Single supply opamp buffer

Discussion in 'General Electronics Chat' started by sgrover, May 12, 2015.

  1. sgrover

    Thread Starter New Member

    Feb 1, 2013
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    I am trying to implement an opamp buffer with a single supply. The voltage being amplified is DC. (This is to isolate a digital output pin that can't supply enough current to the next stage) The opamp I have tried is LF356.

    Now the input voltage is either 3.3 V (high) or 0 V (low). Will this work? I've tried this with an input of 0 V and it doesn't (the ouptut is at 10 V).

    I understand that if an AC signal is being amplified, the ground can be lifted by adding VCC/2 and later removing the DC, but what happens if the input itself is DC?

    I've been under the impression that if an opamp is fed with a single supply (VCC and GND), everything remains the same except that now the opamp rails have now changed. Is this correct? My understanding of single supply opamps is shaky and I'd appreciate inputs. Thanks.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    The usual method of increasing the current output is to use a transistor. An opamp would not be my first choice for such an application. What kind of load are we talking about?
     
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  3. DickCappels

    Moderator

    Aug 21, 2008
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    The LF356 was not designed for single supply operation. Try an LM358 if you want to use a single supply opamp and speed is not an issue. You might have to put a 1k resistor from the output to ground to get really close to zero volts.
     
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  4. crutschow

    Expert

    Mar 14, 2008
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    You need to look at the common-mode range and the output voltage swing of the op amp.
    A single-supply op amp will have a common-mode minimum value and an output swing that goes to the negative rail (which is ground in a single-supply configuration).
    Standard op amps typically only go to within a couple volts of the negative rail for those two parameters, thus cannot work down to zero volts in a single supply setup.
     
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  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I'm with Papabravo. Why not use a transistor (BJT or FET)?
     
  6. sgrover

    Thread Starter New Member

    Feb 1, 2013
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    Thank you for the replies. In case I choose to use a BJT or FET, what should the circuit be?
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    You can use either a common emitter configuration if you want an inversion of the logic, or a common collector (emitter follower) is you don't want the inversion.
    If you use a FET make sure that 3.3 Volts is enough to tune the device fully ON. Again common source or common drain (source follower).
     
  8. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    'Ground' referenced single supply operation requires a device exhibiting a CM input range extending to the negative (cip 0V) rail --- FWIW fast, high Z, Rail-to-Rail devices are abundant, inexpensive and available (e.g. LMC6482)

    That said, depending upon load requirements, I concur with @Papabravo 's assertion that an op-amp may not be your best solution...

    Best regards
    HP
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The TS has yet to tell us what the load is? or what the supply for the load is? Does it need to be driven high or low? How fast? etc...
     
  10. Brownout

    Well-Known Member

    Jan 10, 2012
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    I concur with the above diagnosis regarding rail to rail opamps. I also want to point out that, depending on how much current you need, simply using a CMOS logic buffer or inverter might get you want you need more simply. You can even parallel any number of devices to increase available current by the product of number of devices. For example, by paralleling 4 74HC04 inverter gates, you can draw > 20 mA and maintain CMOS logic levels.
     
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