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Need similar to LM324, that can drive closer to 0V

Other Parts Discussed in Thread: LM324, TLV274, LM7705

Recently I tried using an LM324, thinking it would work where a negative supply wasn't readily available.  Without getting too detailed, the circuit must be able to operate from a 9-12 V supply, and output a DC level that might be anywhere between 0 and about 3 volts. All looked well on the scope and with a meter, but I failed to take into account that I would be driving a circuit that had an existing voltage bias 0f about 1.5 volts.  That bias is extremely "light", so that any typical OP-AMP that had the backing of a dual polarity supply would be able to easily over-ride the bias. But since the 324 outputs are all driven by ordinary UJTs inside the chip, the OP-AM is powerless to "pull" the voltage below .6 volts.

GRRR!!! This was a serious setback. previously I had worked with a complicated circuit using bi-polar op-amps, and a separate OP AMP package to implement an  oscillator and charge pump to provide negative voltage to the original circuit. I had hoped all that would become unnecessary now having the LM324, which I'd hoped could work as needed without a negative supply. Can anyone recommend another OP-AMP, available in a quad package, that can drive closer to 0 VOLTS?

  • TLV274 has a stronger minus rail sink.
    LM324 has a weak rail sink that is about 40uA. After that a strong PNP sink takes over but VOL will be >0.6V
  • Thanks Ron. I actually did discover the TLV274 since I posted, and it is much better. Its still lacking a little a littler though and I think I've reached the practical limit. The problem is that I'm driving a voltage controlled amplifier whose maximum attenuation is at 0V, and full output (0dB) is at some voltage, about 3.2V.  If not for the fact that I'm fighting about 10K pull up to somewhere around 10V, the TLV274 would be perfect. But the best it can do into the pull up load is about 90mV, and at that level the cvoltage controlled amplifier still has some audible output. Its not terrible, but zero should be zero! :-)

    So I think what I'll need to do is develop a negative supply. Even a few volts to work with for the V- rail of just about any OP amp will enable me to use the OP-AMPs negative feedback and gain to do what the designers intended, which is to pull as far negative as is necessary to obtain a 0V output, if 0 is what I need.

    Now here's a puzzle for you (at least it is for me). As it turns out, I'm only using3 of the OP amps in my circuit for creating the final output to my VCA. Well in the past, I have used an OP AMP as a square wave oscillator to drive a charge pump, to create a negative supply for OTHER op amps.... as long as the other OP-AMPs were in a different package. But so far, even with combinations of diodes, I have yet to figure out a way to make an OP-AMP charge pump supply a negative voltage to its own package. Do you think its possible? Not a big deal to use a separate package, maybe even a simple 555 chip to build up a V- supply if it can't be done, but it would save some board space and cost if another package wasn't needed.

  • Hi Peter,

    If I may interject some thoughts on your application. The LM7705 is a low-noise, bias generator that provides a -0.23 V output. It is specifically intended so that a true 0 V level can be obtained from an op amp's output that doesn't quite make it to zero with a single supply configuration. This option will likely provide a lower part count, lower noise negative rail supply compared to alternative approaches using a 555 timer, op amp oscillator, etc. You can view the LM7705 information here:

    By the way, I cover the TLV274. If you have any questions, let me know.

    Regards, Thomas

    PA - Linear Applications Engineering