Help on single-supply buffer OPAMP

Discussion in 'The Projects Forum' started by tifr, Jul 16, 2009.

  1. tifr

    Thread Starter New Member

    Jul 16, 2009
    I need to use a OPAMP as a Buffer ( GAIN of 1) inorder to drive a load of 40mA. The circuit conditions are the following:
    Vin range : 0 - 10 V
    Power Supply available : Single supply of +15 V.
    Output current driving capability of OPAMP: Should be able to drive ~40mA.

    Can anyone guide me on which OPAMP will meet the above requirements.?

    Thanks. PR
  2. bertus


    Apr 5, 2008

    Here is a table from national for larger output current opamps.
    (remove the .txt to read it in excel).
    For 0 - 10 volts look for rail to rail types.

  3. millwood


    40ma is not a whole lot of current for modern opamps, especially current feedback opamps.

    if you want to go the chip route, the BUF family from TI is proven. EL2k from Intersil / Elan are also great - expensive too.

    if you want go discrete, you can build a diamond buffer (most CFB opamps have a diamond buffer as their output stage, thus the high current capability).

    if your prior stage is an opamp, you can incorporate a simple Class A buffer in the output to do the trick.

    too many solutions to list.
  4. tifr

    Thread Starter New Member

    Jul 16, 2009
    Thanks a lot for your replies millwood & bertus,
    I missed out on the other requirement that the OPAMP should be in a DIP package preferably 8 pin.
    My prior stage is a analog signal output from an (NI) National Insrtruments card. And it can drive only a max current of 5mA.

    PS: Iam still searching for the OPAMP to suit my requirements.......
  5. SgtWookie


    Jul 17, 2007
    What is your bandwidth requirement?

    Do you need the output to be rail to rail?

    If your bandwidth requirement is low, you could use an inexpensive opamp with a voltage follower:
    R1 limits Q1's base current.
    R2 limits Q1's maximum output current.
  6. tifr

    Thread Starter New Member

    Jul 16, 2009
    Thanks Sgtwookie,

    Here is the reply to your queries:

    a) I am not so particular of the BW as it is only a DC signal for most part of the time. The signal will vary slowly as per the controlled parameter. 1Mhz BW is fine.

    b) Again, I do not require it to be rail-rail. Signal is 0-10 V. Supply is +15V.

    I will try out your given Circuit. It seems alright. I have to get the IC from the market today.
    But can the transisitor be avoided completely??? I cheked the specs of LM358 and it seems it can provide my required current i.e. upto 40mA of current. Whats your opinion??

    Thanks PR
  7. Audioguru


    Dec 20, 2007
    The minimum output current from an LM358 is 20mA sourcing and 10mA sinking with a 3V loss.
  8. SgtWookie


    Jul 17, 2007
    No, the transistor is required for this inexpensive opamp to be able to provide the current you require. Opamps capable of supplying your required current would cost three or more times as much. Transistors are very inexpensive and easily available anywhere.

    Instead of the LM358/LM2904 you could also use the LM324/LM2902, which is a quad opamp. I've chosen these opamps because they have been available for many years and are still in production by a number of manufacturers.

    Note that you should connect any unused opamp channels as voltage followers (output to inverting input) and ground their noninverting inputs to avoid high-frequency oscillations.

    R1 could be omitted, but it is there to limit the opamps' output current to keep it from overheating in case the load becomes a short to ground. If the output shorts without that resistor, the opamp will be stressed.

    R2 limits the transistors' maximum output current. It also helps reduce the transistors' power dissipation.

    The transistor can be any of a wide variety of general purpose NPN transistors, so long as they are rated for a collector current of 200mA or more, Vceo of 30 or more (collector-emitter breakdown voltage), and a gain (hFE) of 40 or more at 40mA collector current.
    Last edited: Jul 20, 2009
  9. millwood


    buf634 is an in-production buffer.

    all you need is an opamp that's unity stable and does more than 40ma. that are just too many of them to chose from.
  10. tifr

    Thread Starter New Member

    Jul 16, 2009
    hello all,
    I did wire the circuit exactly as mentioned by sgtwookie and it seems to work alright with a dummy load. Thanks for the same.

    I am taking this circuit to do the actual load test. Will keep u guys updated on the progress.

    Also I am geting a slightly different voltage at the junction of the emmiter and RL from what is provided at the Non-Inv. terminal of LM358. It is about 100mV less. Any reasons why this is so? and how to correct this ?

    my selection of components is R1=1K, R2= 150 Ω , Transistor Q1 = BC549C
    OPAMP = LM358AN.