TL071 Single-Sided Minimum Common-Mode Voltage

Discussion in 'General Electronics Chat' started by TrevorP, Mar 1, 2010.

  1. TrevorP

    Thread Starter Active Member

    Dec 8, 2006
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    I'd like to use a TL071 (Operational Amplifier) as a comparator to determine when a signal surpasses a voltage of about 0.5V. Will I have an issue of that voltage being less than 0.7V as it is not a rail-to-rail operation amplifier?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, you will have an issue.

    Why don't you simply use an LM311 single, LM2903 dual, or LM339 quad comparator? They can all sense down to the negative rail, and they were designed for the job you want to do.

    A TL071 is a very good opamp for audio, but using one as a comparator is rather like using a hammer to install screws. It'll work, but there are more appropriate tools to use.
     
  3. TrevorP

    Thread Starter Active Member

    Dec 8, 2006
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    The reason I wanted to use a TL071 was because I'm already using one for an audio-preamp in the same circuit so using a dual pack would save money and space. Basically I'm trying to just have some sort of configurable indicator as to when the output of the pre-amp is past the safe threshold for what it is feeding to. As a sort of clipping indicator. Might there be another way of doing this by placing an LED/resistor in parallel with the negative feedback resistor on a non-inverting op-amp? The only issue would be tuning the LED turn on voltage.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    A schematic of how you had hoped to use the TL071 as a comparator would help.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    A TL07x and TL08x and other Jfet-input opamps have the problem called Opamp Phase Inversion where the output suddenly goes high if an input voltage drops lower than the minimum input common-mode voltage of a few volts.

    I use MC3317x low power single-supply opamps and MC3407x normal power wideband single-supply opamps that have inputs and outputs that work at 0V with a single supply voltage down to 3V. They are much better than LM358 and LM324 low power single-supply opamps.
     
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  6. TrevorP

    Thread Starter Active Member

    Dec 8, 2006
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    Thanks Audio I'll check those out.
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Another option is to shift the signal and reference slightly upwards (1V or something) so it fits in the input range. Depending on how you sense the signal level, could be as simple as tying a resistor divider to the +V rail instead of ground.
     
  8. TrevorP

    Thread Starter Active Member

    Dec 8, 2006
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    Yeah, I had considered that but I wanted a solution that would take the least number of chips so sticking another coupling cap and resistor divider seemed a little too excessive.
     
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