Discussion in 'The Projects Forum' started by Joe24, Aug 26, 2008.

  1. Joe24

    Thread Starter Active Member

    May 18, 2007
    So I was looking at the Data sheet for the TL081CP OP AMP. Specifically at the transitor level schematic, which I have attached. Now, say the output of the op amp to swings from rail to rail ( From say, +10V to -10V).

    Ok, so here is my question. Looking at the output stage of this op amp, which mainly consists of a npn and pnp transistor.When the output of the op amp is positive, then current flows from (+)Vcc, through the npn transistor, and to the output ( It is sourcing current). When the output of the op is negative, the current flows from the output, through the pnp transistor, to
    (-)vcc ( Sinking current). Is this correct? Or am I looking at it wrong.

    Thanks a mil.
  2. SgtWookie


    Jul 17, 2007
    It's not a rail-to-rail output, but we'll go with that for the moment ;)

    As far as your 2nd paragraph - yes, that's correct. :)
  3. Joe24

    Thread Starter Active Member

    May 18, 2007
    Ok, so my second paragraph is correct, thats good.

    Can you elaborate on why it is not rail to rail? It can be configured to be as such no?

  4. Audioguru

    New Member

    Dec 20, 2007
    Opamps with a rail-to-rail output usually are a CMOS type. The TL081 has emitter-follower junction transistors on its output and can swing to within about 1.2V to a supply rail without a load, to within 1.5V with a 1.4mA load and to within about 5V with a 10mA load.

    The TL081 and other old FET-input opamps have a problem that is called phase inversion. If an input gets to within about 3V from its negative supply voltage then its output suddenly inverts.