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LM358: LM358 output distortion

Kevin Chen
Intellectual 390 points
Part Number: LM358
Other Parts Discussed in Thread: RC4558, OPA1678, TLV2170

Hi,

The input (EXC_C) is a sine (0.7V to 4.3V, 2.5V is the middle point),the output (EXC) should be a sine (3.45V to 12.39V, 7.92V is the middle point). But the output had a bit of distortion actually.

Channel 1(yellow) is EXC, channel 2 (green) is the base of 3904.  The output of LM358 at PIN1 is the same with EXC.  The negative feedback doesn't work. I don't know what happened with this circuit. 

channel1 is EXC, channel 2 is EXC_C.

channel 1 is EXC, channel 2 is the input of LM358 at PIN2.

  • 0
    TI__Guru**** 165236 points

    Kevin,

    The LM358 output driver has 3 * VBE voltage change between sourcing current and sinking current. This can be seen in the schematic in the data sheet. (VBE is base emitter diode forward voltage) It takes about 7 us to switch output current drive modes. An output pull down resistor could be added to keep the LM358 output always sourcing current.

    The RC4558 is a low cost alternative that does not have this crossover time delay.

  • 0 in reply to Ron Michallick
    Intellectual 390 points
    Hi Ronald,
    Thank you for reply！And RC4588 can't recognize the input down to 0.7V, full range output swing can't higher than 12V when VCC=+15V，VCC-=0V. So RC4588 can't use in this situation.
  • 0 in reply to Kevin Chen
    TI__Guru**** 165236 points
    Kevin,

    Your input common mode on pin 3 is 4V which is fine for RC4558. Into a lighter load RC4558 has a better VOH than LM358, especially at low temperature. Typical RC4558 VOH at 25C, is VCC - 0.75V - IOUT * 205 ohms (+2mV/C); this is better than LM358 that has VOH = VCC - 1.2V - IOUT * 40 ohms (+4mV/C). VOL is +1.5V + IOUT * 205 ohms; this is weaker than LM358 that has VOL = 0.6V + IOUT * 70 ohms.

    For a rail to rail output, there is OPA1678 or TLV2170 (and many more)