheet (typical application).

The voltage supply was set to 22 V dc. I noticed that the output starts clipping at around 7.5 V (bottom of waveform).

Bias voltage measured at 10.8 V. A 1 kHz sine tone was applied using a function generator to input of amplifier.

A 8 Ohm speaker is used.

Questions:

Why is the output clipping, at the bottom of the waveform, when there is remaining rail voltage available?

Is this behavior normal?

What can I do to prevent my output waveform from clipping at the bottom?

-Thank you in advance

  • 0
    TI__Guru* 98285 points

    Hi Gerardo,

    It seems you're actually reaching the limits of the amplifier. Since there is no EVM to test I ran a quick simulation using the TINA model and the lower limit clips at ~2.5V which is close to your observed ~3.3V.
    For this simulation I used 100Ohm loads to rule out current drive capabilities, but 8Ohm will cause considerably higher current load so the distortion is also expected.

    Here are the simulation details for your reference:

    snam056_test.TSC

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    Hi Ivan,

    Thank you for taking a look at the questions above. 

    From your reply, -Is it normal that the bottom of the waveform starts clipping first? 

    And, Why does the output starts clipping where the red line is? (should it be closer to 0 v?)

    -Thank you

  • 0 in reply to Gerardo T.
    TI__Guru* 98285 points

    Gerardo,

    This amplifier is not specified to be rail-to-rail neither on input or output, so the negative rail limitation could be on either side.
    Based on the simulation results, it seems normal for the waveform to clip at bottom first. As of why I'd have to check if further detail is available, I'll let you know of any findings early next week.

    Best regards,
    -Ivan Salazar
    Applications Engineer